US3252046A - Nanosecond pulse light source - Google Patents

Description

May 17, 1966 Q. A. KERNS ET AL NANOSECOND PULSE LIGHT SOURCE 2 Sheets-Sheet 1 Filed Dec. 27, 1961 GENERATOR N l T U B R T S D K R O W T E N INVENTORS S D M gm y EXNL E mw m W D S M A T L A M N A M L Mi; OGTW Y W B X May 17, 1966 KERNS ET AL 3,252,046

NANOSECOND PULSE LIGHT SOURCE Filed De. 27, 1961 2 Sheets-Sheet z m 59 m r M 5 0 m Fax 5 6/ 57 68 67 66 /2 62 I SPARK GAP /76 F g POWER SUPPLY V #2 k j z 54 i k; f/ J W -u.u W 52 69 r LAMP POWER DELAY SUPPLY 74 RING F POWER SUPPLY I.

IN VEN T ORS QUENTIN A. KERNS By GERALD C. COX

THOMAS G. l/v/vEs W/LL/AM S. FLOOD ATTORNEY United States Patent NANOSECQND PULSE LIGHT SOURCE Quentin A. Kerns, Orinda, Gerald C. Cox, Walnut Creek, Thomas G. Innes, Fullerton, and William S. Flood, Pleasant Hill, Califi, assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Dec. 27, 1961, Ser. No. 162,612 9 Claims. (Cl. 315-62) The present invention relates generally to light sources and more particularly to a very compact electronic device for providing extremely brief controlled light pulses. The invention described herein was made in the course of, or under, Contract W7405eng-48 with the United States Atomic Energy Commission.

Although this invention has diverse uses, as a trigger for high current spark gap for example, the device was first developed as a built-in checking light source for radiation counters of the type which detect light generated by the passage of charged particles through a medium.

In a scintillation counter, for example, charged particles pass into a crystal which responds by emitting a short, faint flash of light. By a somewhat different mechanism, counters of the Cerenkov type also produce light pulses in response to charged particles. The photosensitive device in these counters, generally a photomultiplier tube, resonds to the light by producing an output voltage pulse. The light flash from the scintillation crystal may have a duration of less than one nanosecond (1-10- second) and the photomultiplier tube and associated electronic apparatus is designed to function in correspondingly short time periods. Frequently, many photomultipliers and associated crystals are grouped together in a large array so that a cognizant idea of the spatial relationships of a particle beam, for instance, may be obtained.

Such a detecting array may be very elaborate and extensive, thereby making it difiicult to maintain maximum accuracy. The sensitivity and timing accuracy must be easured and rechecked during both initial adjustment and periodically during the life of the apparatus. Similar multiplier phototubes may have transit time differences up to 25 nanoseconds but frequently the time resolution must be adjusted to within 0.5 nanosecond or less. Generally ambient cosmic rays or particle accelerator beams are utilized for calibrating the phototu-bes, however such procedures are time-consuming, inconv nient and costly. The present invention provides a means by which apparatus as described above can be readily calibrated and periodically tested by self-contained means.

The present invention is a compact and inexpensive lamp which can provide a flash of light having a peak lasting for less than one nanosecond and a total pulse lasting for two nanoseconds. As used with the abovedescribed particle detection apparatus the present invention simulates a nuclear event in the same way that itv would be detected by a scintillation detector. A nuclear event causes a very short pulse of light to appear within the crystal, thus a very short pulse of light is required to simulate such an event. A unit of the present invention is permanently disposed within the view of each photomultiplier tube of the counter, one unit being used for each tube. When the system is periodically checked, the various lights are triggered and the relative time resolution between the tubes, as well as the sensitivity thereof,

is measured. Such an arrangement also provides a complete system check on the amplifiers, delay lines, and connecting cables associated with the tubes.

If there is to be an individual test lamp integrally associated with each photomultiplier tube, it is necessary that the light be compact and inexpensive. Insulation require an external source of Patented May 17, 1966 problems are greatly simplified if the voltage necessary for activating the lamps can be moderately low. The present invention fulfills these requirements by producing light with a corona discharge in a gas across the surface of a small body of material having a high dielectric constant and a resistivity in the range from 1-10 to 1-10 ohm-centimeters. Barium titanate ceramic is the most readily available material which meets the requirements, such material typically having a dielectric constant exceeding 4000. Such material is readily available sinc it is commonly utilized in other ways, for example, as a transducer for converting a varying physical force into a correspondingly varying voltage, as in a microphone or phonograph cartridge. In the present invention as well as in the other applications of the material, better characteristics are obtained by mixing other known substances with the barium titanate to increase the dielectric constant.

