Laser triggering of electric breakdown in liquids

680 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 1 NO. 4, August 1994

Laser Triggering of Electric Breakdown in Liquids

Anders Sunesson,l Peter Bdrmann, Stefan Kroll,

Lund University, Dept. of Physics, Lund, Sweden

Lars Wa If ridsson

ABB Corporate Research, VasterLs, Sweden

ABSTRACT Electric breakdown in semihomogeneous geometries has been studied in n-hexane and transformer oil using a laser triggering method for fields far below the spontaneous breakdown field. The method uses a laser-produced plasma to initiate the breakdown. Strong self focusing in the liquids was limited by increasing the numerical aperture of the laser beam to > 0.20 prior to focusing. Electrical breakdown could be triggered at fields as low as 50 kV/cm in transformer oil and 25 kV/cm for n-hexane. The time lags are between 30 and 200 ps, depending on voltage, liquid, and gap size. Current and light emission measurements of the prebreakdown phenomena are presented and a model for the triggering process is proposed.

1. INTRODUCTION paper describes a. laser-based technique for the

THIS study of electric breakdown in liquids. By focusing a high power h e r pulse in a dielectric liquid subjected to a strong electric field, the onset of an electrical breakdown in the dielectric liquid can be controlled in space and time. We describe the practical implementation and physical cha.racteristics of this approach. Liquid electric breakdown is a phenomenon of great economic and sci- entific importance, since liquid insulation systems are a major design consideration of many electrotechnical devices. Breakdown processes in gaseous media are relatively well understood, and the dielectric properties of gases can be described and predicted accurately [l]. In condensed matter the process on the other hand is much less well understood. For dielectric liquids at present no good theory exists. A variety of breakdown models has been suggested for liquids, but only in recent years have the efforts begun to pay off.

A considerable amount of data has been obtained on point-plane geometries and other inhomogeneous electrode arrangements. The use of shadow techniques, high- speed photography and very sensitive imaging devices has made it possible to study, in detail, the time evo- lution of the prebreakdown events in these geometries [2-51.

However, the majority of these studies has been performed for very inhomogeneous geometries, where the streamer is initiated by the very high field at the inho- mogeneity. The experimental data have confirmed that streamers require a high field for initiation, but that, once initiated, the streamer requires a much lower field to propagate. This means that the study of streamers can be separated into the study of inception and propagation. Many practical insulation problems occur a t fields that are considerably below the initiation field and in these cases the problems must be related to propagation of the

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 1 No. 4, August 1994 681

discharge. Therefore, a way to study the propagation a t low fields is desirable.

Various ways to perform such studies have been proposed. Maksiejewski and Calderwood [6] introduced the trigatron, where a spark plug generates a discharge to trigger the breakdown. This has been used in many experiments, e.g. [7]. A pointed part on an electrode in an otherwise homogeneous geometry can initiate streamers [8]. A conducting particle free to move in the field can initiate breakdown through a microdischarge when it comes in contact with an electrode [9]. All these methods have in common that they require contact with an electrode to perform the streamer inception, yet practical evidence suggests [lo] that breakdown can be initiated anywhere in an electrode gap, and that this is the case also when the electrodes have been covered and when the gap has been filled with barriers.

To be able to initiate a breakdown a t low fields but still not a t the electrodes, a laser-triggering technique has been developed. By focusing a laser pulse a laser- produced plasma can be generated anywhere in an electrode gap, to initiate the breakdown process. Laser- triggered breakdown is a commonly employed method to get reliable triggering of gas- or oil-filled HV switch- es [ll-131, and the technique has also been employed for guiding of discharges and lightning [14] and for studies of discharges in gas or air [15-161. The switching techniques that have been developed aim at achieving low jitter and reliable closure in overvolted gaps, and the lightning experiments are aimed a t diverting the final jump of a lightning discharge to a place where the lightning does no harm, e.g. away from a switchgear or transformer instal- lation. The presently developed technique has the aim to trigger breakdown in undervolted gaps. This means that the emphasis shifts from ascertaining that a breakdown is created in the gap to investigating under which conditions a discharge can propagate in the gap. The technique has been successfully employed to trigger electrical breakdown in transformer oil and n-hexane at semihomogeneous dc fields as low as +50 kV/cm for transformer oil and +25 kVfcm for n-hexane for gaps up to 5 mm. The time delays from laser pulse to breakdown typically are 30 to 200 ps.

This paper describes the technique, its development, and some applications of it. Special emphasis is given to the plasma formation and how the laser beam should be focused to obtain good spatial control of the laser- produced plasma. A simple model of how the technique works is outlined. The experiments were performed at two locations, Lund University and ABB Corporate Re- search, Vasteras. The technical details that are given mostly refer to the measurements and the equipment in

Lund, but the system is Vassterds is similar in most re- spects.

