Investigation of Low Temperature Plasma Generator

ISSN 0018�151X, High Temperature, 2010, Vol. 48, No. 1, pp. 97–125. © Pleiades Publishing, Ltd., 2010.Original Russian Text © E.Kh. Isakaev, O.A. Sinkevich, A.S. Tyuftyaev, V.F. Chinnov, 2010, published in Teplofizika Vysokikh Temperatur, 2010, Vol. 48, No. 1, pp. 105–134.

97

CONTENTSINTRODUCTION1. ELECTRICAL AND THERMAL CHARACTERISTICS OF DISCHARGE1.1. Current�voltage characteristic1.2. Distribution of electric current and of heat fluxes over segments1.3. The part played by radiation in heat flux

2. PROCESSES OF ARC SHUNTING

3. SPECTRAL INVESTIGATIONS OF PLASMA3.1. Automated measuring complex3.2. Results of spectral investigations

4. ELECTRODE PHENOMENA4.1. Erosion of tungsten cathode4.2. High�speed visualization of the cathode region4.3. Investigation of temperature fields of the tungsten cathode surface4.4. Electrode drops of potential

5. PLASMA TECHNOLOGIES5.1. Plasma hardening of metals

REWIEV

Investigation of Low�Temperature Plasma Generator with Divergent Channel of the Output Electrode

and Some Applications of This GeneratorE. Kh. Isakaev, O. A. Sinkevich, A. S. Tyuftyaev, and V. F. Chinnov

Joint Institute for High Temperatures, Russian Academy of Sciences (IVTAN), Moscow, 125412 RussiaReceived February 26, 2009

Abstract—A review is made of experimental and theoretical investigations of processes occurring in low�temperature plasma generators (LTPG) with divergent channel of the output electrode, and the possibilitiesof utilizing these generators in new plasma technologies are analyzed. Comparison is made of the character�istics of discharge (including the current�voltage characteristic) in a divergent channel and in a cylindricalchannel of uniform cross section. The effect of divergent channel of the output electrode and of its expansionratio on the pattern of physical processes in LTPGs of different designs is studied. Investigations are per�formed of the distribution of electric current and heat flux density along a channel with a segmented outputelectrode. The voltaic equivalents of heat fluxes to cathode and anode are determined. The process of “shunt�ing” of discharge is investigated, which causes fluctuations of electric arc�burning voltage. The investigationsinvolving an LTPG with divergent channel reveal that the voltage amplitude in the case of shunting decreaseswith increasing current strength and, at high currents of argon arc, does not exceed 1–2 V. Results are givenof spectral and visual investigations of LTPG. It is demonstrated that, in an LTPG with divergent channel, theplasma temperature in the region of energy input at currents of 300 A and higher exceeds 30 000 K. The sig�nificant part is found which is played by vacuum ultraviolet radiation in the process of closing the arc toanode. The mechanisms of erosion of the tungsten cathode tip are investigated, which play an important partin increasing the cathode service life by way of recirculation of tungsten atoms because of their ionization inthe discharge gap. Results are given of using an LTPG with divergent channel of the output electrode inplasma technologies of surface hardening, cutting, and hard�facing of metals. The technology of plasmahardening of wheel pairs, adopted by the RZhD (Russian Railroads) Joint�Stock Company, provides forincreasing the service life of railroad wheels by a factor of 1.5–2.

DOI: 10.1134/S0018151X10010141

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HIGH TEMPERATURE Vol. 48 No. 1 2010

ISAKAEV et al.

5.2. Plasma cutting

5.3. Plasma hard�facing on Hatfield steel

5.4. Plasma hard�facing on copper

5.5. Some other fields of application of plasma generators

CONCLUSIONS

REFERENCES

INTRODUCTION

The low�temperature plasma generator commandsthe attention of specialists active both in the field ofthe physics of gas discharge and low�temperatureplasma and in the field of high�temperature gasdynamics [1–17]. The temperature range in the dis�charge gap of LTPG varies from hundreds to tens ofthousands degrees. The use of any gases and their mix�tures is of obvious interest from the standpoint ofinvestigating the thermal, electrodynamic, and opticalproperties of gases, as well as of performing variousplasma�chemical reactions.

The possibility of producing homogeneous andheterogeneous flows of gas with desired pressure andtemperature in a wide range of velocities (up to super�sonic) makes the LTPG a unique tool for simulatingthe interaction between high�temperature flows andflying vehicles. Plasma generators are of interest inmetallurgy (melting of metals in electric furnaces, pre�heating of metal in pouring ladles, coke�less reductionof metal), in metal working (cutting, welding, hard�facing, surface modification), in plasma chemistry, inmedicine, in public utilities for processing householdwaste, in heat and power engineering [18–37].

Extensively employed are plasma generators withrod cathodes, with inserts of thermionic material, withself�adjusting length of electric arc, and with vortexflow of plasma�generating gas in the discharge gap.Such a plasma generator was initially made with achannel of uniform cross section. However, it sufferedfrom serious drawbacks, namely, a falling current�voltage characteristic, a low efficiency, and an unstablemode of operation. Later on, various modifications ofLTPG were developed such as generators with mag�netic field�controlled arc length, generators withinterelectrode inserts, and generators with the arclength limited by a step [2, 3]. Analysis of operation ofthese LTPGs led to the conclusion that the main dis�advantage of plasma generator with longitudinal vor�tex flow of plasma�generating gas consists in the lowvelocity of cold gas at the inlet to the discharge gapchannel. Therefore, the transition to divergent outputchannel was expected to have a positive effect on theoperation and characteristics of LTPG [13].

1. ELECTRICAL AND THERMAL CHARACTERISTICS OF DISCHARGE

1.1. Current�Voltage Characteristic

One of the most important characteristics of dis�charge in an LTPG is the current�voltage characteris�tic (CVC) of discharge. The special features of theLTPG scheme define the pattern of its CVC [2].Because of the instability of burning of electric arc, thefalling pattern of CVC for an LTPG with longitudinalflow of gas and self�adjusting arc length entails rigidrequirements imposed on the electric supply forensuring stable burning of the arc. In the case of arcvoltage drop, the arc power increases with increasingcurrent much slower than the current, while the ser�vice life of LTPG is largely defined by current. All thisstrengthens the case for rising current�voltage charac�teristic.

Zhukov et al. [2] suggested a scheme of arc interac�tion with a flow of gas in a long cylindrical channel ofuniform cross section (see Fig. 1). The initial segmentof laminar flow AB, which consists of an arc column 1and boundary layer 2, changes to the region of disinte�gration of boundary layer BCD and terminates in thesegment of arc interaction with turbulent flow of gasDEF. In the initial segment AB, the arc column is sta�ble and located on the channel axis; however, the elec�tric field intensity in this segment is low because thevelocity of flow of cold gas is low and, accordingly, theheat transfer between cold gas and arc column is weak.An entirely different pattern is observed in the segmentDEF: the “technical” intensity of electric field (poten�tial drop per unit channel length) is high, but the arccolumn is very unstable.

Proceeding from the foregoing interaction pattern,it is necessary to provide for the arc burning in laminarflow at a fairly high velocity of cold gas at the inlet.This may be attained by making the arc channel diver�gent [38].

The LTPG exhibits a strong nonuniformity of ther�mophysical and electrophysical parameters. Thecurved lines of electric field in the turbulent segmentproduce a nonuniform magnetic field. In so doing, thecolumn of such an arc may be unstable [12]. It is thedivergent channel that is capable of ensuring the distri�bution of electric current over the surface of outputelectrode and, accordingly, the distribution of the


INVESTIGATION OF LOW�TEMPERATURE PLASMA GENERATOR 99

magnetic field of current contributing to the increasein stability of positive column in the discharge gap. Inso doing, the arc length decreases, and the heat fluxesto the wall become more uniform. In a divergent chan�nel at relatively high velocities of cold gas, the arc col�umn in the vicinity of the point of arc attachment tothe anode may take up a significant part of cross sec�tion, and the flow of current in this case is accom�plished by means of short arcs between the anode andthe electrically conducting core of the flow. The signif�icant decrease in local density of current and specificheat flux to the anode makes for reduction of erosionof the output electrode.

The stability is primarily associated with actingforces. Hence follows that it is the channel expansionratio providing for constant dynamic pressure of gas orfor constant Mach number along the channel that maybe optimal.

A series of one�dimensional calculations of flows atconstant Mach number produced the channel profile[13] with angle α = 6° between the generatrix of out�put electrode and the channel axis. The experimentalinvestigations performed for different values of αrevealed that the foregoing reasoning is correct [39,40]. The effect of divergent channel on the CVC of dis�charge was demonstrated in channels with α = 0° andα = 6° [40] (the LTPG supply had no�load voltage of330 V and provided for smooth adjustment of currentwithin 80–500 A). Upper limits with respect to cur�rent were set on the experiments with cylindricalchannel because of marked erosion of the electrode,and the lower limits—because of the extinction of dis�charge, when the arc voltage fluctuations reached theno�load voltage of the supply. The most importantadvantage of LTPG with divergent channel is the risingCVC (for cylindrical channel, this characteristic isfalling). Such advantages of LTPGs with divergentchannel provided the possibility of their efficient oper�ation within a wide range of variation of parametersusing the same equipment. In order to support thisinference, extensive diverse investigations were per�formed of physical processes in the channel of anLTPG with segmented output electrode [41], which isshown schematically in Fig. 2. Coaxial shunts installedon each one of ten anode segments were used for cur�

rent measurements. Differential optical dividers wereused for measuring the arc voltage.

The system of determining the foregoing parame�ters is based on the use of L�Card, L761, and L783analog�digital cards which provide for simultaneoussampling of 16 measuring channels with a samplingfrequency up to 3000 kHz; this enables one to measureboth the instantaneous values of current and voltageand the time�averaged values with recording and withdigital and graphic display of information.