It is required that the lamp have very low time jitter, that is, the light should consistenly flash at a corresponding time in each cycle of impressed voltage. Other devices that use a gas discharge to provide a light flash depend upon a source of radiation to initiate an ionelectron avalanche through the gas. In most cases randomly occurring external radiation as from cosmic rays is depended upon to start the discharge. In the present invention, only field-emitted electrons are needed for this purpose. Such field-emitted electrons are predictable and need only suflicient electric field to be emitted. Thus, as soon as some particular voltage value is applied, a flash of light occurs. The lamp in the present invention can generate flashes of light of shorter duration than that of the applied electrical pulse. Such a characteristic simplifies supplying satisfactory initiating voltage pulses to a large array of lamps.

A housing may be provided for mounting the lamp with an associated photomultiplier tube, the housing shielding the lamp from the sensitive surfaces of other nearby tubes, providing physical protection and providing for electrical connections.

The invention is also highly useful for precisely controlling the firing of high current spark gaps. Spark gaps radiation to initiate breakdown. If the breakdown time of a large spark gap is to be precisely controlled, a source of radiation with no time jitter is required. The present invention produces ultraviolet light which is elfective in initiating breakdown of a spark gap.

Therefore, it is an object of the present invention to provide a high speed lamp providing an extremely brief flash of light.

It is another object of the invention to provide a lamp with very low time jitter when activated by pulses with relatively long rise times.

It is another object of the invention to provide a lamp which can produce flashes of light shorter in duration than the activating electrical current pulse.

It is another object of this invention to provide a high speed lamp which is inexpensive to manufacture and which is extremely compact and suitable for permanent installation on a phototube as a test light source therefor.

It is a further object of the invention to provide a high speed lamp requiring a relatively low activating voltage.

It is another object of the invention to provide an accurately timed trigger for a high current spark gap.

The invention will be better understood by reference to the accompanying drawing of which:

FIGURE 1 is a section view of the fast light source with a block diagram of accompanying circuitry.

FIGURE 2 is a cross-sectional view of the light source taken at line 2-2 of FIGURE 1,

Referring now to FIGURES 1 and 2 in conjunction, I

there is shown a light source 11 having an envelope 12 of glass or a similar insulative transparent material. A

pair of lead-in wires 13 and 14 pass through one end of the envelope 12 within a stem 16 which projects into the envelope. Within envelope 12 the lead-in conductor 13 projects from stem 16 and supports an annular strap 17 which is spot welded thereto. The strap 17 encircles and firmly clasps a solid cylinder of barium titanate 18 at one end thereof, leaving the other end of the cylinder exposed. The outer surface of the barium titanate 18 is coated with a layer 20 of conductive silver, except for an exposed end 19 which is left unsilvered. One end of a short length of tungsten wire 21 is spot welded to the end of the second lead-in conductor 14 within the envelope 12, the opposite end of the tungsten wire 21 contacting the center of the exposed unsilvered barium titanate surface 19. The contact between the tungsten wire 21 and the surface 19 of the barium titanate is a spring loaded pressure contact, the end of tungsten wire 21 being held against the surface 19 solely by the resiliency of the slightly curved wire. The end of the tungsten wire 21 is arranged to press against the unsilvered surface 19 of the barium titanate with a force of 1 to 3 grams thereby insuring electrical contact therewith. Generally, the inside of the tube envelope 12 is evacuated and filled with hydrogen at /2 atmosphere pressure, although other gases may be selected to secure a particular spectral response, optical decay time, or chemical behavior.

The light sources are preferably supplied with a pulse voltage between 700 to 3000 volts. Considering now the circuitry associated with the lights for providing such activating potentials, a pulse generator 22 provides a two nanosecond long pulse of approximately 3000 volts amplitude. A coaxial cable 23 feeds the output of the pulse generator 22 to a distribution network 24, such network generally being a splitting transformer which provides for an output of 1500 volts to each lamp from the 3000 volt input. The signals from each of the four outputs of the distribution network 24 are carried by coaxial cables 26 to individual light sources 11 of which one is shown in FIGURE 1 and has been described. Each coaxial cable 26 from the output of the distribu tion network 24 is terminated in its characteristic int-- pedance by a terminating resistor 27. The impedance of the resistor 27 combined with the parallel resistance of the lamp 11 is matched to the impedance of the coaxial cable 26 so that no reflections or other electrical aberrations will occur.