Lens System

I Computer I Figure 1. Experimental setup.

2. SETUP AND EQUIPMENT The basic experimental setup is shown in Figure 1.

The laser, a Q-switched Nd:YAG laser (QuantaRay DCR1) that operates a t 1064 nm wavelength, produces pulses with a pulse energy variable from 30 to 600 mJ, a pulse width of M 8 ns, a diameter of 8 mm, and a divergence of - 1 mrad. The laser can also, by frequency doubling, produce pulses of 532 nm wavelength with pulse energies between 5 and 200 mJ. The emitted beam is focused into the test cell via an optical isolator (Electro-Optics Tech- nology Inc., model 1845-lo), dichroic mirrors and a lens system. In an optical isolator optical radiation can only be transmitted in one direction, light propagating in the opposite direction will not be transmitted. The isolator is needed to avoid that laser radiation scattered in the reverse direction primarily via stimulated Brillouin scat- tering [17,18] is reflected back into the laser cavity. Such radiation would be amplified by the gdin still present in the laser rods, and it could severely damage the laser. To create a small intense laser, produced plasma at the desired focal spot the beam is expanded before focusing it. The significance of this procedure is described in Section 3.1.

The test cell is made of stainless steel and supplied with windows on four sides for easy optical access. A hemispherical electrode system with a radius of curva-. ture of 10 mm for the electrodes is placed in the cell. HV is supplied by a 0-65 kV FUG supply via a 150 Ma re- sistor. A 100 pF capacitor battery connected to ground,

682 Sunesson et al.: Laser Triggering of Electric Breakdown in Liquids

buffers the HV electrode during the prebreakdown phase so that propagation of a streamer will not be charge limited. Electrode gap distances of 2 and 5 mm were used. The cell was filled either with Nytro lox transformer oil or n-hexane purum grade for the experiments. The liquids were used as received without degassing or further purification.

Prebreakdown currents during the whole period from the laser pulse arrival to the breakdown can be monitored on a Tektronix TDS 540 digital oscilloscope. When a breakdown occurs, in order to prevent any damage to the oscilloscope, the voltage level is limited by a protection circuit. It is important that the protection does not limit the bandwidth of the current measurement. The circuit used consists of several steps that progressively brings the voltage down to a level safe for the input circuits in the oscilloscope. A preamplifier can be used to monitor very weak currents. Input and output imped- ance are matched to the transmission lines (50R) in order to prevent reflexes and not restrict the bandwidth. The bandwidth of the protection circuit was measured to be > 700 MHz, i.e. above the analog bandwidth of the digital oscilloscope (500 MHz).

Additiona.1 protection is provided by a large 30 cm wide copper sheet from the cell to the oscilloscope. This sheet provides a common ground for the current measurement electrode, the protection boxes, the cable shields, the oscilloscope and the ground of the lab. The main purpose served here is again to protect the internal circuitry of the oscilloscope. The large current flow during electrical breakdown may induce a large voltage spike in the ground loop consisting of the shielding of the measurement cable and the copper foil. The cables should therefore be laid as tight as possible to the copper sheet to re- duce the area between them. The internal ground plane in some instruments can be very weak. The copper foil shall therefore be connected both to the ground input of the power cord a.nd to the ground of the coaxial inputs a t the front panel of the oscilloscope to avoid a potential across the internal circuit that may cause severe damage.

transmitted through optical fibers to the appropriate de- tector. Both current and light signals can be recorded on the digital oscilloscope.

The experiment a t ABB Corporate Research, Vasterbs, used a similar laser source that gave infrared radiation with pulse energies up to 450 mJ and a 10 mm plane- plane electrode gap and a similar current measurement system.

3. MEASUREMENTS 3.1. SELF FOCUSING

During the initial measurements, when a setup with direct focusing of the unexpanded laser beam into the cell was used, it was found that self focusing of the laser beam [17,18] could severely limit the applicability of the technique. The experiments showed that the beam is broken up into thin plasma bars of 2 to 5 cm length, and that the plasma bars occur randomly within a volume - 2 mm in diameter. This means that energy deposition in the gap becomes difficult to quantify, and that the spatial position of the trigger point cannot be specified accurately. Therefore, self focusing must be minimized in order to fully control the laser triggering process.

Filament

Figure 2. Self focusing geometry. Solid lines are actual beam contour, dotted lines are beam contour for pure geometrical focusing in the absence of self focusing. z s f is the self focusing point calculated from the text, and zg is the position of the geometrical focus. At z,j the beam collapses to a filament and a plasma bar is formed.