Figure 3 gives the CVCs of the plasma generatorunder investigation for two working gases, namely,argon and nitrogen. One can see that the CVC for bothgases exhibits a weakly defined rising pattern. Yetanother feature of the CVC given in this figure is theexpansion (compared to arcs in cylindrical channels)of the zone of stable burning of discharge in the low�current region.

1.2. Distribution of Electric Current and of Heat Fluxes over Segments

In [41], results are given of an investigation of dis�tribution of electric current and of heat fluxes along asegmented channel of output electrode. The heatfluxes were registered by measuring the flow rate andtemperature of water at the inlets and outlets of water�cooled parts of LTPG. The temperature was measuredby transistor�based temperature sensors with measur�ing accuracy of ~2% in the temperature range fromzero to 100°С Flow meters were used for determiningthe cooling water flow rate.

It was found that one or two segments “operate” ateach instant of time because of the arc shunting; in sodoing, the arc as a rule stretches out in successive tran�

Cathode

Anode

0 1

2

A B C D EF

Fig. 1. The scheme of arc interaction with gas flow.

1200 20 40 60 80 100mm

1

2

3 (10 pieces)

4

5

Fig. 2. An LTPG with segmented output electrode:(1) nozzle, (2) cathode, (3) anode segments, (4) nozzleholder, (5) insulator.

100


ISAKAEV et al.

sition from one segment to another. Then the arc ter�minates at one of the segments, and the process isrepeated. The pattern of arc shunting depends on thetype of gas and its flow rate. The current extraction inargon is usually accomplished by the first two segmentsin a wide range of variation of current from 70 to300 A; in nitrogen, under the same conditions, all tensegments of plasma generator may be involved in cur�rent extraction.

Different possibilities exist for the closing of arc toanode. For example, the current from the first segmentmay reach only the fifth one; after shunting, the cur�rent appears on the second or third segment rather

than on the first one. Figure 4 gives average values ofcurrent over segments for the arc current of 70 and300 A. One can see that the increase of gas flow rate inthe nitrogen arc significantly affects the distribution ofcurrent along the channel: at a current of 300 A andflow rate of 2 g/s, almost the entire current flows to thefirst segment; with a flow rate of 5 g/s, seven segments“operate”, and the maximal current flows to the fifthand sixth segments.

Figure 5 gives a typical oscillogram of the values ofcurrent to individual segments of anode and therespective voltage drop on the arc. The current makessuccessive transitions from the first to ninth segment

30024018012060

180

160

140

120

100

80

60

U, B

G = 2 g/s

(a)

30I, A

80

70

60

50

40

30

G = 2 g/s

(b)

70 110 150 190 230 270 310

Fig. 3. The voltage�current characteristics of a discharge: (a) nitrogen as working gas, (b) argon as working gas.

10987654321

250

200

150

100

50

0

I, A

1

2

3

4

1

2

3

4

10987654321

250

200

150

100

50

0

i

(b)(a)

Fig. 4. The distribution of current over the anode segments for different values of flow rate of the working gas (nitrogen) and arccurrent (a) I = 300 A and (b) I = 70 A: (1) G = 2 g/s, (2) 3 g/s, (3) 4 g/s, (4) 5 g/s.



and then, as a result of shunting, jumps over to the firstsegment. The oscillograms may be used for exactlydetermining the electric field intensity. In our case,this intensity is about 1300 V/m, and the arc burningvoltage increases almost linearly with the arc length.

The apparent discrepancy between Figs. 4 and 5 isexplained by the fact that, rather than giving instanta�neous values of current to individual segments, Fig. 4gives its average values during the entire time of theexperiment with the given flow rate of gas.

The distribution of specific heat flux along thechannel is given in Fig. 6. The service life of the outputelectrode depends on specific heat flux; this is of great

importance when using LTPG in plasma technologies.One can see in Figs. 4 and 6 that the dependences ofspecific heat flux and of current distribution on thechannel length exhibit the same pattern.

In Fig. 7, comparison is made between specificheat fluxes to segments for two modes of operation ofLTPG. With an arc power of 50.7 kW and a high flowrate of nitrogen, the maximal heat flux to the anodesurface (second–fourth segments) is lower than that tothe first segment of anode channel with an arc powerof 8 kW and lower flow rate of gas. This emphasizes thedecisive importance of the choice of geometry andoperating mode of LTPG for increasing its service life.

300

200

100

U, V

U, V

950009485094700945509440094250Time, µs

400

300

200

100

0

I, A

I1I2

I3I4

I5I6 I7

I8 I9

I10I20

I30I1

Fig. 5. An oscillogram of currents in the anode segments and the arc voltage: I = 300 A, G = 5.0 g/s, the working gas is nitrogen.

109876543210

2800

−200

2300

1800

1300

800

300

q, W/cm2

(a)

109876543210i

2500

2000

1500

1000

500

(b)

Fig. 6. Specific heat fluxes to segments, nitrogen as the working gas: (a) I = 300 A, (b) I = 70 A; designations are as in Fig. 4.

q, W/cm2

i

102


ISAKAEV et al.

Investigations revealed that the length of electricarc in molecular gas varies in a wide range and signifi�cantly depends on the gas flow rate and the dischargecurrent. These features of LTPG facilitate their effi�cient uses in various plasma technologies.

1.3. The Part Played by Radiation in Heat Flux

It is only the combined heat flux to the surface ofelements of LTPG structure that may be determined asa result of experimental investigations. The combinedheat flux to the structural elements which are at zeropotential consists of radiant and conductive fluxes. Inaddition, the heat delivered by ions from the electrodelayer is transferred to the electrodes, and Joule heat isreleased in the electrode during the flow of current.The results of Belevtsev et al. [42, 43] were used toobtain the dependences of axial values of electrontemperature on longitudinal coordinate z (z = 0 corre�sponds to the cathode end), as well as the radial distri�butions of electron temperature Te(r) in the zones cor�responding to average cross sections of the nozzle (z =2.5 mm, d = 4.5 mm), of the first segment of anode(z = 10 mm, d = 6.0 mm), of the second segment(z = 16.5 mm, d = 7.3 mm), and of a number of subse�quent segments.

In [41], the radiant flux at a current of 300 A andargon flow rate of 3 g/s was estimated. The radiant fluxto the surface of each one of the anode segments isformed both by plasma self�radiation within the givensegment and by radiant fluxes from neighboringregions. In view of the fact that the segment length sig�nificantly exceeds the typical radius of radiatingplasma filament, the self�radiation flux to the sur�face of the ith segment may be estimated using the

Π i

equation of radiant energy transfer written in cylindri�cal coordinates. As a result, we have

, (1)

where li is the segment length, Ri is the average radiusof side surface, and the specific power lost by theunit volume of plasma is related to the density of radi�ant energy flux S by the known relation [44]

. (2)

The contribution by radiation of the first segmentto the radiant flux delivered to the side surface of thesecond segment may be estimated in view of thefact that R1/l1 1 and R1/l2 1,

(3)

The integral in Eq. (3) is calculated in elementaryfunctions and in application to the case under consid�eration leads to the relation . Obviously,it is possible to absolutely similarly calculate the con�tribution by inter�illumination between any twoneighboring segments.

The radial distributions of volumetric radiationpower of argon plasma (divergence of radiant flux)were obtained using the experimental data of Evansand Tankin [45] with correction for VUV radiation ofArI, ArII, and ArIII lines [46] which was not recordedin [45]. This correction does not exceed 20% at maxi�mal values of observed temperature Te = 26 kK andbecomes insignificant at Te < 15 kK.

The radiant fluxes qRad(z) calculated using Eq. (3)were compared with the calorimetrically measuredheat fluxes delivered to the nozzle surface and to anumber of anode segments. The actual radiant fluxesto the wall in the zone of energy input (nozzle, anodesegments) are defined by the effective coefficients ofabsorption of radiation by the channel surface and callfor inclusion of repeated reflection of plasma radiationfrom the anode walls. In view of the fact that in thenozzle region the heat removed by cooling water in afirst approximation corresponds to the radiant fluxthat arrived to the nozzle surface, it is possible to esti�mate the importance of arrival to the nozzle of radia�tion from the region of the first segment (according toEq. (3)): this fraction amounts to about 20%.

Figure 8 gives comparison of calorimetrically mea�sured heat fluxes along a divergent channel withrespective values of radiant fluxes. In view of the errorof estimation of radiant fluxes, which is at least 20–30%, one can speak of the decisive part played by radi�ation transfer in the plasma–anode surface energy bal�ance apparently up to z = 40 mm, where conductiveheat transfer replaces radiant heat transfer.

Π = π ρ ρ ρ∫0

2 ( )iR

i i il q d

ρ( )iq

ρ = div( )iq S

Π12

� �

Π = Π+ +

∫ ∫2 12

2 112 1 2 2 2 3/2

1 1 2 20 0

.2 [( ) ]

l l

R dxdxl x x R

Π ≈ Π12 110%

109876543210Nos. of segments

1000

800

600

400

200

qn, W/cm2

1

2

Fig. 7. Specific heat fluxes to individual segments, nitrogenas the working gas: (1) G = 2 g/s, power of 8 kW; (2) G =5 g/s, power of 50.7 kW.



In the temperature range T ≥ 20 000 K, the partplayed by the radiation of lines of singly (ArII, NII)and doubly (ArIII, NIII) charged ions becomes pre�vailing; in so doing, the lion’s share in the total diver�gence of radiant flux falls on the optically dense VUVradiation of ArII and NII lines. This radiation islargely transferred in the far wings of resonance lines,diffuses to the boundary of plasma volume, and isabsorbed in its peripheral region.

2. PROCESSES OF ARC SHUNTING

The arc shunting and the emergence of character�istic fluctuations of voltage in a plasma generator withself�adjusting arc length were studied for quite sometime [47, 48]. However, no valid explanations wereavailable for a long time for physical mechanismswhich lead to the emergence of fluctuations of arcburning voltage in plasma generators of this type andenable one to obtain quantitative dependences ofobserved fluctuations on the principal parameters ofarc discharge. Experiments produced qualitatively dif�ferent dependences of characteristics of pressure fluc�tuations (in particular, oscillation frequency) on theworking parameters of plasma generator such as dis�charge current, flow rate of plasma�generating gas,channel diameter. No comparison was made of thecharacteristics of voltage fluctuations with experimen�tal observations for different channels and gases. Thequantitative characteristics of fluctuations of arc burn�ing voltage in channels of uniform and variable crosssections may significantly vary even for the same flowrates of working gas and the same currents.