The envelope 12 serves a dual purpose of enclosing the light source and as a coil form for a coil 28. Such coil 28 is connected in series with the light source 11, the series combination being connected in parallel with the terminating resistor 27. The coil 28 is provided to give the effect of a higher output voltage from the distributionnetwork 24. Since prior to discharging the light source 11 appears electrically as a capacitance, before the light is produced there is in effect a capacitor and inductor in series, the voltage across the individual com-.

ponents of a series resonant circuit being higher than the impressed voltage. Thus under ideal conditions the voltage across the light source-11 can be made nearly twice that of the output from the distribution network 24.

Referring now to FIGURE 3, there is shown an enlarged section view of the exposed surface 19 of the barium titanate 18 and the end of the tungsten wire 21.

Electrical field lines 31 are indicated between the wire 21 and the surface 19, the field lines being perpendicular to the wire 21 and surface 19 at the surfaces thereof. The field intensity is greater near the end of the wire 21 most adjacent the surface 19. The assembly may be considered as an arc gap in series with a small capacitance, the arc gap being the space between the'wire 21 and the surface 19 while the capacitance is simulated by the barium titanate. Only a very limited number of electrons can flow to the surface 19 .before it is charged to the same potential as the wire 21, thus effectively acting as a small capacitance. When a voltage is applied, a potential difference is created between the Wire 21 and silver layer 20. Assuming the wire 21 is negative, a few field emitted electrons will pass from the wire 21 to the surface 19, following the field lines 31. The majority of the electrons flow where the electric field is strongest, near the end of the wire 21. In the hydrogen atmosphere through which the field lines pass, gas mole cules are struck by the electrons and ionized, the ions and additional released electrons adding to the total current. An avalanche effect rapidly occurs since the hydrogen ions and additional electrons-in turn ionize' more hydrogen atoms and the total current increases to a maximum value almost instantaneously to provide a corona discharge. The hydrogen is excited and light is emitted from the resultant corona.

Since the barium titanate has high resistivity, the electrons coming to the surface 19 quickly charge such surface to the potential of the wire 21 and the electron flow ceases, hence, the effect is similar to that of a small series capacitance. Some of the hydrogen atoms are still sufficiently excited to emit light, but the adjacent barium titanate 18 provides a heat sink effect whereby the excited hydrogen atoms are quickly de-energized or cooled .by coming in contact with the barium titanate. Thus the avalanche effect causes the light to appear suddenly while charging of the surface 19 and the heat sink effect very rapidly extinguish the light. With these combined effects the light pulse can be shorter in duration than the activating voltage pulse if such voltage pulse is longer than the time it takes for the barium titanate to accumulate a charge. The light appears first in a small circle around the end of the wire 21, the circle rapidly expanding outwardly for a short distance toward the layer 20 of conductive silver, thus the abovedescribed process does not occur simultaneously across the entire surface 19, but starts first at the center and progresses outwardly. However, the electric field gradient decreases with distance from the wire 21 and is not sufficient to sustain a discharge for more than a short distance therefrom, thus concentrating the light at the end of the wire 21 in a circle typically about 0.01 inch in diameter. The time interval from the time the light first appears in the small inner circle until the light is extinguished is in the order of a nanosecond (1'10- second).

After the cessation of the voltage pulse, the charge accumulated on the surface 19 of the barium titanate 18 will leak off to the wire 21 and conductive surface 20, since both are then at the same potential. The resistivity of the barium titanate is sufliciently low to permit leakage of the charge before a subsequent voltage pulse is applied, yet the resistivity is sufiiciently high that the corona discharge electrons quickly charge the surface 19, quenching the corona.

The high dielectric constant of barium titanate aids in providing a light with a lower impressed voltage. v A material with a high dielectric constant concentrates the electric field near the tip of the wire 21. The higher the dielectric constant, the more intense the electric field. Thus, a lower driving voltage can be utilized with such materials and a dielectric constant in the rang-e from 500 to 10,000 is preferred.

