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Temporal and spatial spectroscopy of the plasma formation in crossed electric andmagnetic fieldsV. I. Miljević and D. Dj. To?ić Citation: Journal of Applied Physics 51, 2520 (1980); doi: 10.1063/1.327973 View online: http://dx.doi.org/10.1063/1.327973 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/51/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Temporally and spatially resolved measurements of multi-megagauss magnetic fields in high intensity laser-produced plasmas Phys. Plasmas 15, 122701 (2008); 10.1063/1.3035909 Cross-field plasma acceleration and potential formation induced by nonlinear Landau damping of electrostaticwaves in a relativistic magnetized plasma Phys. Plasmas 10, 3939 (2003); 10.1063/1.1612498 CrossField Plasma Acceleration and Potential Formation Induced by Electromagnetic Waves in a RelativisticMagnetized Plasma AIP Conf. Proc. 669, 816 (2003); 10.1063/1.1594055 Bound states of spatially separated electrons in crossed electric and magnetic fields Low Temp. Phys. 27, 1014 (2001); 10.1063/1.1430844 Instability of a Partially Ionized Plasma in Crossed Electric and Magnetic Fields Phys. Fluids 6, 382 (1963); 10.1063/1.1706743
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Temporal and spatial spectroscopy of the plasma formation in crossed electric and magnetic fields
v. I. Miljevi6 and O. OJ. To~i6 a)
Atomic Physics Laboratory, Boris Kidric Institute of Nuclear Sciences, Belgrade, Yugoslavia
(Received 3 October 1979; accepted for publication 4 December 1979)
The formation of argon plasma in crossed electric and magnetic fields in a cylindrical diode with an incandescent cathode has been studied by means of the delay time of the anode current pulse and photon pulse (corresponding to the optical transitions) and the shape of the voltage collapse. The working conditions were: pressure p = 10-5_10-3 Torr, anode voltage Ua = 800 V, and maximum magnetic field Bmax = 1200 Gs. Photoelectrical recording of spectral lines was performed with a monochromator in the wavelength range 3600-6000 A, and the total optical spectrum was recorded simultaneously on a photoplate in a separate spectrograph in the wavelength range 2000-10 000 A. The delay time of the anode current pulse and photon pulse are approximately the same and are in the millisecond range. The delay time of the photon pulse does not depend on the wavelength. Simultaneously the spectral lines of the working gas (A II), residual gas (0 II), and tungsten (W I) appear. Tungsten atoms appear at the moment of breakdown as a result of ion bombardment of the cathode. Neutral atomic lines of the working gas (A I) have not been observed. Radial analysis shows that the delay time of the photon pulse does not depend on the radius. Spectroscopic results have been analyzed in terms of excitation and ionization processes during the formation time. The shape of the voltage collapse suggests the streamer breakdown mechanism.
PACS numbers: 52.20. - j, 52.70.Kz, 31.70.Hq, 34.80.Dp
I. INTRODUCTION
In a great number of papers, 1-3 the plasma formation in crossed electric and magnetic fields has been studied. The model of electron movement along a cycloidal path in crossed fields has been generally accepted. It is assumed that electrons, while they move along the cycloid, reach energies necessary for the ionization of neutrals. It is usually considered that the paths are the same for all electrons and that electron-atom collisions are nonelastic. The more general case is when the energy distribution of electrons is taken into account.
The above-mentioned papers are the result ofinvestigations by means of experimental setups with a cold cathode.