It is obvious that the amplitude of voltage fluctua�tions and oscillation frequency are associated with theprocesses of elongation of current filament and ofelectric arc shunting to the anode wall. As distinctfrom earlier experiments, computers were employedin [47–55] for recording the parameters in performingexperimental investigations of processes in plasma

generators of different geometries. This provided forobtaining significantly more quantitative characteris�tics of processes and for performing analysis of elec�trodynamic interaction of current filament in thechannel with the flow of plasma�generating gas. Therapid Fourier transformation of signals and their sub�sequent computer processing enabled one to obtain alarge volume of scientific information and, first of all,data on the amplitude�frequency characteristics of fastprocesses in atmospheric�pressure plasma. The dataon fluctuations of electric arc burning voltage, whichare experimentally observed in channels of differenttypes, in different working media, and at differentpressures in the channel were generalized in [53–55].

A typical pattern of fluctuations of electric arcburning voltage in a plasma generator with self�adjust�ing arc length in nitrogen at currents of 200 and 300 Ais given in Fig. 9. Figure 10 gives typical Fourier spec�tra of voltage fluctuations for different flow rates ofplasma�generating gas and different currents whosefrequency corresponds to the process of arc shunting.The data similar to those in Fig. 10 may be used fordetermining the dependences of the fundamental fre�quency of voltage fluctuations on the arc current andthe gas flow rate.

The saw�tooth pattern of voltage fluctuations(Fig. 9) is often explained by the “stretching” of elec�tric arc by the gas flow, as well as by electric breakdownbetween the hot core of electric arc burning along thechannel axis and the anode (at the anode point locatedat a smaller distance from the cathode than the hotcore) (Fig. 11). Doubtless, the “stretching” of electricarc by the gas flow and the reconnection of electric arcto a new anode spot exist; however, the mechanism ofshunting involves a more complex combination ofphenomena. Traditionally referred to as electricbreakdown is the formation of a current�conductingchannel due to various elementary processes such asionization by electron impact, photoionization, ther�

1800

1400

1000

600

200

q, W/cm2

60 7050403020100z, mm

25

20

15

10

5

T, kK

nozzle 1 2

3 4

1

2

3

Fig. 8. (1) The electron temperature on the axis, (2) the total heat flux, and (3) the radiant flux along the channel.

104


ISAKAEV et al.

mal ionization, and so on. It may be demonstrated thatit is the mechanisms of force interaction of electriccurrents in different part of the arc that play the mostimportant part in the “breakdown” observed in themode of burning of self�adjusting arc in a flow of gas.

In a plasma generator with self�adjusting length, theelectric discharge burns in the inner channel of outputelectrode, and the gas flow usually additionally has thetangential component of velocity because of the deliv�ery of gas to the plasma generator inlet. In the channel,

2440164084040

(a)200

150

100

50

0

V, V

(b)

2440164084040

200

150

100

50

0

Fig. 9. Fluctuations of arc burning voltage of LTPG in nitrogen at P = 105 N/m2, d = 5 × 10–3 m, G = 3 g/s; (a) I = 200 A,(b) 300 A.

50403020100

V, V

9

6

3

(a)

kHz50403020100

V, V

9

6

3

(b)

kHz

11

2 2

Fig. 10. Spectra of fluctuations for different values of gas flow rate: (a) G = 0.72 kg/s, (b) 1.07 kg/s; (1) I = 100 A, (2) 220 A.

(c)(b)

I(S1)

C1

IS S1

I(S)

SK

ϕ

C1

dL(t)C

K

(a)

KC

Cathode

A Anode A A

Fig. 11. The position of current filament of electric arc in the plasma generator channel at different instants of time.



the current filament initially oriented along the chan�nel bends because of the need for closing the current tothe channel wall serving as electrode and is stretchedby the transverse gas flow (Fig. 11).

The interaction of electric currents flowing in dif�ferent parts of the arc and the dynamics of reconnec�tion of the anode end of the arc to a new spot wereconsidered in more detail by Sinkevich [55].

The turn of the current filament segment, whichwas initially in position shown in Fig. 11a and adjoin�ing the anode spot at point A at angle ϕ(t) (Fig. 11b),causes a variation of the pattern of interaction betweendifferent segments of current filament. This interac�tion may be readily understood if we single out andconsider the force interaction of two segments of cur�rent filament, of which one is at point C located on theaxial part of the arc and the other one—at point C1

located on the radial part of the arc CA. It must betaken into account that one and the same total electriccurrent I(S) maintained by a stabilized external supplyflows through the cross section of current filament atpoints C1 and C at all instants of time. In the state atϕ(t) ≠ 0, a current component arises which is directedin antiparallel with current I(C), and nonzero force ofinteraction of currents I(C1), I(C) leads to an increasein deformation of current filament and shunting of theanode end of the arc. The shunting process causes adecrease in the arc burning voltage.

We use the notion that the process of shunting ofelectric arc with self�adjusting length in the channel ofplasma generator with gas flow rate is defined by elec�trodynamic interaction of electric currents in the seg�ments KC1 and C1A of current filament and find thedependence of the fundamental oscillation frequencyon the external parameters of the problem. For thispurpose, we will consider the dynamics of element ofarc of the current filament segment of length δL,which is located at point S and interacts with the arcsegment located in the vicinity of point S1 on the arcpart closing to the anode wall. The dynamics of ele�ment of arc of the current filament segment of lengthδL are defined by the balance of inertial and Ampereforces. In this case, the cross section�averaged equa�tion of motion for the element of arc volume may bewritten as

. (7)

The left�hand part of Eq. (7) is the part of the over�all gas flow rate which flows via current filament (thekth fraction of flow rate G of plasma�generating gas inthe plasma generator channel) and may be given as

. (8)

The magnetic field induction appearing on theright�hand side of Eq. (7) is produced by all currentsflowing in the remaining parts of the current filament;however, it is by segment CA of the filament that the

ρ = ∇ +∫∫ ∫∫ ∫∫d VdS PdS dSdt

j × B

ρ =∫∫ ( )VdS k t G

main contribution is made. In this case, the right�handpart of Eq. (7) may be written as

j (9)

where ξ1 = × BdS/( is a dimensionless

coefficient allowing for the effect made by currentI(S1, t) flowing in the segment C1A, by means of themagnetic field generated by this current at point S1, oncurrent I(S, t). We will determine the characteristicoscillation frequency as

. (10)

In this case, if we ignore the first term on the right�hand side of Eq. (1), Eqs. (1)–(3) yield

. (11)

It follows from our experiments that the dimen�sionless coefficient appearing in formula (11) ξ1 is~2⎯3 for a wide range of variation of the gas flow rateand electric current. One can see that the characteris�tic frequency is inversely proportional to gas flow rateand increases as the square of electric current (seeFig. 12).

The experimentally obtained slight variation ofcoefficient ξ1 for a wide range of variation of the flowrate and electric current may count in favor of theforce mechanism of arc shunting.

In determining the characteristic oscillation fre�quency f from relation (11), the effect of pressure gra�dient in Eq. (8) was disregarded; this disregard is justi�fied if the value of parameter of magnetohydrody�namic interaction N = IBd/P is low compared to unity.This is not so in the general case: in the case of weakcurrents, the effect of pressure gradient may be deci�sive. For arbitrary values of parameter N from theequation of motion, the following generalization offormula (11) may be obtained:

. (12)

Here, ξ2 is a dimensionless coefficient allowing for theeffect of redistribution of pressure on the current fila�ment motions, and ρ0 is the density of plasma�gener�ating gas.

Relation (12) qualitatively corresponds to theexperimentally obtained dependence of characteristicfrequency of arc voltage fluctuations on parameterG/d3, given in [2, 9]. For high values of plasma�gener�ating gas flow rate and fixed values of electric currentof the arc, where N 1, the oscillation frequencymust exhibit a linear dependence on parameter G/d3,

f = ξ2G/ρ0d3, (13)

as was observed in experiments for values of G/d3 ≈(10–12) × 103 kg/m3 with N ≈ 10–2 [50]: an increase inelectric current results, in the case of constant flow

∫∫ j = = ξ μ /21 1 0( ) ( ) ,dS S B S I d×B

∫∫ j µ/ 20 )d I

( )=

1d kGf

dt G

= ξ μ /21 0f I Gd

= ξ μ + ξ ρ/ /2 31 0 2 0f I Gd G d

�

106


ISAKAEV et al.

rate, in the change of mode of fluctuations of electricarc burning voltage in the channel (see Fig. 9). High�amplitude low�frequency oscillation is observed,which is apparently associated with the above�described mechanism of electric arc shunting on theanode.

These same frequencies may be determined by themethod of dimensional analysis [52, 54]. Two frequen�cies may be obtained proceeding from the parametersof the problem, which include G, r, m, I, d, and f,

, (14)

where the coefficients a and b are determined fromexperimental data.

Detailed analysis of the processes describing saw�tooth variations of voltage on the arc at constant valueof electric current, which involves the derivation andsolution of model equations, is a complex computa�tional problem. The numerical solution of this prob�

μ= =

ρ

20

1 2 3,I Gf a f bGd d

lem is based on solving the complete system of equa�tions of continuity, motion, and energy for plasma,Maxwell equations, and equation of external electriccircuit in view of Ohm’s law.