Repetition rates of 20,000 light pulses per second have been obtained. A kilocycle sine wave generator has 11 is disposed with each photomultiplier been utilized as the power source, although the time jitter of the light pulses is increased. It is expected that repetition rates of several million flashes of light per second will be attained when adequate pulse generators are obtainable. Negative polarity on the wire gives a lower threshold voltage than a positive polarity and less amplitude fluctuation at a given voltage. The light appears blue to the dark adapted eye. A lowered pressure of the hydrogen within the envelope 12 gives more light output and good amplitude stability but lengthens the decay time of the light. Less than percent amplitude jitter is encountered in any single lamp although greater variations between lamps is encountered.

While it is not necessary that the end of the wire 21 actually contact the exposed end 19 of the barium titanate 18, the construction of the lamp is greatly simplified if contact is made between the two elements. The resistivity of the bariurn titanate is suificiently high that, in effect, a gap does exist between the wire 21 and any part of the end 19 not actually in contact with the end of the Wire 21. By having the end of the Wire 21 spring loaded against the barium titanate 18, the ruggedness of the lamp is improved and uniformity of performance of the lamps is more easily obtained.

When used as a test light for a phototube, the envelope 12 may be enclosed in a light shield having a small opening atone end whereby the light can be directed toward the phototube or other light sensitive device. In usage the light source 11 is disposed near and at one side of the scintillator crystal viewed by a photomultiplier tube so that radiation particles will not be stopped by the lamp instead of entering the scintillator crystal. If a single large crystal of barium titanate is utilized, the light can be viewed at the surface as described and also from the opposite end of the crystal since such a crystal is transparent and thus allows light to be emitted therethrough to suitably disposed detecting devices. One light source tube and crystal. In a large array of tubes, as used in certain particle detectors, there may be more than one hundred tubes, each with a separate light source.

The light output of the corona lamp contains ultraviolet radiation snfiiciently energetic and of sufiicient intensity to stimulate consistent emission of photoelectrons from the surfaces of such metals as copper and tungsten during light pulses as short as two nanoseconds.

Such a metal surface may be the cathode of a high potential electric gap having a dielectric of air or other suitable atmosphere. The photoelectrons emitted by this cathode during illumination by the corona lamp will initiate a spark discharge of the gap provided that voltage gradient in the gap is previously or simultaneously raised above a sharply defined threshold value which will sustain electron multiplication in the gas. Such critical potential is equal to the molecular ionization potential of the gas divided by the mean free path of an electron.

Some mechanism of cathode illumination in conjunction with over-voltaging of the gap is a well known and conventional means of triggering spark discharges in atmospheric gap switches with minimum time uncertainty. However, the use of the present invention to provide initial free electrons coincident with electrical pulsing of a low capacitance electrode in the gap to switch the gradient above the critical threshold executes this triggering mechanism with a small fraction of the trigger power required by conventional methods. The two conditions for initiating a spark discharge are met separately and the expenditure of trigger energy for each minimized in the mode described. curacy better than that of the best conventional methods, and which realizes greater power gains for the gap than any triggering method which effects timing accuracy in the nanosecond range.

The voltage increment necessary to switch decisively through the spark discharge threshold region is small amounting to about one tenth of the threshold voltage for The result is a triggering mode with timing acelectrode 54.

a typical gap with air at atmospheric trio.

For gaps in the range from two thousand to thirty thousand volts this voltage excursion is of the same order of magnitude as that required to fire the corona lamp. The two effects: illumination of the gap cathode by the corona lamp and overvoltaging of the gap or a portion thereof can be accomplished'within a few nanoseconds by small vacuum tube amplifiers and other presently available electronic switching devices.

Referring now to FIGURES 4 and 5 there is shown the structure of a spark gap, switch utilizing the present invention as a triggering means. The apparatus is contained within a cylindrical insulative housing 51 having a first circular end plate 52 secured to one end and a secondcir-cular end plate 53 secured to the opposite end. A first electrode 54 is formed as an integral part of end plate 52, the electrode being an annular tubulation projecting into the housing 51 along the axis thereof and having an axial bore 56.