Self focusing is a nonlinear optical effect that occurs because at optical frequencies the permittivity and hence also the refractive index of a medium depends on the electromagnetic field strength of the optical field. The index of refraction can be expressed approximately as

Further, care should be taken to avoid ground loops from the laser to the oscilloscope. The laser flashlamp discharge will otherwise induce a voltage spike in this (1)

+ - n M no + n2EE

loop that may cause the zero level of the current measurement to fluctuate. Best is to totally disconnect the laser electrically from the oscilloscope.

where no is the line_ar, intensity independent, part o f the refractive index, E the electromagnetic field strength, and n2 the nonlinear part of the refractive index. n2

Light pulses from the prebreakdown phenomena have been monitored by an EM1 9558 photomultiplier tube (PMT) or a fast photo diode (PD) (Electro-optics Tech- nology 2010) [19]. The frequency response of the photo- diode is from DC to 400 MHz. The optical signals are

is very small, therefore the intensity dependence of the refractive index normally cannot be seen. However, high power lasers can produce instantaneous electric fields of 100 kV/cm which can be sufficient to give rise to signif- icant intensity dependent effects. If n2 > 0 the effect on

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 1 No. 4, August 1004 683

a laser beam will be that its more intense (central) parts encounter a higher refractive index than the less intense (outer) parts. This causes the medium to act as a lens, and it will focus the beam. An already convergent beam will then focus a.lready before it reaches its expected geometrical focus. See Figure 2 for a schematic view of laser beam focusing when self focusing is present. A rig- orous treatment of the self-focusing problem [18] shows that a laser beam is broken up into filaments where the intensity is so high that plasma formation occurs along the filaments. We therefore get plasma bars in the self focusing medium, in agreement with the observed phenomena. From Equation (1) it is clear that self focusing depends on the laser field strength, and to limit self focusing the field should be kept low. However, to generate a laser-induced breakdown the field should be as high as possible. The solution to this dilemma is to expand the beam before it is fina.lly focused in the dielectric liquid. This means that a high numerical aperture is used in the focusing process. The numerical aperture N A is defined as N A = d / f, where d is the beam diameter before focusing and f is the focal length of the lens(es) used. A high N A keeps the field low until one gets very close to the focal point so that the self focusing point, zsf in Fig- ure 2, essentially will coincide with the geometrical focal point.

Figure 3. Lens system for minimizing the influence of self focusing. a is a negative lens that expands the beam, b and c are positive lenses for collimation and focusing of the beam.

It was found experimentally that a numerical aperture of 0.20 or larger was required to avoid the effects of self focusing. To accomplish this, a new optical system was constructed, using three lenses instead of a single lens for focusing. This is shown in Figure 3. The laser beam is expanded using a plano-concave lens, and two plano- convex lenses are used to focus the beam to the desired focal point. The lenses are all oriented so that (spheri- cal) aberration is minimized. The focal point is moved by changing the negative lens position. The system can be used as a free-standing lens setup on an optical table, or built into an 'optical conduit', where the lenses are placed in a tube that is inserted into the oil-filled test cell. The optical conduit allows distances from the last lens to the focus of - 15 to 35 mm whereas the free-standing setup

gives distances up to 200 mm if N A > 0.20 is main- tained. The laser-produced plasmas consistently occur at the same point if the numerical aperture condition is fulfilled. The formed plasmas are, as viewed by the eye, cigar shaped and of 2 to 5 mm length. At the lowest energy level the plasma appears point shaped. Self focusing may be a partial reason for the cigar shape at higher power levels but the reason may also, and more probably, be geometrical: the high-field region around the focus, where the field is at or above the limit for plasma formation will be approximately cigar shaped.

A further point is that the oil and the optics absorb and reflect the laser radiation, giving losses. It was found that the absorption coefficient of the oil for the infrared laser radiation was 4.9 m-l, and that the absorption coefficient for the green laser radiation was < 0.05 m-l. Reflection losses in the optics are N 4% per glass-air interface for the lenses. The cell window had a transmission of 86% including reflection losses at the input air-glass interface. The total (reflection and absorption) losses for the setups in this paper are between 30% for the optical conduit case and 65% for a case where all lenses are placed outside the cell. Pulse energies pertaining to the laser-produced plasma given in the article are unless clearly stated corrected for reflection/absorption losses, and thus represent actual pulse energies a t the focal point (neglecting multiphoton absorption effects).

Duration / ns Figure 4.

Light emitted from a laser-produced plasma.