3. SPECTRAL INVESTIGATIONS OF PLASMA

The scheme of LTPG and the scheme of measure�ments of spectra, as well as of the emissivity of thecathode and adjacent plasma, are given in Fig. 13.High�current (150 to 500 A) atmospheric�pressureargon and nitrogen arcs [56–60] in a plasma generatorwith divergent channel of the output electrode [39]were investigated. The observation of the region ofinteraction between electrode plasma and tungstencathode was performed via openings in the plasmagenerator nozzle (see Fig. 13) with the minimal nozzlediameter of 4 mm. For providing an adequately largezone of observation and for obviating the hydrody�namic disturbance of the gas flow delivered to the noz�zle with tangential swirl from a header enclosing thecathode, two observation openings in the nozzle hadthe shape of an ellipse with axes of 2.5 and 1.5 mm; inso doing, the major axis was oriented along the Z�axisof the plasma generator. The cathode, which was madeof a lanthanized tungsten rod 5–8 mm in diameterembedded and brazed in a massive water�cooled cop�per case, terminated in a cone with a vertex angle of80–90°.

A sharp image of the plasma�cathode region sized2.5 by 1.5 mm was simultaneously projected byGelios�40 objective onto the matrix of VS�FAST high�speed camera with magnification of 2 : 1 and onto theentrance slit of DFS�452 spectrograph on a 1.6 : 1scale using a quartz condenser and a Dove prism fororienting the image of horizontally burning arc alongthe entrance slit.

3.1. Automated Measuring Complex

The system of spectral measurements consists of aDFS�452 diffraction spectrograph and a MOAS�2multichannel optical spectrum analyzer whichincludes two Toshiba TDS1250A photodiode CCDarrays [58]. The spectral range of sensitivity is 180 to

2.01.51.00.50G, g/s

3025201510

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4003002001000I, A

3025201510

5

(a)f, kHz f, kHz

I = 100 A

I = 200 A

I = 300 A

G = 0.5 g/s

G = 0.7 g/s

G = 1.0 g/s

G = 1.4 g/s

(b)

Fig. 12. The current dependence of dimensionless oscillation frequency for different values of gas flow rate.

CCD

DFS

1

2 5

37

6

4

Z

Fig. 13. The scheme of measurements in the cathoderegion: (1) cathode, (2) nozzle with observation opening,(3) anode, (4) Dove prism, (5) VS�FAST CCD camera,(6) CCD array, (7) DFS�452 spectrograph.



1000 nm. The central part of plasma image was pro�jected onto the photodiode array located along thespectral sweep. The array arranged in the perpendicu�lar direction recorded the distribution of illuminationintensity over the slit height; when a sharp image ofplasma filament is projected onto the spectrographslit, this distribution corresponds to the distribution ofthe intensity of radiation in the arc cross section and,when the Dove prism is used in the optical scheme, tothe intensity distribution along the arc.

The wavelength standards were provided by radia�tion of lamps of high�frequency exciter of spectraPPBL�3M or of discharge tube with hollow cathodewith vapors of mercury, copper, silver, and other met�als, ensuring reliable identification of plasma spectrain the entire range of 200–1000 nm under investiga�tion. The brightness standard was provided by a TRSh�2880 small�size tungsten ribbon�filament lamp, whosebrightness temperature on pyrometric wavelength was2400–2500 K. In the case of absolute calibration ofspectral intensities of radiation of plasma, the standardlamp was positioned in place of the plasma generatorwith all of important parameters of the recording sys�tem being retained.

The Spec_Mcd.100 system based on Mathcad 7.03Professional mathematical software by Mathsoft wasdeveloped for processing the spectrometric informa�tion obtained by the recording equipment describedabove [58].

Data bases in the Microsoft Excel format on spec�tral lines of atoms and ions of elements under investi�gation were developed for interpreting the recordedspectra [61]. As regards the plasma under investiga�tion, such data bases contain hundreds of lines of ArI,ArII, ArIII, NI, NII, NIII, CuI, CuII, WI, and WII.

The experimental investigation of local radiativecharacteristics as principal carriers of information

about the state of nonuniform plasma is the funda�mental objective of spectral diagnostics. Better (com�pared to the use of CCD arrays) prospects for suchinvestigation are opened by the use of megapixel CCDmatrices at the output of spectral instruments [60].

The Andor matrix (1024 × 256 pixels sized 26 by26 μm) sensitive in the UV range was used for obtain�ing radial distributions of spectral line intensities: itsspectral resolution with DFS�452 in operation withgrating of 1200 grooves per millimeter is approxi�mately 0.04 nm, and the space resolution (with theobject image on the entrance slit on a 1:1 scale) is50 μm or better. Given the local contours of spectrallines, it becomes possible to simultaneously and inde�pendently determine the local values of temperatureand concentration of electrons. For this purpose, it isnecessary (a) to record two�dimensional images ofsuch a set of spectral lines with known atomic con�stants, which satisfies the condition , and(b) to perform correct transformation of recordedchord distributions of spectral line intensities againstthe background of continuous radiation (continuum)to radial distributions while providing for spectral res�olution which fits the problems of analysis of contoursof these lines.

Figure 14 gives an isometric two�dimensionalimage of one of the spectral regions of radiation ofnitrogen plasma, which was obtained using a CCDmatrix.

In order to make the spectrogram more informa�tive, the reimposition of the first and second orders ofdiffraction was performed during recording; this isshown in the interpretation of a number of lines ofNII, NIII (II order of diffraction), and NI (I order ofdiffraction).

Δ ≥k eE kT

800I order

400II order

805 810 815 820 825 830 835 840

402 404 406 408 410 412 414 416 418 420 λ, nm

λ, nm

NI 821, 63 nm

NII 417, 62 nm

NIII 409, 73 nm

NI 818, 49 nm

NII 404, 13 nm

NII 403, 51 nm

NII 402, 61 nm

Fig. 14. A matrix spectrogram of nitrogen plasma in the arc cathode region at I = 350 A and G = 1.5 g/s.

108


ISAKAEV et al.

3.2. Results of Spectral Investigations

Plasma generators with vortex stabilization ofworking gas and divergent channel serving as the elec�tric arc anode enable one to make significant progressin the field of generation of highly ionized argon andnitrogen plasmas of extremely high parameters [56,57, 62]. In particular, specific energy inputs of~100 kW/cm3 could be realized at currents I = 400–500 A. In so doing, the electron density ne ≤ 2 ×1017 cm–3 and electron temperature Te ≤ 3.0 eV wereattained in the cathode region; this provides for com�plete single and partly double ionization of the axialplasma of argon and nitrogen [56, 57]. Note that Pel�lerin et al. [63–65] and Mar et al. [66] performed adetailed investigation of the parameters and atomiccharacteristics of cathode plasma of argon arcs atsomewhat lower currents.

A much larger volume of spectroscopic informa�tion (data on hundreds of lines of atoms, ions, anddoubly charged ions) and, consequently, much moretime and effort were required for obtaining the results(given in Fig. 15) of determining the temperatures ofpopulation and distribution of argon and nitrogenplasma at a current of 300 A in the zone of energyinput. Therefore, the problem of determining theparameters of plasma using a smaller amount of spec�troscopic information is very important. For obtainingthe simplest estimates of plasma parameters ne and Te,it is expedient to have a small (as regards spectralextent) region in which lines of different degrees ofionization with prevailing Stark broadening arepresent. The region of 328–332 nm containing lines ofArII and ArIII is recommended as such a region forargon plasma. In the case of nitrogen plasma, thespectral region of “express diagnostics” on lines of NIIand NIII corresponds to the range of 407–420 nm. Inso doing, it is necessary to provide for good spectralresolution of line contours (in the case at hand, it wasbetter than 0.01 nm for the width of instrument func�tion of 0.02–0.025 nm and characteristic width of ionlines of 0.05–0.1 nm and lines of doubly charged ionsof 0.03 nm).

The resultant integrated experimental data on thepopulation of a very large number of excited states ofsingly and doubly charged ions of argon and nitrogenprovide a reliable experimental basis for further devel�opment of methods of collisional�radiative kinetics ofnonequilibrium plasma, in particular, the fundamen�tal theory of Biberman–Vorob’ev–Yakubov [67], inapplication to highly ionized spatially nonuniformplasmas with predominance of singly and doublycharged ions. Therefore, modern automated systemsfor acquisition and processing of spectroscopic infor�mation were used to demonstrate the possibility bothof statistically reliably determining the temperatureand concentration of electrons and of obtaining a database of unique volume and accuracy on radiationintensities, transition probabilities, and Stark broad�

ening constants of hundreds of spectral lines of ArI,ArII, ArIII, NI, NII, and NIII [68].

The following features of relaxing nitrogen plasmawere found as a result of investigation of nitrogenplasma outside of the region of Joule heating [69–73]:

(1) the electron component of plasma is overcooled: the electron concentration is higherthan its equilibrium values corresponding to those cal�culated by the Saha formula with measured electrontemperature;

(2) predomination of atoms with deficit of mole�cules is observed in discharge,

[N0(T)] ≤ [N], [N2] [ (T)], [N] [N2];

(3) singly charged atomic ions dominate overmolecular ones,

ne ≅ [N+] [ ].

These features lead one to the inference of practicalimportance, namely, that the basic excited particles inthe axial region of relaxing plasma with Te ~ 6–7 kKand ne ~ 1016 cm–3 are provided by metastable atoms ofnitrogen in 2D and 2P states; for the foregoing values ofne and Te, these atoms are in thermodynamic equilib�rium with the ground state of nitrogen atom. The con�centrations of metastable atoms of nitrogen are foundfrom the equations

~ 1017 cm–3,

~ 5 × 1015 cm–3.

The obtained values of metastable atoms of nitro�gen demonstrate that the condition ne ~ n+ ~[N(2D)] � 1016 cm–3 is valid; this condition supports ahigh plasma�chemical activity of relaxing nitrogenplasma.

4. ELECTRODE PHENOMENA

Most extensively employed at present are plasmatechnologies with the process duration of severalhours. In our opinion, the major limiting factor is theshort service life of the electrodes, especially, of thecathodes. No problems of cathode replacement arisein the processes of plasma treatment of metals (weld�ing, hardening, spraying, hard�facing), all the more sobecause the LTPG power in this case is low and itsstructure is rather simple. Further development ofplasma techniques calls for the development of long�life high�power (1 MW and higher) LTPG. Hence fol�lows the need for developing cathodes with a servicelife of the order of 1000 hours. Largely employed inLTPG is the hot cathode of lanthanized tungsten, theinvestigation of the mechanisms of whose erosion is amatter of significant urgency.