A second cylindrical electrode 57 projects into housing 51, along the axis thereof, from end plate 53, the electrode having a threaded stem 58 engaged in a threaded bore 59 at the center of the end plate. The inner end of electrode 57 is of the same diameter as electrode 54 and has aconcavity 61 conforming to axial passage 56 of electrode 54. Electrode 57 is spaced from electrode 54 to form the spark gap 62, the gap spacing being adjustable by rotation of electrode 57 in the threaded end plate bore 59. To assure good electrical contact between electrode 57 and end plate 53, a sleeve 63 is disposed coaxially around stem 58 and secured to the end plate, the opposite end of the sleeve receiving the electrode and making a pressure as a dielecsliding contact therewith.

A tungsten ring 64 is positioned midway in gap 62, coaxially with respect to the electrodes, and is supported by a conducting arm 66 which projects radially through an insulative bushing 67 in the sidewall of housing 51.

A protective metallic lamp housing 68, containing a lamp 11 as previously described, is disposed within the axial bore 56 of the first electrode 54, the housing being formed to leave lamp 11 exposed to the surface of electrode 54 so that ultra-violet rays from the lamp 11 can illuminate the gap facing surfaces of the first electrode 54. It is essential that the lamp housing 68 be sufiiciently spaced from the gap 62 that arcing does not occur to the lamp. The larnp 11 preferably has a quartz envelope 12' to improve the transmission of ultra-violet light. In some instances it is preferred that the lamp 11 operate without an envelope to increase the intensity of the ultraviolet, however in this case it may be necessary to operate the entire spark gap assembly at other than atmospheric pressure to obtain satisfactory operation of the lamp. The lamp housing 68 where used is physically supported by a clamp 69 of cylindrical shape and a retainer 71, threadably engaged in the axial bore of the clamp, which pass through the axial bore 56 of the first A ring power supply 72 provides activating voltage for the ring 64 and, through a delay circuit 73, triggers a lamp power supply 74 connected to the lamp 11 so that the lamp 11 is flashed shortly after a potential is applied to the ring 64. A spark gap power supply 76 is connected in series with a load 77 to the first and second electrodes 54 and 57. The load may, for example, be an air core magnet or similar component operated under pulsed conditions.

Considering now the operation of the switch, assume that the spark gap power supply 76 provides a potential just below the spontaneous breakdown potential across the gap 62. Thus no current flows as yet through the load 72. A 20 percent overvoltage pulse is then supplied by the ring power supply 72 to the ring 64. If the lamp 11 were not present, the spark gap would function as a conventional three electrode spark gap wherein a spark 7 first forms from the ring 64 to one of the electrodes 54 or 57 and then across the gap 62. Time jitter would occur in such case since breakdown would await initiation by radiation from some external source. In the present invention, the flash ofultra-violet radiation from the lamp 11 is a controlled source of radiation which causes emission of photoelectrons from the gap electrodes, causing immediate sparking. Thus the onset of ionization is initiated by controlled radiation rather than being initiated by randomly occurring ambient radiation.

In a typical embodiment of the spark gap, 20 thousand volts is applied across the gap 62 at atmospheric pressure. After a time delay of 15 nanoseconds the current through the spark gap has reached 10 percent of full current with a time jitter of plus or minus two nanoseconds. The rise time of the current is determined principally by the reactive characteristics of the load 77 and spark gap power supply 76.

While the invention hasbeen disclosed with respect to a small number of typical examples it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention and thus it is not intended to limit the in vention except a defined in the following claims.

What is claimed is:

1. In a pulsed light source, the combination comprising a body of material of the class having a high dielectric constant and a high electrical resistivity, a first conductor contacting an extensive first portion of said body of material, a second conductor adjacent a second relatively small portion of said body, an envelope surrounding said body and said first and second conductors, an ionizable gas within said envelope, and a voltage pulse generator connected to said first and second conductors and providing a voltage difference therebet-ween of a magnitude less than that required to initiate an are between said first and said second conductors.

2. In a pulsed light source, thecombination compris ing a block of material having a resisitivity in the range of about 1-10 to 1-10 ohm-centimeters and having a dielectric constant in the range of about 500 to 10,000, and means establishing a high electric field across a small portion of the surface of said block which field is of an intensity less than that required to initiate an arc.