3.2. PLASMA FORMATION A laser-produced plasma is by definition an extreme-

ly violent state of matter. It could therefore severely disturb the breakdown process, apart from initiating it. Measurements show, however, that the plasma is very short lived (Figure 4) compared to the total process leading to breakdown. The lifetime of the plasma emission is < 100 ns, negligible compared to even the shortest delay to breakdown times, - 30 ps. After the plasma has cooled down, the stability of the electrode gap may be affected both by an increase of local charge density, and by an expanding gas bubble formed by the plasma. The


increase in local charge density may be created by either photoionization, by charges ejected from the plasma, or by charges drawn out from the plasma by the electric field. The expanding gas bubble is simply liquid vapor- 2 5 q

\ I

40t Although the average intensity a t the focal point may P be estimated reasonably accurately the instantaneous in- 2 ’ tensity at the focal point can not be determined easily. Generally, the intensity (in W/m2) of a focused beam a t the focal spot is given by

the liquid that are brought together in clusters by the laser field [25].

ized by the heat from the plasma.

w 3 q

EP T T W 2

I = - (2)

tion, and ‘w the radius of the focal spot. The electrical 0 I00 200 300 400

Pulse Energy / rnJ

0 where E p is the laser pulse energy, T is the pulse dura-

field (in V/m) can be calculated from Figure 5

(3)

where n is the refractive index of the medium, €,E, is the permittivity, and c the speed of light. The 8 ns wide pulses are however not smooth Gaussian pulses in time, but have random amplitude modulation from mode beating during the emission, see e.g. [20]. Therefore the instantaneous intensity during the pulse is not regular from pulse to pulse, rather it consists of a sequence of short and intense peaks on some background level. The minimum pulse energy for plasma formation is - 30 mJ for the laser used but due to the pulse shape irregularities, only the average intensity, not the field can be calculated. In a more accurate measurement to determine the minimum peak power to obtain plasma formation, an injection- locked Nd:YAG laser from Continuum (type NY 82) was used. The laser emitted a smooth pulse profile with a half-width of 28 ns. The pulse energy a t the focal spot needed for plasma formation was then 90 mJ. The focal point diameter was estimated by projection of the focal point via a microscope objective onto a screen. From the magnification the focal point diameter was estimated to be 100 pm. From these values the peak power could be calculated to 3.2 MW, and the intensity 40 GW/cm2. These values correspond to a focal point electrical field of the order of 4 MV/cm, or of the same order of magnitude as the fields needed for initiation of streamers in dielectric liquids [21,22]. The plasma is probably formed in a two-step process. The high electric field provided by the laser pulse causes ionization in the liquid by multiphoton ionization, ionization around impurities or some other process. The liberated electrons are then accel- erated by the field and cause impact ionization in the liquid, ultima.tely causing plasma formation [23,24]. An alternative explanation is that the electron avalanche develops in gas bubbles formed by microbubbles present in

Transmission of a laser pulse through a cell with transformer oil as a function of input laser pulse energy. The unfilled symbols are the transmission without focusing; the filled symbols are the transmission with focusing and plasma formation.

A measurement of the transmitted pulse energy is shown in Figure 5. The unfilled symbols, approximately falling along a straight line, shows the transmitted pulse energy when the laser beam is passed through the cell without focusing, and the filled symbols show the transmission when the beam is focused and a plasma is formed. Above an input energy of 250 mJ pulse energy, the transmitted energy no longer increases for the focused beam. Due to absorption of the laser light during the propagation in the liquid, the remaining pulse energy at the plasma formation point is x 60 mJ. This corresponds to the energy needed for plasma formation at the present focusing condition. Above this pulse energy the laser-induced plasma is either absorbing or reflecting the rest of the radiation. We believe that the ionization degree increases in the plasma during the laser pulse until the electron concen- tration reaches the value where the plasma frequency [26] equals the light frequency. At this point the plasma becomes opaque and the later parts of the laser pulse then will not be absorbed. The plasma becomes larger for higher pulse energies and the cavitation sounds [27] from the imploding gas bubble are also more intense for the higher pulse energies. This can be explained by assuming that, a t higher pulse energies, the critical field for plasma formation is obtained in a larger volume, and the plasma will therefore be larger. A rough calculation of the electron density from the plasma frequency and the laser wavelength used gives an electron density of - 10” ~ m - ~ . The interelectron distance then becomes - 1 nm, which means that the plasma is nearly metallic and most molecules are consequently ionized.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 1 No. 4 , August 1994 685

Figure 6

Photograph of laser-produced plasma in oil. A is the anode, B is the cathode, and C is the laser- produced plasma. The pulse energy at the focal point is - 80 mJ. The laser beam direction is top to bottom in thc figure.

Figure 7 Photograph of a laser-initiated breakdown in transformer oil. A is the anode, a t +30 kV potential, B is the cathode, C is the laser-produced plasma, and D is the breakdown arc. Note the branched structure towards the cathode. The laser beam direction is top to bottom in the figure.

3.3. ELECTRIC B R E A K D O W N To start an electrical breakdown, the laser plasma was

initiated midgap between the electrodes. When H V was applied to the test cell, and the laser was fired, it was indeed possible to start a breakdown process using the laser-produced plasma. Photographs of the process are shown in Figures 6 and 7. Figure 6 shows the laser plas- m a when no voltage was applied, whereas Figure 7 shows the plasma and the breakdown arc channel when a breakdown has occurred. Gap distance for these photos was 5 mm, and the applied voltage for breakdown was +28 k V dc. The total pulse energy at the focal spot was 80 mJ. The photos were taken with a normal camera with

the shutter left open during the whole process and the light in the laboratory dimmed so as not to overexpose the film. The photos therefore represent time-integrated images of the whole breakdown process from initiation to breakdown arc.