�0( )e e en n T

� N02 �

�+N2

⎛ ⎞−= ⎜ ⎟⎝ ⎠eV

2

0

2.38( ) exp,

DgN D Ng T

⎛ ⎞−= ⎜ ⎟⎝ ⎠ eV

2

0

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PgN P Ng T



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NII

TII

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NI

NIIIEI+ + EI

++EI+

10 80 90Ek, rel

I, II, III, eV

Fig. 15. Relative populations of atoms, as well as of ions and doubly charged ions of (a) argon and (b) nitrogen. The arrows indi�cate the straight lines by whose slopes the respective population temperatures were determined: for argon, TII/I = 1.747 ±0.026 eV, TIII/I = 1.97 ± 0.06 eV, TII = 1.79 ± 0.08 eV, Trel = 1.93 ± 0.03 eV, TIII/II = 2.141 ± 0.03 eV, TIII = 2.2 ± 0.6 eV;for nitrogen, TII/I = 2.250 ± 0.022 eV, TIII/I = 2.560 ± 0.002 eV, TII = 2.55 ± 0.08 eV, Trel = 2.566 ± 0.024 eV, TIII/II = 2.820 ±0.024 eV, TIII = 2.8 ± 0.3 eV.

110


ISAKAEV et al.

In the case of LTPG with divergent channel of theoutput electrode, the arc in the cathode region exhibitsa high current density because of higher electrical con�ductivity and electric field intensity compared tocylindrical channel of uniform cross section. Accord�ingly, the hot cathode is under more stressed condi�tions.

The processes occurring in the cathode and on itssurface cannot be separated from the phenomenaoccurring in the region of transition from the cathodeto thermally equilibrium plasma of the arc column.Important characteristics such as the cathode poten�tial drop, thermal�field emission current, ion current,and return electron current of plasma depend on thecathode temperature and on the parameters of cath�ode plasma. All of these quantities are important inconsidering the region of cathode plasma which con�sists of the collisionless zone of space charge, the zoneof relaxation to ionization equilibrium, and the zoneof relaxation to thermal equilibrium [74–77].

It is only in considering the self�consistent problemof determining the parameters of plasma in the cath�ode layer and the temperature distribution in the cath�ode and on its surface that one can determine the cur�rent�voltage characteristic of the cathode layer andestimate the electrode erosion. An important problemin determining the service life of the cathode is that ofidentifying the mechanisms of formation and dynam�ics of cathode spot and of quantitatively describingthese mechanisms. In spite of numerous attempts atsolving this problem [77], no internally consistentconcepts are as yet available of the mechanism of for�mation and motion of cathode spot (note that thequestion of the so�called retrograde motion of cathodespot is still being discussed). The formation of cathodespot essentially varies the pattern of cathode erosionand its emission characteristics. In turn, increasederosion promotes the emergence of the mechanism ofrecycling of cathode material and the increase in theservice life of the cathode.

In the plasma generator under investigation, thewater�cooled copper cathode had an insert of thori�

ated or lanthanized tungsten. The service life of LTPGlargely depends on the service lives of the cathode andanode. Therefore, researchers give much attention tostudying complex electrode processes [74–85].

4.1. Erosion of Tungsten Cathode

Results of investigation of erosion of tungstenunder the conditions identified above are given in [82–84]. The procedure of measuring erosion is based onmicrophotography of the working surface of the cath�ode with small depth of focus (10 μm) of the opticalscheme. A series of photographs taken under condi�tions of controlled travel of cathode along its axis rela�tive to the microscope objective enables one to con�struct the topographic map of the working surface. Asa result of comparing topograms of the cathodeobtained in a series of successive tests, the space pat�tern of measurements of the working surface is regis�tered, as well as the magnitude of cathode erosion.With the cathode working surface 2 mm in diametertaking up the entire image field of POLAMR�111microscope, the accuracy of measurement of elemen�tary volume is about 103 μm3, which corresponds to2 × 10–8 g for tungsten.

The erosion of sintered tungsten was investigatedfor a conic cathode (Fig. 16) with a tungsten insert2 mm in diameter press�fitted in a copper case. Theerosion was determined after one�hour tests at a cur�rent of 350 A and nitrogen flow rate of 3 g/s. After suc�cessive tests of one and the same cathode, visual andinstrumental analysis of its state leads one to assumethat the erosion is caused by at least two mechanisms.The first one of these mechanisms is the evaporation oftungsten. Apparently, this was the principal mecha�nism during the first two hours of tests. One can seethis from clearly observed traces of flashing�off oftungsten surface and of successive deepening of thecircular zone limited by central projection and, on theoutside, by the conic surface of the insert. In the pro�cess of cathode operation, a deepening crater withhigh projection at the center is gradually formed; how�ever, the top of the central projection hardly variesduring the second hour of testing. The erosion isapproximately the same during the first and secondhours and amounts to ~5 × 10–10 g/C. During the thirdhour of testing, the erosion increases four�fold. Thecrater depth increases to approximately the sameextent as during the first two hours of testing; the cen�tral projection disappears. One can assume that thishappens as a result of spallation. Therefore, the mech�anism of erosion is associated with the evaporation ofthe cathode material, its mechanical destruction, andthe carryover of large fragments of material of thecathode tip.

Series of tests of cathodes with a sintered tungstentip of the same diameter were performed under thesame conditions (Fig. 17). The end face of tungsten tip

Fig. 16. A conic cathode with tungsten insert 2 mm indiameter press�fitted in a copper case.



is flat and located at the level of inner edge of the cop�per case. There is a radial gap of 0.2 mm to a depth of5 mm between the side surface of tungsten insert andthe copper case. The measurements revealed that thespecific erosion of cathode with radial annular gap ishigher than that in the former case (Fig. 18).

We have later performed investigations with a cath�ode of initial shape using fused tungsten which was notthoriated or lanthanized. After one hour of operation,it was found that the cathode spot travels if judged bythe arc traces. The travel starts from the cathode cen�ter, proceeds on a spiral path, melts through a thinlayer of metal, and “perishes” at the edge of a flat zone1.5 mm in diameter (Fig. 19). The pattern of the sur�face of circular zone indicates that the erosion in thiszone is caused by the evaporation of material. Notethat this is possible only in the case of a very high den�sity of current to the cathode. A peripheral zone existsin addition to the above�identified central zone; in sodoing, the central zone is “raised” by 100–150 μmabove the peripheral zone. The cylindrical side surfaceof the raised part is filled with tungsten droplets 10–60 μm in size. The peripheral region of the cathodedoes not exhibit traces of fusion and is a circular zonepermeated with deep cracks which separate fragments100–300 μm in size. The peripheral circular region issubject to erosion in the form of large fragments whichbreak off. The area of the central zone amounts toapproximately 35% of the area of the entire tungstentip of the cathode. We can assume that the erosionassociated with the evaporation of metal is propor�tional to the area of the central zone and likewiseamounts to 35% of the entire erosion of the cathode.

4.2. High�Speed Visualization of the Cathode Region

The system of high�speed digital video filmingbased on the VS�FAST camera (manufactured by Vid�eoSkan, Moscow) involves a 1.3 megapixel (1280 ×1024) color CMOS sensor (manufactured by Micron)with diagonal of 19.7 mm (see Fig. 20). The systemprovided for the input of images at a rate of up to 488

full frames a second. The increase in the frame fre�quency may be provided by reducing the number N oflines of image being inputted, ν = 5 × 105/N.

A remarkable property of the system with colorCMOS sensor is the possibility of presenting the colorimage of object in the form of three digital compo�nents corresponding to the blue, green, and red com�ponents of the integral pattern. For improving thecolor reproduction and cut�off of long�wave regions oftransmission of three sensor filters, an additional cut�off IR filter is employed which is located at the systeminput and does not transmit radiation of wavelengthλ > 740 nm. The following mode of recording videoimages was used for ensuring the optimal values ofspace and time resolution: the number of image lines,150; the number of frames, 3000 s–1; and the time offrame exposure, 2 μs; the space resolution in view ofthe scale of image and of matrix pixel size of 12 μm wasnot worse than 30 μm.

The use of the VS�FAST camera in life tests of aplasma generator with nitrogen plasma at currents of150–450 A revealed [84] that, in operation with sin�tered tungsten, the cathode loses 5 to 20 fragments10⎯3 to10–2 mm3 in volume during the first hour, andthe overall carry�over of large fragments during thishour of plasma generator operation amounts toapproximately 3 × 10–10 g/C. No separation of largefragments (d ≥ 0.1 mm) is observed and no significantvariations of the shape of the front part of the cathodeoccur after the first hour of continuous operation ofnitrogen arc with a current of 150–450 A. In all likeli�hood, the modification of the microstructure of thecathode tip, which occurred during the first hour ofoperation, results in improved cooling of the tip,reduced average temperature of its surface, and (veryprobably) reduced consumption of the cathode mate�rial. The weighing of the electrode prior to and after

Fig. 17. A cathode with tungsten insert 2 mm in diameterwith a flat end face and with an annular gap of 0.2 mm to adepth of 5 mm.

4321Time, h

102

101

100

10−1

Specific erosion per hour, kg/C, 10−12

1

2

1

11

2

2

Fig. 18. Comparison of erosion of (1) press�fitted conictungsten cathode insert with that of (2) insert with anannular gap between the tungsten insert and copper case.

112


ISAKAEV et al.

the tests and the estimation of the observed loss of itsvolume lead to the value of specific consumption oftungsten gW ≥ 2 × 10–9 g/C. By and large, the lossof material during the two�our test amounted to atleast 3 mg, which corresponds to the specific con�sumption of tungsten of 1.2 × 10–9 g/C. About half ofthis loss is apparently due to the evaporation of tung�sten from the surface of liquid film in the zone of arc ata temperature exceeding the melting temperature oftungsten.