The other type of the experimental setup with crossed electric and magnetic fields consists of an incandescent cathode as a source of electrons. Such a device is used, for instance, as a very effective ion source.4
The investigations performed so far have shown some of the pecularities of this discharge. The cathode heating current induces a tangential Bio-Savart magnetic field, which is also perpendicular to the radial electric field, thus forming an additional crossed field system.5
.6 Spectroscopic
investigations demonstrate the intense excitation of ion levels in both pulsed and steady-state discharges in crossed field. 7
-IO
The presence of crossed electric and magnetic fields, together with an incandescent cathode as a strong source of electrons and the influence of the magnetic field generated by the cathode heating current, cause the complexity of the system. Theoretical papers concerning plasma formation
·"Now at the Faculty of the Electrical Engineering, University of Belgrade.
under such conditions are not available in the literature. In this paper the formation of an argon plasma in
crossed electric and magnetic fields in a cylindrical diode with an incandescent cathode has been studied by means of the delay time of the anode pulse, photon pulse (corresponding to optical transitions), and the shape of the voltage collapse. On the basis of these results the mechanism of plasma formation through excitation and ionization processes has been analyzed.
II. EXPERIMENTAL DETAILS
An optical method has been used for studying the plasma formation by observing the time development of optical transitions of excited atoms and ions. In the case of photon emission, electrons must have enough energy for the excitation of atom energy levels, and therefore by means of the optical method only the electron component is studied. Depending on the choice of atom or ion spectral lines with different upper energy levels, the time dependence of electron energy during the process of the plasma formation can be followed.
The optical method offers the possibility not only of time but also of space analysis of the light emitted. A relatively small volume of the interelectrode space can be observed.
A cylindrical diode 80 mm long with an anode 34 mm in diameter and the incandescent cathode 1 mm in diameter, through which we passed an alternating current (the rms intensity of which was approximately 55 A), is used in the experiments. The diode was submitted to an axial magnetic field which is varied between 0 and 1200 Gs. Such a diode is usually called a magnetron diode. A capacitor of 5 J.LF, 800 V is used as a supply source. The pulse repetition time varies
2520 J. Appl. Phys. 51 (5), May 1980 0021-8979/80/052520-04$01.10 © 1980 American Institute of Physics 2520
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between 2 and 20 s. The diode is filled with argon at a low pressure (10-3-10-5 Torr).
The scheme of the experimental setup, Fig. 1, permits optical investigations to be performed from directions coaxial and orthogonal to the magnetic field. The axial direction was used for photoelectric measurements. A monochromator with the accessories for fast photoelectric recording in the wavelength range 3600-6000 A was used for the study of the time-dependent phenomena. Photoelectric recording is sufficiently fast in relation to the formative time which, in our case, is in the millisecond range. The spectral lines in the range 4000-4500 A were usually used for the analysis, as the sensitivity of the photomultiplier is maximum in this range. The signal from the photomultiplier was taken to a cathoderay oscilloscope.
By means of a spectrograph with a wavelength range 2000-10 000 A the spectra were recorded on photoplates. The light of the incandescent cathode caused difficulties with the recording of spectra. The discharge pulses are very short, and the light emitted in one pulse yields faint photographic records. If a large number of pulses are directly ~ecorded, the cathode continuum blackens the photographic plate and obscures the line spectrum because of low-duty cycle (the repetition time varies between 2 and 20 s). This is why we have introduced a mirror chopper between the light source and the entrance slit of the spectrograph in such a way that the exposure time was somewhat longer than the anode pulse. The mirror chopper is schematically shown in a dashed-line circle in Fig. 1.
The chopper consisted of a relay electromagnet with a mirror fixed to the moving arm. The relay was triggered with a thyratron discharging a condenser and used as a main timing device to avoid the electromechanical delay of 3.6 ms. The contacts of the relay were adjusted to start the discharge with the mirror in position to illuminate the entrance slit of the spectrograph. Details of the chopper and diagram of the triggering system are given in Ref. 8.
Apart from that, the shapes of the voltage collapse and anode current pulse have been recorded simultaneously on the cathode-ray oscilloscope.
As the cathode was ac heated, in addition to the external axial magnetic field there are two parasitic fields: axial electric (due to voltage drop along the cathode) and tangential Biot-Savart magnetic field (due to heating current flow).