3. In a pulsed light source, the combination comprising a body of high dielectric constant material having a resistivity in the range from about 1-10 to about 1-10 ohm-centimeters, a conductive coating disposed on the surface of a first portion of said body and surrounding a second uncoated portion of said body, a probe conductor adjacent'said second portion of said body, a voltage supply connected to said conductive coating and to said probe conductor to provide a voltage difference therebetween of an intensity less than that required to initiate breakdown between said conductive coating and said probe conductor, a transparent envelope enclosing said conductor and said body, and an ionizable gas disposed within said envelope.

4. In a device for producing a rapid flash of light, the combination comprising a body of high dielectric constant material having a resistivity in the range of about 1- 10 to about 1 10 ohm-centimeters, a conductive coating on said body surrounding an uncoated surface portion on said body, a conductive probe contacting said. body at said uncoated surface portion, a voltage supply connected to said conductive coating and to said probe and creating a voltage difference therebetween of a magnitude less than that required to initiate an arc breakdown between said conductive coating and said conductive probe, a transparent envelope enclosing said body, and an ionizable gas disposed within said envelope.

8. 5. A device for producing light as described in claim 4, the combination further comprising an inductance'connected in series with said conductive coating and said probe across said voltage supply.

6. A device for producing light as described in claim 4 further characterized by the dielectric constant of said body having a value in the range from 500 to 10,000.

-7. In a pulsed light source for triggering breakdown across a spark gap of the class having at least two spaced apart electrodes, the combination comprising a body of material having a high dielectric constant and having a resistivity in the range from about 1-10 to about 1-10- ohm-centimeters, said body being disposed in view of at least one of said electrodes whereby light created along the surface of said body illuminates such electrode, and means establishing a high electric field across a portion of said body of an intensity less than that needed for an arc discharge.

8. A spark gap switch comprising, in combination, an annular insulative housing, a first cylindrical electrode disposed coaxially within said housing at a first end thereof and having a concavity formed in the center of the innermost end thereof, a second cylindrical electrode disposed coaxially in the second end of said housing and having an innermost end spaced from said innermost end of said first electrode to form said spark gap, a third electrode extending within said gap and spaced apart from each of said electrodes, a body of material disposed in said concavity of said first electrode and having a high dielectric constant and high electrical resistivity, a conductive coating disposed on an extensive area of the surface of said coating, :1 probe conductor contacting the surface of said body away from said coating thereon, a first voltage source applying a potential difference between said first and second electrodes, a second voltage source for applying a pulsed potential to said third electrode, and a third voltage source applying a potential difference between said probe conductor and said conductive coating upon the application of said pulsed potential to said third electrode, said potential dilference applied by said third voltage source being of a magnitude less than that required to initiate an arc discharge between said probe conductor and said conductive coating.

9. In a pulsed light source, the combination comprising a cylindrical body of material composed principally of barium ti-tanate, a conductive coating disposed on the .cylindrical surface of said body, a conductive probe contacting a central portion of an end of said body, a transparent envelope enclosing said body, an ionizable gas in said envelope, an inductance coil disposed around the outside surface of said envelope and connected in series with said conductive coating and said conductive probe, and a power supply connected across said series connected inductor and said conductive coating and said conductive probe, said power supply providing a potential of a magnitude less than that required to initiated an arc discharge between said conductive coating and said conductive probe.

References, Cited by the Examiner UNITED STATES PATENTS DAVID J. GALVIN, Primary Examiner. ARTHUR GAUSS, Examiner.

C. R. CAMPBELL, Assi tant Ex miner.

Claims (1)

1. 7. IN A PULSED LIGHT SOURCE FOR TRIGGERING BREAKDOWN ACROSS A SPARK GAP OF THE CLASS HAVING AT LEAST TWO SPACED APART ELECTRODES, THE COMBINATION COMPRISING A BODY OF MATERIAL HAVING A HIGH DIELECTRIC CONSTANT AND HAVING A RESISTIVITY IN THE RANGE FROM ABOUT 1.108 TO ABOUT 1.1015 OHM-CENTIMETERS, SAID BODY BEING DISPOSED IN VIEW OF AT LEAST ONE OF SAID ELECTRODES WHEREBY LIGHT CREATED ALONG THE SURFACE OF SAID BODY ILLUMINATES SUCH ELECTRODE, AND MEANS ESTABLISHING A HIGH ELECTRIC FIELD ACROSS A PORTION OF SAID BODY OF AN INTENSITY LESS THAN THAT NEEDED FOR AN ARC DISCHARGE.

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