A significant distinction of the closing of nitrogenarc to the cathode surface from that of argon arc is the

much smaller diameter of its attachment to the cath�ode: shown in Fig. 20 is the glow of cathode plasma ofnitrogen and argon for the same arc currents and gasflow rates. No carryover of large fragments of the cath�ode material is observed in argon plasma even at cur�rents of 200–400 A.

4.3. Investigation of Temperature Fields of the Tungsten Cathode Surface

The thermal state of the surface, as well as thedegree and pattern of erosion of thermionic cathodesof high�current plasma systems, define their workingcapacity and service life [80, 86]. The nonuniformitiesof temperature fields of the surface of thermioniccathodes may cause the emergence of zones of localoverheating, evaporation of the material, its destruc�tion as a result of cracks, and to other phenomena andprocesses defining the cathode service life [81, 85, 86].For this reason, space�time analysis of temperaturefields on the high�energy�density surface of the cath�odes of high�current electric arcs presents an impor�tant research problem.

The possibility of displaying the glow pattern of thecathode and adjacent plasma in each one of three col�ors, namely, red (R), green (G), and blue (B), trans�forms the high�speed VS�FAST camera employed byus into a three�color pyrometer with high time (of theorder of microseconds) and space (not worse than30 μm) resolution. The temperature field on the cath�ode surface was determined by the ratio of local emis�sivity (brightness of glow) of the cathode on theselected wavelength λ' to emissivity of standard sourceon the same wavelength,

(15)

λ λ ε λ=λ η λ ε λ

⎛ ⎛ ⎞⎞ε λ≅ −⎜ ⎜ ⎟⎟λ ηε λ⎝ ⎝ ⎠⎠

c c c c c

st st st st st st

c

st c st st

2

( ', ) ( ', ) ( ', ( , , ))( ', ) ( ', ) ( ', )

( ', ( , , ))1 1exp .' ( ', )

w

w

b T U T T x y zb T U T T

T x y zcT T T

Fig. 19. Photographs of a fused tungsten cathode 3 mm in diameter prior to and after 30�min tests.

Fig. 20. The cathode plasma of nitrogen (top image) andargon (bottom image) arcs at I = 300 A. The value of scaledivision in scale inserts is 1 mm.



In expression (15), bc(λ', Tc(x, y, z)) and bst(λ', Tst)denote the local value of brightness of cathode glowless the “screening” plasma radiation and the value ofbrightness of standard source of radiation with truetemperature Tst, respectively; с2 = hc/k is a constant; ηis a coefficient which takes into account the differ�ences between the conditions of recording of theobject and of the standard; εw(λ', Tc(x, y, z)) and εst(λ',Tst) are the emissivity factors (on the investigatedwavelength λ') of the cathode surface as functions oflocal temperature Tc of the cathode and of tungstenribbon at the true temperature Tst of the latter; andUc(λ', Tc) and Ust(λ', Tst) denote the Planck spectraldensity of radiation energy of the cathode surface andof the standard lamp on the wavelength λ', respec�tively.

Local values of cathode surface temperature Tc(x,y, z) may be determined from the solution of Eq. (15),

. (16)=⎛ ⎞λ ε λ λ− η⎜ ⎟ε λ λ⎝ ⎠

stc

st st st c c

c c st st2

( , , )' ( ', ) ( ', )1 ln

( ', ) ( ', )

TT x y zT T b T

c T b T

Equation (16) was solved numerically relative to Tc

using the literature data of [87, 88] for εw(λ, Tc).

The “working” surface of the cathode after the testsexhibited metallic luster and no traces of oxides. Theprincipal problem in determining local values of thecathode surface temperature Tc(x, y, z)) consists incorrect extrapolation of the brightness of plasma glowto the region of glow of the cathode surface for therespective colors (RGB). This procedure was per�formed using computer codes in the Mathcad envi�ronment, and the identified component of brightnessof cathode glow was used for calculating the tempera�ture field on the cathode surface.

Examples of the resultant temperature fields on thecathode surface with respect to the “red” componentof radiation are given in Figs. 21 and 22. Note that theisotherms T > 3500 K in Fig. 21b correspond to theregion of molten metal on the cathode surface (glare inFig. 21a). Apparently, because of much lower specificenergy inputs to argon plasma compared to those tonitrogen plasma, the maximal temperature on thecathode surface in argon arc significantly differs from

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ISAKAEV et al.

the melting temperature of tungsten and does notexceed 3200 K.

4.4. Electrode Drops of Potential

The cathode and anode potential drops in electricdischarges have always attracted researchers' attention[1, 73, 88], because the electrode processes are of keyimportance as regards understanding the mechanismof maintaining a discharge. At low pressures, wherethe electron free path is commensurable with the sizeof discharge gap, the probe methods make it possibleto obtain information about the electrode regions ofdischarge. At close�to�atmospheric pressures, theprobe methods become ineffective. The size of even anuncooled probe significantly exceeds the electron freepath. Because of high heat fluxes, a probe must beeither cooled or high�speed one. In both cases, theinformation obtained in probe measurements mayhardly be used for understanding the mechanism ofelectrode processes.

The spectral methods of investigation of plasma arelikewise ineffective from the standpoint of investiga�

tion of electrode processes, because the size of elec�trode region, especially, of the cathode region, is com�parable with the free path of charged particles. There�fore, one of the principal methods of experimentalinvestigations of electrode processes under conditionsof arc discharge is the determination of the depen�dence of heat flux on current strength. In [41], the heatflux to the cathode was measured by calorimetry. Fig�ure 23 gives the values of voltaic equivalent of heat fluxto the cathode (the ratio of heat flux Qc to currentstrength I) ϕc = Qc/I, which are in the range from 1.5to 4 V. The voltaic equivalent of heat flux to the anodeϕА (argon as working gas) is given in the table. One cansee that this equivalent is independent of both currentstrength and gas flow rate.

It was demonstrated in [46] that the closing of cur�rent to anode in a high�current electric�arc plasmagenerator with divergent channel of the output elec�trode may be accomplished owing to photoionizationof plasma�wall transition layer by VUV radiation gen�erated in the highly ionized core of the arc. The uni�formity and diffuse pattern of the passage of current inthe anode region are due to the combined effect ofradial electric field in transition layer Er ~ 40–50 V/cmand of volume photoionization of the layer by VUVradiation, which maintains the necessary level of con�ductivity of the transition layer.

The investigations revealed that the characteristicfeatures of high energy�density arcs in molecular gasesadd significant importance to the problem of correctchoice of gas flow rate and discharge current strength,especially, in developing LTPG for use in plasma tech�nologies.

5. PLASMA TECHNOLOGIES

5.1. Plasma Hardening of Metals

The operating stability of metal articles under con�ditions of friction and high wear largely depends on theproperties of surface layers. Numerous large�size andcomplex articles may be made of ordinary steels, andthe working surfaces must be modified in some orother way.

One of the most efficient methods of modifying thesurface of metals is that of thermal hardening. Theefficiency of operation of articles after thermal hard�ening depends on the strength and hardness of the

26021016011060 I, A

5

4

3

2

1

ϕc, V

1

2

3

4

5

6

7

8

Fig. 23. The voltaic equivalent of heat flux to the cathodeas a function of current strength: (1, 3, 5, 7) argon, (2,4, 6, 8) nitrogen; (1, 2) flow rate of 2 g/s, (3, 4) 3 g/s,(5, 6) 4 g/s, (7, 8) 5 g/s.

Voltaic equivalent of heat flux to anode (argon as working gas)

F, g/s 2 3 4

I, A 89 152 201 252 302 88 150 200 250 302 87 150 200 250 300

ϕA, V 6.45 6.04 6.34 6.06 6.16 6.34 5.93 6.07 6.30 6.29 6.21 5.93 7.06 7.67 7.61

F, g/s 5 6

I, A 88 150 202 251 301 87 150 202 252 301

ϕA, V 6.15 5.90 6.10 6.36 6.68 6.26 6.21 6.63 7.07 7.28



hardened zone of metal, on the uniformity of thestructure and stability of the properties, on the pres�ence of a smooth transition region providing for thestrength of adhesion between the hardened layer andbase metal, and on high resistance to destruction aris�ing, as a rule, because of incipient crack. Variousmethods of thermal hardening are known, namely,gas�flame method, high�frequency current, electron�beam, laser, and plasma methods [25].

The plasma method takes a special place amongvarious methods of thermal hardening. High rates ofheating and cooling, the short duration of residence ofmetal at above�critical temperatures, and the possibil�ity of simultaneous realization of chemical and ther�mal processing define the advantages of the plasmamethod over induction hardening and flame treat�ment.

Compared to laser and electron�beam processing,the plasma method is favorably distinguished for itseconomic efficiency, which is of fundamental impor�tance in the case of quantity production. However, thenarrow strip of hardening by a highly nonuniform heatflux of LTPG with cylindrical channel fails to providefor stable properties of material on the surface and overthe article depth. The processing of surface with a gapbetween neighboring strips being hardened or withtheir overlapping suffers from serious disadvantages[90] associated with nonuniform structures of harden�ing.

A special flux transformer was developed for solv�ing the problem of hardening in a wide strip and byrequired uniform heat flux [61, 91]. This transformeris a hydrodynamic transition region where a cylindri�cal jet of low�temperature plasma issuing from thedivergent channel of LTPG (Fig. 24) is transformedinto a plane jet and delivered to the article surfacebeing processed.

This flux transformer was used in hardening a partof the rim and ridge of railroad wheel pairs [92–96],ridges (flanges) of street�car wheel pairs [97], and met�allurgical equipment [98] (Fig. 25).

Figure 26 gives the cross section of the rim of awheel subjected to plasma processing, on which thehardened zone is identified by etching. This zone hasthe form of a continuous strip up to 60 mm wide cov�ering a part of the rolling surface (up to 35 mm) and ofthe ridge (up to 25 mm). The maximal thickness ofhardened layer on the rolling surface and on the ridgeis 2.5 and 4 mm, respectively.