J 9 801

601 1064nm i' Spontaneus I

Breakdown Voltage

11 , , 1

3 10 30 100 High Voltage / kV

Figure 8

Weibull plot of a triggered breakdown experiment in transformer oil. The electrode gap was 5 mm and the experiment was conducted as a constant- voltage experiment with ten trials a t each voltage level. T h e spontaneous breakdown voltage was 60 kV.

Experiments to determine the breakdown probabilities have been performed for HV. The measurements were carried out as constant-voltage experiments where the voltage was kept constant and the laser was fired l o x for each voltage level. The time between the laser shots was 1 minute if no breakdown occurred, and after each HV reapplication after breakdown, the system was allowed 1 min to stabilize. The field between the electrodes is not totally uniform, and field calculations show an 20% higher field a t the HV and ground electrodes. According to [28], a t the given geometry and voltage levels, ions are transported through the gap before they relax, so that no space charge is accumulated in the gap at stationary conditions. The number of breakdowns for each voltage level was recorded, calculated into relative probabilities, and finally plotted on Weibull paper assuming a Weibull 2-parameter distribution. A straight line was fitted to the points in the diagram with the least squares method. A typical example for transformer oil is given in Figure 8. The graph is very steep, and thus the expo- nent in the Weibull distribution is high, normally 2 10. For some experimental runs the probability distribution could indeed have been replaced by a step function, since the statistical character of the breakdown process totally vanishes. The voltage level required for spontaneous breakdown was +60 kV in Figure 8. The voltage for the triggered case that led to breakdown in 50% of the trials ( U ~ O ) was as low as +22 kV, or 37% of the spontaneous

686 Sunesson et al.: Laser "kiggering of Electric Breakdown in Liquids

breakdown voltage.

- -..- Spontaneus Breakdown Voltage

40 t 1 30 t I \

z t 20;

L

10 1 \

L 0 1

0 20 40 60 80 100 120 Trig pulse energy / rnJ

Figure 9. The 1150 voltage (as defined in the text) measured from a series of constant-voltage experiments in transformer oil for different laser pulse energies. Pulse-to-pulse variation of the laser pulse energy < 5%. The gap length was 2 mm, and the spontaneous breakdown voltage was 45 kV. The line is for guidance of the eye only.

The dependence of Us0 on the laser pulse energy was investigated for transformer oil and a 2 mm gap. The laser pulse energy was increased from the onset of plasma formation until the maximum possible energy. For each pulse energy level a triggered breakdown experiment was run to determine the U50 level. For the breakdown experiments the laser pulse was applied l o x per voltage level as above, and the parameters from the least squares fit were used to calculate the U50 voltage level. The resultant curve is shown in Figure 9, where the spontaneous breakdown voltage has been indicated. The pulse-to-pulse fluctuation of the laser pulse energy was < 5%. As can be seen, the needed voltage level decreases sharply from > 30 to - 8 kV at 60 mJ and above, where the curve levels out. It is interesting that no further lowering of the breakdown voltage occurs above 60 mJ pulse energy.

To investigate the influence of excitation wavelength, experiments with the second harmonic frequency from the laser, 532 nm wavelength, were performed. The re- sults for transformer oil and n-hexane are summarized in Figures 10 and 11. As can be seen for transformer oil (Figure 10) the breakdown voltage for 1064 nm triggering is reduced from 45 to - 8 kV, whereas for 532 nm triggering no real lowering of the breakdown voltage appears to occur. The two plots also differ in spread: The 532 nm line has a much larger statistical spread. For n-hexane (Figure 11) the wavelength change simply displaces the plot to a higher voltage region, leaving the spread low. The laser-produced plasmas were monitored for each of

99t

1064 nm / I

I

S pont aneus (Breakdown ' Voltage

1, i

3 10 30 100 High Voltage / kV

Figure 10. Weibull plots for two constant-voltage experiments in transformer oil performed at 1064 and 532 nm. The gap distance was 2 mm, and the spontaneous breakdown voltage was 45 kV.

99 t

1064 nm 532 nm

Spontaneus Breakdown

Voltage

10 30 100 High Voltage / kV

' t , . , , . 3

Figure 11. Weibull plots for two constant-voltage experiments in n-hexane performed at 1064 and 532 nm. The gap distance was 5 mm, and the spontaneous breakdown voltage was 60 kV.

the four cases, and (by unaided ear) it was noted that the cavitation sounds [27] that are produced when the plasma-produced gas bubble implodes always appeared very prominent except for the 532 nm produced plasmas in transformer oil which caused much lower sounds than the other cases.