- .... ~;
.~ MONOCHR. PHM 0 CRO
'---
FIG. 1. Experimental setup: A, anode; C, cathd'de; dashed-line circle, mirror chopper.
2521 J. Appl. Phys., Vol. 51, No.5, May 1980
FIG. 2. Oscilloscope traces of the anode voltage (upper beam) and photon pulse (lower beam): Uu = 800 V, P = 2 X 10-4 Torr, B = 740 Gs (the time scale is 0.1 ms!cm).
Those fields influence the plasma formation time and change considerably the measured time delay.6 In order to eliminate this influence, a special device for the synchronization ofthe anode voltage pulse has been used so that the phase and intensity of the cathode heating current are constant.
III. RESULTS Experimental results obtained by the optical method
have been divided into two parts: results concerning the time analysis and those concerning the space analysis of the plasma formation.
A. Time analysiS
In our case it is convenient to introduce the term "delay time" as the time interval between the potential application and the appearance of the photon pulse. Since the delay time does not depend on the choice of the spectral line, the expression "the delay time of the photon pulse" is used.
Typical oscilloscope traces of the anode voltage pulse (upper beam) and the photon pulse (lower beam) for Ua = 800 V, P = 2 X 10-4 Torr, and B = 740 Gs (time scale 0.1 ms/cm) are shown in Fig. 2. It may be seen that the photon pulse appears suddenly and at the end of the formative time, when the breakdown occurs.
The delay time of the photon (upper beam) and anode current (lower beam) pulses for Ua = 800 V, P = 3 X 10-5
Torr, and B = 100 Gs (time scale 0.2 ms/cm) are shown in Fig. 3. It may be seen that the current pulse appears insignificantly earlier than the photon pulse, but the difference between them is much smaller than the delay time.
The anode voltage (upper beam) and anode current la
--~'
~
liI~ii;I FIG. 3. Oscilloscope traces of the photon pulse (upper beam) and the anode current pulse (lower beam): Uu = 800 V, P = 3 X 10-5 Torr, B = 200 Gs (the time scale is 0.2 ms/cm, Iu = 10 A/cm).
V.1. Miljevic and O.Oj. To!\ic 2521
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FIG. 4. Oscilloscope traces of the anode voltage (upper beam) and the anode current (lower beam): Ua = 800 V. P = 5 X 10-5 Torr. B = 340 Gs (the time scale is 0.5 ms/cm. fa = 10 A/cm).
(lower beam) for Ua = 800 V, P = 5 X 10-5 Torr, and B = 340 Gs (fa = 10 A/cm, time scale 0.5 ms/cm) are shown in Fig. 4. The anode voltage is approximately constant until the anode current pulse appears, i.e., until the collapse. A slight decrease of the voltage before the breakdown is caused by the discharge of the capacitor through the corresponding resistors for the capacitor filling. It may be noticed that the voltage collapse is sudden and complete at the end of the formative time.
The dependence of the delay time on the magnetic field [Ua = 800 V: (l)p = 6x 10-5 Torr, (2)p = 8x 10-5 Torr, and (3) p = 2 X 10-4 Torr] is shown in Fig. 5. The delay time increases with the increase of the magnetic field and the decrease of the gas pressure. It may be relatively long, even a few milliseconds.
B. Space analysis of the plasma formation
A typical dependence ofthe delay time on the radius is showninFig.6(Ua = 800V;p = 3X 10-5 Torr;B = 210Gs; C, cathode; A anode). The broken line corresponds to the case when the slit of the spectrograph is lighted from the magnetron diode along the radius (from the cathode to the
BIGs)
FIG. 5. Dependence of the delay time on the magnetic field for (1) p = 6 X 10-5 Torr. (2) p = 8 X 10-5 Torr. (3) p = 2 X 10-4 Torr. Anode voltage Ua = 800 V.