Five regions of microstructures of different typeswere revealed in the investigation of the microstruc�ture of thermally hardened and transition layers(Fig. 27).

We will consider a thin austenitic structure up to20 μm thick. It is tightly bound to the wheel metal anddoes not come off. Its hardness is somewhat lower thanthat of the adjacent layer of acicular structure. It doesnot appear possible to determine the exact value of

microhardness because of the small thickness of thelayer. An electron�microscopic study revealed thepresence in this layer of nitride phases formed as aresult of saturation of the surface layers of metal withnitrogen because of characteristic features of the tech�nology of plasma processing. The presence of a layer ofaustenite in the vicinity of the surface leads one toassume that the nitrogen content in the surface layermay reach 1.5–2.0% by weight.

1

2

3

4

5

Fig. 24. A setup for hardening: (1) cathode, (2) nozzle,(3) anode, (4) flux transformer, (5) article.

Fig. 25. A setup for hardening railroad wheel pairs.

Hardened layer

Fig. 26. Cross�sectional view of the rim of a thermallyhardened railroad wheel.

116


ISAKAEV et al.

The width of layer with the structure of low�tem�pered “pile” martensite with microhardness НV0.981 =500–740 units reaches 1 mm. The thin structure of thislayer includes two components, namely, twinnedplates and thinner strips of martensite. Dispersed car�bide particles 50–100 μm in size are observed withintwinned crystals of martensite.

The width of the layer which is the region of inter�mediate inhomogeneous structure consisting of a mix�ture of regions of troostomartensite with НV0.981 = 420and of martensite with НV0.981 = 610 is likewise about1 mm. The intermediate structure of troostosorbitewith НV0.981 = 300–390 up to 1 mm wide smoothlymoves towards base metal.

The structure of base metal of the wheel is tem�pered sorbite with regions of ferrite on the boundariesof grains with microhardness НV0.981 = 300–320.

Therefore, the plasma processing of the ridge androlling surface of the wheel provides for smooth tran�sition from hardened structures to structures of thebase metal of the wheel.

For estimating the effect of hardening on the ser�vice life of seamless rolled wheel pairs, observationswere performed in the Moscow Railroad Depot ofgroups of 300 each of hardened and non�hardenedseamless rolled wheel pairs put into service at approx�imately the same time at the beginning of the observa�tion period (for about three months) [99, 100].

Figure 28 gives average values of mileage countedas the wheel pairs are removed from service.

A similar experiment was performed [101] forwheel pairs with tires; in so doing, simultaneous obser�vations were performed of 300 each of hardened andnon�hardened wheel pairs and wheel pairs providedwith automatic ridge lubricators (ARL). The observa�tion results are given in Fig. 29.

The metal of street�car wheel pairs is close inchemical composition to that of railroad wheel pairs.However, the structures of rails and street�car wheels,as well as the operating conditions, call for the harden�ing of both the inner and outer surface of flanges. Theplasma flux transformer developed by us makes it pos�sible to harden railroad wheels in one revolution. As tostreet�car wheels, the other side of the flange may behardened only in the second revolution after appropri�ate readjustment of the flux transformer. These fea�tures were taken into account when the decision wastaken of hardening a pilot lot of street�car wheels forMosgortrans (Moscow City Transportation Com�pany). The operation of this pilot lot demonstrated[97] both the significant reduction of intensity offlange wear as a result of hardening and the effectmade by the manner in which hardened and non�hardened wheel pairs are arranged in a car, as is shownin Fig. 30.

3.02.52.01.51.00.50L, mm

800

700

600

500

400

300

200

HV 0.981

Nitrides,austenite

MartensiteMartensite +

troostomartensite TroostosorbiteTemper sorbite

Fig. 27.The distribution of microhardness over the hardened layer thickness.

300250200150100500Number of wheel pairs

200

150

100

50

Average mileage, thou. km

1

2

Fig. 28. Average values of mileage of (1) hardened and(2) non�hardened wheel pairs in groups as the pairs areremoved from service.



5.2. Plasma Cutting

A number of plasma technologies (cutting, weld�ing, hard�facing, melting, and others) involve the useof LTPG with external electrode, which is shownschematically in Fig. 31. An arc compressed by thewall and blown over by a flow of gas may be used toobtain high specific heat fluxes directed towards anarticle. The external electrode may serve the functionsof both cathode and anode. The former case involvesthe use of the effect of cathode cleaning of the articlesurface by ion bombardment.

The principal techniques of thermal cutting ofmetals include the oxygen, plasma, and laser cuttingtechniques [28, 34]. The laser cutting is an advancedmethod; however, for economic reasons, it is presentlyemployed only for cutting metals of small thickness.The oxygen cutting is possible only for steel and tita�nium alloys, and the plasma cutting may be employedfor cutting any metals. Therefore, the economic per�formance is to be compared for oxygen and plasmacutting of low�carbon and low�alloy steels.

Raising the efficiency of cutting metals of standardthickness is likewise an urgent objective for a numberof reasons including higher requirements of the qualityof the surface of cut such as tolerances for size, per�pendicularity of cut, degree of surface roughness andof the depth of zones of thermal effect.

Usually employed for plasma cutting of metals areLTPGs in which the article being cut serves as theexternal electrode. In so doing, the electric arc isimmersed into the region of cut, where the bulk of heatrelease occurs. It was pointed out in [28] that the prin�cipal criterion defining the efficiency of cutting is theratio of current to nozzle diameter . The maximalpermissible value for cylindrical nozzles of uniformcross section is = 1400 A/cm. This value is limitedby the emergence of a cathode–nozzle–article double

I d

I d

arc which causes destruction of the nozzle. For a longtime, the numerous methods employed for raising the

ratio did not include the variation of the shape ofnozzle channel. Given below are the results ofinvestigations [102–103] of the effect of divergentchannel of the nozzle on the efficiency of plasmacutting and on the possibility of raising the ratio(in so doing, d is the diameter of the minimal crosssection of the nozzle).

The limiting permissible values of the ratio arelimited by double arcing and are made evident by theabrupt increase in the heat flux to the nozzle. One cansee in Fig. 32 that the double arcing occurs at α = 0°much earlier than at α = 6°. The investigationsrevealed that the current�to�diameter ratio for a diver�gent nozzle may amount to 2000 A/cm.

I d

I d

I d

Non�hardenedwheel pairs 35970 km

Wheel pairs provided with ARL

Hardened wheelpairs

54081 km

99353 kmN

on�

AR

L

Har

dene

d

100000

90000

80000

70000

60000

50000

40000

30000

20000

10000

0

km

Fig. 29. Average mileage of locomotive wheel pairs.

All hardened1�hardened;

3�non�hardened

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0 In

ten

sity

of w

ear

of fl

ange

th

ickn

ess,

mm

/10

000

km

Fig. 30. The service life of street�car wheel pairs.

har

den

ed

118


ISAKAEV et al.

The investigations resulted in the development ofan LTPG (Fig. 33) for cutting metals of large thick�ness. With a current of 900 A, this LTPG is capable ofcutting copper 120 mm thick and stainless steel200 mm thick at a rate of 8 m/h while providing forhigh reliability and efficiency of production.

5.3. Plasma Hard�Facing on Hatfield Steel

Various powders are extensively employed inplasma technologies of hard�facing and spraying. Thequality of articles subjected to such processing islargely defined by the degree of heating and accelera�tion of particles. A setup for the delivery of powderalong with the working gas of LTPG was developed[104] for raising the efficiency of heating and acceler�ation of particles and for raising the efficiency of theprocess. In this setup, the powder flow rate and,accordingly, the process efficiency may be increasedwhile providing for the uniform injection of powderover the arc discharge cross section and in the zone ofspraying.

High�manganese G13L steel (Hatfield steel) findsextensive application for parts operating under condi�tions of intense shock loads and abrasive wear owing tothe capacity of high�manganese austenite for beingsubjected to strain hardening and for forming a solidwear�resistant surface without the variation of the vis�cosity of article as a whole [105]. The reduction of arti�cles of Hatfield steel by means of hard�facing involvesserious difficulties associated with the properties ofsteel. Hatfield steel exhibits inadequate structural sta�

bility under heating and cooling. The combination ofthermal properties unfavorable for heating, namely,low thermal conductivity and high coefficient of ther�mal expansion, makes G13L steel very sensitive to theformation of thermal cracks [106].

An especially urgent objective is that of increasingthe service life of railroad frogs manufactured of Hat�field steel. The cost of manufacture of such frogs (thematerial of these frogs, as was indicated above, con�tains a significant amount of manganese) is increasedas a result of high operating costs of replacing frogs onrailroad tracks and because of the need for interrup�tions of railroad traffic.

The properties of metal of hard�facing when usingvarious methods identified above largely depend onthe depth of melting of base metal and on the degree ofmixing of the hard�facing material and base metal. Asa rule, the larger is the depth of melting and the higherthe degree of mixing, the poorer are the properties ofthe layer of hard�facing and its bond to the base. Of allknown methods, including the most universallyemployed method of hard�facing, i.e., the electric�arcone, plasma hard�facing provides for the least meltingof base material [29].

The experimental investigations were performed inan updated series�produced UPN�303 facility (seeFig. 34) which accommodated an LTPG with diver�gent nozzle and external electrode [107–111].

Analysis of the investigation results revealed thatthe best results are obtained in the cases of using G13powder for hard�facing and of hammering of eachlayer. The effect of hammering is opposite to that ofhard�facing. It causes stretching strain and developscompressive stress, which may balance to a degree thetensile stresses. In addition, the hammering helpsincrease the density of metal subjected to hard�facing,heals minor cracks, and closes pores. For increasingthe effect of hammering, it must be performed usinglarge�mass impactors and at a high temperature of thelayer of hard�facing; this significantly facilitates thework of deformation. It is necessary to make the pro�

5

4

1

2

3

6

Fig. 31. A schematic view of LTPG with external elec�trode: (1) internal rod electrode, (2) thermionic tip,(3) nozzle, (4) device for injection of working gas, (5) insu�lator, (6) external electrode provided by the article.