Current and light measurements have been performed for both transformer oil and n-hexane. A typical case is shown in Figure 12, where the current from a laser- triggered breakdown in n-hexane, 5 mm gap, and $35 kV HV is recorded. The delay to breakdown time here is - 70 ps. The current trace in Figure 12 consists of irregular pulses of very short ( X 10 ns) duration, followed by a large pulse after M 65 ps. The large pulse is preced- ed by a rapid sequence of small pulses. The ringing in the detection system after the large pulse occurs because the protection circuit (see Figure 1) is activated. After the large pulse a continuous current superposed by one

IEEE Transactions on Dielectrics and Electrical Insulation Vola

-2 - Time / ps I i Figure 12

Current measurement in n-hexane. The gap distance was 5 mm and the voltage was +35 kV. The laser pulse occurs at 0 ,us, and the breakdown occurs at 70 f i s .

short spike flows and develops into a runaway current that ends with breakdown a t 70 ps. This current signa- ture is typical for the triggered breakdown measurements in n-hexane: a few small pulses of increasing amplitude a t fairly regular time intervals, followed by several pulses at irregular time intervals with larger, approximately constant, amplitude. After a time, which depends on the applied HV, a burst of small pulses or a continuous current appears. This is immediately followed by a large, saturating pulse accompanied by ringing. After the large pulse a continuous current superposed by pulses or a dense burst of pulses of increasing amplitude appear, both of which eventually lead to breakdown.

In Figure 13, a combined current-light graph is shown. The measurement is made for transformer oil in a 5 mm gap and voltage of +35 kV. The upper graph is the measured current, and the lower graph is the PMT signal that was recorded simultaneously. The recordings show that there is a good correlation between light and current pulses, something that has been found for most experiments measuring current and light from streamer propagation e.g. [3,4]. This means that the current pulses are light-emitting discharges. A few current pulses that are not accompanied by light pulses occur, but the reverse has not been observed. The plasma emission saturates the PMT, which explains the large, long pulse in the be- ginning. After the PMT has recovered it again can record the light pulses. A large light pulse is recorded simultaneously with the large current pulse described above. It is stronger tha.n a.11 other light pulses except the laser- produced plasma pulse and the breakdown arc emission. The PMT, like the protection circuit, becomes overload- ed by this pulse and the bulge after the large pulse is an effect of this. The recording begins with a burst of small pulses starting - 5 ps after the laser pulse, followed by larger pulses. For the electrode gap and voltage used in

Electrode current

1 No. 4, August 1994 68 7

I

++ 10 ps. I Div.

Figure 13 Current/light measurement in transformer oil. The gap distance was 5 mm and the voltage was $35 kV. Current is the upper graph, and light is the lower graph. The laser pulse occurs at the first large light pulse.

Figure 13 a large pulse that saturates the circuits appears after 55 to 60 ps, followed by a burst of pulses with increasing amplitude that ends with breakdown. The PMT trace looks different because the PMT responds slower than the current measurement system and does not re- turn to zero as fast. Therefore the light emission during the last - 5 ps before breakdown looks quasi continuous. We believe that this is just a consequence of a very high pulse rate.

The signatures presented here are typical of the laser- triggered breakdown technique. The delay from laser pulse to breakdown varies with the voltage applied. For transformer oil it decreases from 200 ps for $20 kV to - 30 psi for +60 kV for a 5 mm gap. In n-hexane the delay time varies between 95 ps for $20 kV and 30 ps for +50 kV. The transformer oil measurements also some- times showed a fast rising continuous current before the large pulse, but in contrast to what was the case for n-hexane there was no case where a continuous current leading to breakdown could be observed. In all recordings discussed the trigger point was roughly in the middle of the gap. The distance between the large pulse and breakdown was also voltage dependent, much in the same way that the total delay to breakdown was voltage dependent. Measurements on a 10 mm plane-plane gap performed in Vasterbs showed similar current signatures except that a continuous current before the large pulse almost always was recorded for positive HV.


4. DISCUSSION HE laser-produced plasma may influence the elec- T trode gap in several ways, as mentioned in Section

3.2. The laser beam itself may produce charges, and the plasma formation process creates a hot essentially metallic plasma consisting of rapidly moving electrons and almost stationary, heavier ions. The ions are made up of molecular fragments of the liquid molecules. Some of the electrons may escape into the surrounding liquid before the plasma recombines and, consequently, a localized space charge density can be formed at the plasma location. The fast electrons may also through collisions ionize liquid surrounding the plasma. UV and X-ray radiation from the plasma may also contribute to ionization of liquid molecules. Within - 100 ns the plasma has re- combined as evidenced by the radiation (Figure 4) and the heat generated by the spark forms a high-pressure gas bubble containing vaporized molecules and probably also free electrons and fragments of the liquid molecules. The plasma also causes a shock wave in the liquid.