2522 J. Appl. Phys .• Vol. 51. No.5. May 1980
0.6 r--
0.4 r--
'7 i.
c I I
-
-
I I I I RADlJS A
FIG. 6. Radial analysis of the delay time of the photon pulse (p = 3 X 10-5
Torr. Ua = 800 V. B = 210 Gs).
anode), while the full line corresponds to the radial analysis. It may be noticed that the delay time does not depend on the radius.
In the spectrum which has been recorded on the photoplate, ion spectral lines of the working gas (A II) and impurity lines for ionized oxygen (0 II) (present as a residual gas) and tungsten atomic lines (W I) (resulting from the ion bombardment of the cathode) have been identified. However, argon atomic lines have not been found.
IV. DISCUSSION
Different physical processes occur from the moment when the potential difference is applied between the electrodes of a certain gas system to the moment when some type of a discharge is established.
Electron paths are considerably changed when the magnetic field is applied to a cylindrical vacuum diode. If the magnetic field is smaller than the critical one, the electron paths are slightly curved and the magnetron diode behaves like an ordinary diode. However, when the magnetic field is larger than the critical one, electrons cannot reach the anode and in the case of ideal vacuum the electron paths are closed. The critical magnetic field is given by Hull's relation II Be = (8Uam/e)1!2/R (R is the radius of diode; m and e are the electron mass and charge. In our case the critical magnetic field is Be = 112 Gs.
In the cylindrical magnetron diode (with directly heated cathode) filled with gas at low-pressure, electrons perform a complex motion: along a cycloidal path they rotate around the cathode (due to external magnetic field) and in axial directions (due to parasitic fields) until they collide with neutrals. In collisions they obtain a radial velocity component in the direction of the anode. With a strong magnetic field and low gas pressure the electron transit time is expected to be much longer than in a diode without magnetic field.
During electron-atom collisions the following excitation and ionization processes are possible:
A + e--+-A' + e, (1)
A + e --+- A * + e,
A + e --+- A + + e + e,
V.I. Miljevic and D.Dj. To!!ic
(2)
(3)
2522
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where A, A ',A *, and A + stand for the atom in ground state, metastable state, excited state (allowable transition), and ion in ground state, respectively.
If the transport of electrons is caused by some of the above reactions, then we may conclude that owing to the increased probability for the collision and the considerably extended electron paths in the crossed fields, the cumulative processes play an important role. Consequently, we must take into account the following processes also:
A' + e --+ A * + e, A' + e --+ A + + 2e,
A * + e ---+A + +2e,
A + + e ---+ (A +)* + e,
A + e --+ (A +)* + 2e.
(4)
(5)
(6)
(7)
In practice this means that within the mentioned time interval light emission would result of either A I or A II spectral lines, for in our case argon is the working gas. Apart from that, the successive collisions of the type (4), (5), and (6) would result in excitation and ionization much earlier than tl and probably on the radius smaller than the anode radius, and the delay time would depend on the radius. Recent investigations have shown that noble gases have large cross sections for simultaneous ionization and excitation. For example, in direct electron collisions with ground-state argon atoms, the preferential excitation of 4p ion levels occurs, and cross sections for individual levels have rather high values. 12 Therefore for fast electrons process (7) should be taken into account.
However, it has been proved experimentally that the spectrum does not contain argon neutral atomic lines 7 and the delay time is the same for all the lines and does not depend on the radius, i.e.,
tI(A II) ~ trIo II) ~ tI(W I)'
This delay approximately equals the delay time of the anode current pulse tu,,)' The rise time of the photon and anode current pulses is very short, i.e., much smaller than tf .
In our case the working gas is argon with oxygen as a residual gas. Tungsten results from the evaporation and ion bombardment of the cathode. In the interval between the anode voltage pulses tungsten evaporates from the cathode heated at the temperature of 2600-2800 K. At these temperatures the tungsten vapor pressure is of the order of 10-6
Torr, which is below the residual pressure at which, for instance, it is not possible to establish the discharge at the given conditions.