8007006005004003002001000I, A

0°

6°

3500

3000

2500

2000

1500

1000

500

q, W/cm2

Fig. 32. The current dependence of heat fluxes to nozzlesof different geometries.



cess of forging the layer of hard�facing closer in time tothe instant of hard�facing.

Two frogs of the P65 type, reduced by plasma hard�facing using the technology developed by us, were sentto the Moscow Railroad for performance tests. Thesefrogs sustained the passage of 115 and 173 million tonsin gross weight while a new frog on the average sustainsthe passage of 90 million tons [111]. The experimentalresults make it possible to develop a new bimetallicfrog with the base of ordinary carbon steel and theworking zone of hard�facing wear�resistant coating.The advantages of such a frog include the lower con�sumption of scarce and costly manganese and thehigher maintainability (the metal of cast frogs made ofHatfield steel suffers from numerous defects). It is fur�ther advisable to develop mobile units for plasmareduction of frogs without removing them from rail�road tracks.

5.4. Plasma Hard�Facing on Copper

One of the main units of continuous casting plantsis the mold. The principal requirements imposed onthe mold walls include the high values of thermal con�ductivity and wear resistance. Proceeding from theformer one of these requirements, the walls are madeof copper and copper alloys. The service life of suchmolds amounts to 100–150 heats.

Chemical and electrochemical methods of apply�ing coatings are employed for extending the service lifeof mold walls. However, the small thickness and thequality of coatings obtained by such labor�consumingmethods fail to provide for significant increase in theservice life. Extending the service life of the walls ofcopper molds involves high expenditure of copper fortheir manufacture and repairs. For performing theprocess of hard�facing, the article surface must have a

temperature which provides for wettability, and thehard�facing material must be in the molten state [29].The high thermal conductivity under conditions ofhard�facing identified above requires appropriateenergy input. The article is heated to high tempera�ture; in so doing, cuprous and cupric oxides areformed on the surface of copper, which adverselyaffect the wettability of the article surface. Under theseconditions, the hard�facing must involve the use ofpowders which do not form intermetallic compoundswith copper which adversely affect the quality of hard�facing. These are nickel�based powders. The plasmahard�facing is characterized by complex processesinvolving the solid, liquid, and gaseous phases. Underreal conditions, the hard�facing depends on numerousparameters. Therefore, in developing plasma technol�ogies of hard�facing on copper, one has to be satisfiedwith the knowledge of only the qualitative depen�

3

4

1

5

2

Fig. 33. PR�03 LTPG for cutting metals of large thickness: (1) cathode, (2) tungsten tip, (3) insulator, (4) gas swirling device,(5) nozzle.

Fig. 34. Setup for plasma reduction of railroad frogs.

120


ISAKAEV et al.

dences of the effect of various parameters on the pro�cess of hard�facing.

Under these conditions, the LTPG with divergentnozzle and injection of powder along with working gasturned out to be a convenient tool for required controlof the process of hard�facing. The relevant technologymakes it possible to obtain coatings 4–5 mm thick persingle pass [112]. The hardness on the surface of hard�facing layer is 75–82 HRB. The hardness smoothlydecreases with increasing distance from the coatingsurface and reaches the value of 70 HRB at a distanceof 1 mm from the interface between the hard�facinglayer and base metal (Fig. 33). A zone of decreasinghardness is observed at a distance of 5 mm from thehard�facing surface, which covers approximately1⎯1.5 mm of the sample thickness and is indicative ofthe presence of transition layer between the hard�fac�ing metal and copper base. The hardness decreasesdown to the level of hardness of copper, i.e., by a factorof five–eight. Estimates indicate that, in the case ofthis technology, one can expect an increase in theoperating stability of the molds of machines for con�tinuous casting of blanks (MNLZ) up to 500–1000heats [113].

5.5. Some Other Fields of Application of Plasma Generators

Interest in the synthesis of carbon nanotubes andnanofibers has abruptly increased in recent years. Oneof the advanced methods of preparing nanotubes is theplasma�jet method [114, 115]. In [116], use is made ofthe device of [106] with the injection of the startingmaterials for the synthesis of nanotubes (finely andultra�finely divided particles of carbon and metalliccatalysts) into the working gas of LTPG with divergentchannel of the output electrode. The objective of pro�jected experimental investigations is to reveal the spe�

cial features of the mechanism of synthesis of nano�tubes and the dependences of their structure on thepattern of production process and to study the possi�bilities of preparing significant amounts of nanomate�rial.

The plasma technologies are successfully employedfor intensifying the parameters of blast in the blast�fur�nace process. From the standpoint of metallurgy, it isof interest to search for other technologies of iron pro�duction, including methods of reduction of iron frommineral stock bypassing the sintering�blast�furnaceprocess. The conventional method of pig iron produc�tion using this process has presently been brought to ahigh degree of perfection. The depletion of reserves ofcoking coal and the urgent need for tightening therequirements of environmental protection give rise tointerest in alternative methods of reduction of iron.One such method is that of plasma reduction, includ�ing the idea of developing an energy�metallurgy com�plex [37, 117, 118]. The scheme of an energy�metal�lurgy complex with plasma reduction module on thebasis of LTPG with divergent channel of the outputelectrode is given in [119], where it is suggested todirectly deliver finely divided ore into the LTPG alongwith working gas. The working gas in LTPG may beprovided by natural gas with simultaneous feed offinely divided coal. This technology appears to be ofinterest both from the economic standpoint and fromthe standpoint of environmental safety of the processof plasma reduction.

An important place in metallurgical productionbelongs to the process of metal casting. An importantrequirement in preparing quality steel is that of main�taining a constant temperature of liquid metal in theintermediate ladle of machine for continuous castingof blanks [120]. The technology of maintaining thesteel temperature in the intermediate ladle using aplasma generator is already employed by numerouscompanies such as CORUS (Britain), DUNAMETANVAL (Sweden), KAVASAKI (Japan) and others.The reliability of operation and the possibility of con�trolling the LTPG make up the necessary condition forsuccessful casting of quality steel. An LTPG withdivergent channel of the output electrode meets theserequirements; therefore, relevant investigations [121–123] are doubtless of practical interest.

The plasma spraying using various powders is char�acterized by an extremely broad spectrum of applica�tions [20–22]. However, the low productivity of theprocess and the poor quality of spraying place limits onits practical application. The quality of sprayingdepends on temperature and velocity of powder parti�cles. The generally employed method of introducingparticles by cold carrier gas into the LTPG jet fails toprovide the possibility of their adequate accelerationand heating. The use of the method of injecting pow�der with working gas, especially under conditions ofsupersonic flow of plasma in the LTPG channel, leads

76543210Distance from the surface, mm

90

80

70

60

50

40

30

20

10

CoatingNi–Cr

BaseCu

HRB

Fig. 35. The distribution of hardness over the depth ofhardened layer.



one to hope for the solution of the foregoing problemsand for significant increase in productivity of the pro�cess. The process of efficient conveyance of finelydivided materials using an LTPG with divergent chan�nel of the output electrode may turn out to be promis�ing for use in heat and power engineering associatedwith the combustion of coal mixture in the furnaces ofsteam boilers [24, 124, 125].

Note the possibility of using LTPG with divergentchannel of the output electrode for solving the prob�lem of treatment and burial of low�radioactive waste ofnuclear industry. In such technologies, it is the LTPGwith divergent channel of the output electrode thatwill provide for the desired service life and automationof the process of treatment of low�radioactive house�hold and industrial waste [126, 127].

CONCLUSIONS

Investigations of the physical processes occurringin an LTPG with divergent channel of the output elec�trode revealed that

—the current–voltage characteristic in the diver�gent electrode channel is weakly increasing;

—the range of existence of discharge compared tothat of discharge in cylindrical channel expandstoward both low and high currents;

—owing to the high stability of burning of the dis�charge, the range of its existence may be realized atfairly low currents;

—owing to a more uniform distribution of heatflux along the discharge channel, the LTPG is capableof operating at high currents;

—the temperature in the region of energy input atcurrents of the order of 300 A reaches the value of30 000 K for nitrogen and argon;

—the VUV radiation of axial plasma effects thephotoionization of the wall layers and makes for theclosing of current to the anode;

—the temperature of the cathode surface in thecase of operation in the atmosphere of nitrogen ishigher than that in the case of operation in the atmo�sphere of argon; and

—the amplitude of arc shunting in divergent chan�nel is lower compared to that in cylindrical channel ofuniform cross section. At fairly high currents, the arcshunting in argon is actually absent (the amplitude ofvariation of arc voltage is 1–2 V).

The LTPG with divergent channel of the outputelectrode turned out to be a convenient tool for use indeveloping various plasma technologies, namely,

—a setup for plasma hardening of the ridges of rail�road wheel pairs has been developed and is extensivelyemployed; as a result of hardening, the service life ofwheel pairs increases by a factor of 1.5–2;

—an LTPG for plasma cutting of metals of largethickness has been developed; at a current of 300 A,

this LTPG is capable of cutting copper 120 mm thickand stainless steel 200 mm thick at a rate of 8 m/h;

—a unit has been developed for plasma reductionof railroad frogs; trial operation in the tracks of theMoscow Railroad revealed that the service life ofreduced frogs is 1.5 times that of new frogs;

—a technology of hard�facing on copper has beendeveloped; estimates indicate that the service life ofthe molds of machines for continuous casting ofblanks in the case of hard�facing on copper of theirwalls, may be increased to 500–1000 heats.

Research is under way of other advanced applica�tions of LTPG with divergent channel of the outputelectrode, namely,

—preparation of carbon nanotubes and nanofi�bers,

—plasma reduction of ore,—plasma heating of steel in the intermediate ladle

of MNLZ, and—plasma spraying with various materials with sub�

strates of diverse materials.

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