The experiments show that the laser-triggered breakdown method to study electric breakdown can give electric breakdown a t fields substantially below the spontaneous breakdown fields. The Weibull plots show that the process has gone from a process with relatively high statistical spread, i.e. spontaneous breakdown, to a process with low statistical spread, i.e. a triggered breakdown process. The triggering process is very reliable, and the transition from no breakdown to breakdown occurs over a narrow voltage range. The breakdown fields indeed are of the same order of magnitude as can be found in real design cases for transformers or other electrotechnical devices [lo, 291. The fields also are of the same order of magnitude as the fields that are found in spark-plug triggered measurements [6), which emphasizes the similarity between the methods. Photographs of breakdown arcs in some cases show evidence of streamer propagation towards an electrode. For example in Figure 7, a branched structure extending from midgap towards the cathode is discernible. This structure is probably a streamer seg- ment that was formed prior to the arc formation and managed to propagate to the cathode.

The measurements on the dependence of U50 on pulse energy (Figure 9) clearly show that there is a saturation effect in the process; above a certain pulse energy the electrode gap system is so disturbed that breakdown fol- lows as long as a critical field is exceeded. The reason for this saturation effect may be found in the separation of breakdown in initiation and propagation in triggered breakdown techniques. Once the initiation condition has been fulfilled by the plasma, propagation to breakdown will occur if the field is high enough. It is then not possible to increase the propagation probability by making

the plasma larger unless it is made very large. Then the bubble may grow to cover the entire gap distance and cause a gas discharge between the electrodes that breaks down the gap. A third alternative is that the plasma above a certain pulse energy limit can not absorb more energy, which would limit the process.

The measurements at different wavelengths show a shift in breakdown voltage but unchanged spread for n-hexane whereas for transformer oil there both is a shift up to the approximate spontaneous breakdown level, and an enlargement of the spread. This can be interpreted as different plasma forming efficiencies. In the 1064 nm case, the plasmas are consistently ‘fat’, whereas the green plasmas appear more fragmented, as evidenced by the difference in cavitation sound. The reason for this can be that self focusing is 4 x as prominent at 532 nm since the critical power for self focusing can be shown to scale as the inverse square of the frequency [18]. Thus, a doubling in frequency entails four times lower critical power. Since self focusing is stronger, it will be more difficult to control the plasma formation process. For n-hexane the boiling point is lower than for transformer oil, which could account for the difference between transformer oil and n-hexane for 532 nm.

The current and light measurements show that there is a large delay time from laser pulse to breakdown. These delay times are incompatible with any possible streamer propagation speed, but they are of the same order of magnitude as spark-plug triggered measurements [7,30]. The reason for the discrepancy with the delay time for streamer propagation probably is that the triggered breakdown process is a multi-step process. The process appears to include some conditioning of the gap until a streamer propagates, and this excludes the possibility that a streamer propagates right away. In spark-plug triggered measurements, shadow photography of the process [30] has revealed that the trigger spark generates a high- pressure gas bubble. The laser-produced plasma also will lead to the formation of a high-pressure gas bubble. The plasma is approximately cylindrical in shape, so the bubble initially should be cylindrical too. When the pulse energy is absorbed in the plasma it will heat and vaporize liquid. Not all energy is used for the vaporization. Some is used for ionization and dissociation of the liquid, some is lost as radiation, and the remaining energy is used to heat the gas that has been vaporized. An approximate value for the size of the cavity for a known pulse energy, for a cylindrical approximation can be calculated from the latent heat of vaporization of the liquid and the volume expansion from liquid to the hydrostatic pressure. This can be performed for the transformer oil case.

Calorimetric measurements for 100 mJ laser pulse en-


ergy in transformer oil give an estimate of the energy absorbed in the plasma of - 27 mJ. This amounts to an absorption of 30%, if losses are taken into account. For a cylinder-shaped plasma of 50 p m radius and 4 mm length, and a latent heat of vaporisation for transformer oil of - 2 x IO8 J/m3, - 7 mJ would be needed for total vaporization. The rest, maximum 20 mJ, would be used mainly to heat the vaporized gas above the boiling point, 280°C. The final temperature using a specific heat a t constant volume c, of 2 J/gK is - 900 K. From these values a gas pressure of - 250 MPa before the expansion starts can be calculated using the general gas equation pV = nRT and assuming that the deviation is not too large. The final cavity radius assuming 100 kPa pressure, or the approximate hydrostatic pressure in the liquid, can be calculated if an adiabatic radial expansion (very fast process) of the cylinder is assumed. For such a process the equality p V n = constant holds. The constant K, = c p / c , will be very close to 1 due to the large molecules. These conditions will give a final cylinder radius of - 0.8 mm at 100 kPa pressure. It is reasonable to expect some overshoot due to inertia, and a final radius of 1 m m therefore seems reasonable. This represents a fairly large bubble, and clearly further effort must be put into quantifying the process. Photographic records show weak evidences of bubbles of size - 1 to 2 m m diameter, which is compatible with this very rough estimate. After reaching its maximum size the cavity will be compressed if its internal pressure is low enough. This can occur if the vaporized liquid condenses, which must happen when it cools off. Then the internal pressure will be reduced to zero, and a collapse occurs.