After the breakdown, because of the great ion velocities (the magnetic field does not influence them), an intensive ion bombardment occurs and a considerable number of tungsten atoms are found in the discharge. 10
The relatively long delay time tf during which there is no light emission shows that electron energies in that time interval are small. It may be assumed that electrons have a certain energy distribution, with the characteristic high-energy tail, but the number of such electrons is small and their contribution negligible. The mean cycloid pitch is a small part of the anode radius, and a great number of collisions with neutrals is necessary for the electrons to reach the an-
2523 J. Appl. Phys., Vol. 51, No.5, May 1980
ode. It means that in the time interval 0 < t < tl the mechanism of plasma formation through the ionization of neutrals (ionization wave) cannot be accepted as the mechanism of plasma formation in the magnetron diode with the incandescent cathode. If such a wave were to exist the ionization would be followed by the excitation of discrete levels whose energies in the case of argon are close to the ionization energy. Then the ionization process would be followed by light emission, and the delay time of the photon pulse would depend on the radius. Also, in this case the increase of the light intensity with time would be slower. For primary avalanches, for instance, the light intensity increases exponentially with the time.
The absence of the ionization or excitation of atoms of the working gas (absence of the neutral atomic or ion spectrallines) in the interval of the delay time 0 < t < tI , the independence of the photon pulse delay time on the radius, and the very long delay time of the anode current and photon pulses (of the order ofms) point out the peculiarity of the plasma formation in the gas magnetron diode. Following the application of the anode voltage to the electrodes, we can differentiate two successive processes:
(1) Electron transport from the cathode towards anode within the time interval 0 < t < t (. Electron energy is insufficient for the excitation and ionization of neutrals ("dark discharge").
(2) Plasma formation. Owing to the electron transport the potential distribution is modified and probably by means of the streamer mechanism the breakdown is suddenly established. It is followed by a very intense ionization and excitation of the gas and a sudden drop of anode voltage. Electron energies are high and sufficient even for simultaneous ionization and excitation of the atoms. As a result the line spectrum of the working gas (A II) and impurities (0 II,W I) appears almost in the same time ("bright discharge").
The presence of tunsten atomic lines (W I) in the spectrum seems to suggest simultaneous excitation and sputtering of tungsten by ion bomardment.
We have planned further spectroscopic measurements and rf spectrum analysis concerning the plasma formation and origin of fast electrons in crossed electric and magnetic fields.
'H.A. Blevin and S.c. Haydon, Aust.1. Phys. 11, 18 (1958). 2R. Haefer, Acta Phys. Austriaca 7, 52 (1953). 'S.c. Haydon, in Proceedings of the International Conference on Ion Phenomena in Gases, Munich, edited by N. Maecker (North-Holland, Amsterdam, 1961), p.1.
48. Cobie, D. Tosic, and 8. Perovic, Nucl. Instrum. Methods 24, 358 (1963).
'D. Tosic, 1. Electron. Control 17, 623 (1964). "D. Tosic and Y. Miljevic, Int.1. Electron. 30, 175 (1971). 7y. Miljevic and D. Tosic, 1. Phys. D 4, 1545 (1971). Ky. Miljevic and D. Tosic, in Proceedings of the Second International Conference on Gas Discharge, London, Conference Publication Number 90 (Institute of Electrical Engineers, London, 1972), p.347.
"V. Miljevic,in Proceedings of the Eleventh International Conference on Ion Phenomena in Gases, Prague, edited by L Stoll (Institute of Physics of the University of Zagreb, Yugoslavia, 1973), p.453.
Illy. Miljevic, in Proceedings of the Seventh Yugoslavian Symposium on Physics of Ion Gases, Rovinj, edited by Y. Yujnovic (Institute of Physics of the University of Zagreb, Yugoslavia, 1974), p.317.
"A.W. Hull, Phys. Rev. 18, 31 (1921). "P.N. Clout and D.W.O. Heddle, 1. Phys. B 4,483 (1971).
V.1. MiljeviC and DDj. To~ic 2523
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