In [30], Chadband et al. have measured lifetimes of cavities in a spark-plug triggered measurement. Lifetimes of the order of 100 to - 500 ps were found for spark energies of 20 to 100 mJ. These values are of the same order of magnitude as the observed delay to breakdown times in laser-triggered breakdown measurements, where the laser pulse energy is 30 to 100 mJ. Therefore, it appears reasonable that the largest part of the delay to breakdown time in laser-triggered breakdown measurements is related to the expansion of a gas-filled cavity.

A hypothesis for the spark-plug triggered breakdown process is proposed in [30], where the cavity caused by the trigger spark grows until it either bridges the electrode gap and thus causes a gas breakdown, or until an electrohydrodynamic instability develops at the surface of the cavity, causing the initiation and propagation of a negative streamer. For the laser-triggered breakdown measurements these are possible processes, but a problem is that no contact with the electrodes is expected if the gap is large enough. The hypothesis that the generated cavity can bridge the gap may work for the 2 mm gap

distance. For the 5 and 10 mm gaps, however, no contact between the cavity and the electrodes is expected.

\ \ I I I / / I :- ----< ' i I I I \ '

2 -

4 - 1

7

8

+

Figure 14 Proposed timing sequence of the triggered breakdown process. The laser beam is directed hori- zontally and the field direction is vertical in the figure. Steps 1-8 refer to the text.

An alternative model is proposed, based on detailed analysis of current and light measurements performed for a 5 mm gap and current measurements performed for a 10 mm gap. The model is outlined below, and a picto- rial description is given in Figure 14. After the laser- produced plasma is extinguished (step l), a gas-filled cavity is formed (step 2). The short pulses preceding the large pulse in the current graphs can be interpreted as gas discharges (partial discharges). They must therefore occur in the expanding cavity, in the field direction (step 3). The discharges will create free charges inside the cavity, and these charges will tend to migrate to the surface of the cavity [31]. Electrostatic forces will then distort the cavity in the field direction so that it becomes more pointed in this direction (step 4). After each suc- cessive discharge more and more charge is expected to accumulate a t the cavity walls. The discharges will not contribute to the growth of the cavity, but they may contribute to a change of shape (step 5). After a number of


discharges, the cavity may have become so pointed in the field direction that a gas discharge can start a streamer in either direction. Assume that a positive filamentary streamer sta.rts towards the cathode (step 6). Evidence for this would be a continuously flowing current [3], and this has been found in the current graphs. This streamer would then propagate towards the cathode (step 6). Cal- culations of the velocity of this streamer have been made based on the 10 mm gap measurements in transformer oil, and they give a.n approximate value of 2 km/s, which is a possible positive streamer propagation speed [3,8]. When the cathode is reached, the positive charge in the

ACKNOWLEDGMENT Fruitful discussions with Thorsten Schiitte of ABB Cor-

porate Research, Sweden concerning the physical mech- anism of the laser triggering method are gratefully ac- knowledged. Thorstens contribution has been very valu- able and is very much appreciated. In the construction of the protection system for the current measurement Kenneth Johansson of ABB Corporate Research, Sweden gave indispensable support. Figure 13 has been kindly provided by J . Backman from his diploma work.

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For the n-hexane measurements, the reverse seems to be true: For positive voltages the negative streamer gets ignited first, and it is followed by a positive streamer towards the cathode.

This model explains the observed phenomena fairly well, and therefore seems credible. To verify it, imaging measurements on the plasma-formed cavity expansion and its transformation into streamer growth should be performed, as well a.s a more detailed investigation of the energy balance in the plasma/cavity forming process. Such measurements are planned and they will help further understanding of the breakdown process. Further, spectral studies of the plasma formation process and the events leading to breakdown are planned. Special atten- tion will be given to the question of charge production due to the pla.sma. and the discharges, as well as the question of charge production resulting from a laser beam.

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The author is employed by ABB Corporate Research but sta- tioned at Lund University This paper is based on a presentation given at the 11 th Internation- al Conference on Conduction and Breakdown in Dielectric Liquids, Baden-Dattwil, Switzerland, July 1993.

Manuscript wan received on 20 October 1993, in final form 13 May 1994.

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Laser triggering of electric breakdown in liquids