CA1039346A - Spark sources with electronic switching tubes - Google Patents

Abstract

ABSTRACT:

A spark source is disclosed having an analytical spark gap, a capacitor, a power supply for charging the capacitor, and an electronic switching tube connected in a series circuit between the capacitor and the spark gap. When the tube is rendered conductive by a triggering pulse or other signal, the capacitor is discharged through the spark gap and the switching tube, which carries the entire discharge current. The switching tube is preferably of the gaseous type,having an anode, a cathode and a control electrode. The switching tube is prefer-ably of the type containing hydrogen, but other types contain-ing other gases or vapors may be employed. Thus, for example, the switching tube may contain argon or mercury vapor. The switching tube is preferably connected in the circuit with its cathode connected to ground. One electrode of the analytical spark gap is also preferably grounded.

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Description

1~)39346 This invention relates to spark sources, which may be employed for various purposes, particularly the production of light for spectroscopic analysis. The material to be analyzed is introduced into an analytical spark gap, across which sparks are produced. The sparks vaporize the material to be analyzed and excite the resulting vapor to produce light which can be analyzed spectroscopically to determine the constituents of the material being analyzed.
One object of the present invention is to provide an improved spark source of the general type disclosed and claimed in U.S. patent No. 3,749,975, issued July 31, 1973, upon the application of John P. walters, one of the applicants herein.
Such patent discloses a spark source in which a capacitor is charged to a high voltage, and is then discharged across an ' analytical spark gap. A control gap is also employed in series ~ - -with the analytical gap. To control the waveform of the discharge ~~
current across the analytical spark gap, first and second variable ;
coils or inductive elements are provided in series with the capacitor and the spark gaps. A shunting diode is connected ~ ~ -across the series combination of the analytical spark gap and the second inductive element. ~
In the spark source of such walters patent, a gaseous ~ -electronic control tube is connected in parallel with the control gap. The discharge of the capacitor across the analytical spark gap can be initiated by applying a control pulse or other signal to the control electrode of the electronic control tube, so as to . .
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~039346 render it conductive,which has the effect of momentarily short-circuiting the control gap. The increased voltage across the analytical gap then causes the initiation of the discharge of the capacitor across the analytical gap. After a very brief interval, the control gap also breaks down, so that it carries the main discharge current between the capacitor and the ana-lytical spark gap.
A further object of the present invention is to eliminate the control spark gap of such Walters patent, and to provide a new and improved spark source which employs an electronic switch-ing tube in an advantageous manner to control the timing of the analytical sparks.
In accordance with the present invention, the spark source comprises a spark gap including a grounded electrode and an ungrounded electrode, a capacitor having positive and negative -terminals adapted to be positively and negatively charged, a power supply for charging the capacitor and having positive and negative terminals, means connecting the positive and negative terminals of the power supply to the positive and negative ter-minals of the capacitor, a discharge circuit connected between the negative terminal of the capacitor and the ungrounded electrode of the spark gap, an electronic switching tube having an anode, a thermionic cathode and a control electrode, means connecting the anode to the positive terminal of the capacito~, means connecting the cathode to the grounded electrode of the spark gap, a heating circuit for supplying heating power to the thermionic cathode, and means for supplying control signals between the control electrode and the cathode of said electronic 1~)39346 switching tube, said signals being of such magnitude and polarity as to render the tube conductive so as to cause the discharge of the capacitor across the spark gap, sparks thereby being pro-duced across the gap in precisely timed relationship to the control signals.
The discharge circuit preferably includes first and second inductive elements connected in series with the spark gap, and a shunting diode connected across the series combination of the second inductive element and the spark gap for modifying the wave form of the spark current. A third inductive element may be con- -nected in series with the shunting diode. The third inductive element may be adjustable for adjusting the wave form of the spark current across the spark gap.
In one embodiment, the shunting diode may be reversely polarized relative to the polarization of the electronic switch-ing tube, whereby the shunting diode is reverse biased and not conductive during the first half cycle of the capacitor discharge current.
In another embodiment, the shunting diode may be polarized ~
the same as the electronic switching tube and thereby is forward -biased and conductive during the first half cycle of the capacitor discharge current.
The spark source preferably includes diode rectifier means connected between the anode and cathode of the electronic switch-ing tube, the diode rectifier means being polarized oppositely with re~pect to the polarization of the electronic switching tube.
The electronic cwitching tube is preferably of the gaseous type containing an ionizable gas or vapor, preferably hydrogen.

:, ' , , ' ,' ' ''' " . , , Further objects, advantages and features of the present invention will appear from the following description, taken with the accompanying drawings, in which:
Fig. 1 is a schematic circuit diagram of a spark source to be described as an illustrative embodiment of the present invention.
Fig. 2 is a fragmentary schematic circuit diagram showing a modified spark source in which the shunting diode is reversed in polarity so as to be forward biased rather than back biased during the first half cycle of the capacitor discharge current.
Fig. 3, which contains parts a, b, c and d, is a family of waveform diagrams illustrating the operation of the spark source shown in Fig. 1, with the shunting diode back-biased.
Fig. 4, which contains parts a, b and c, is a family of waveform diagrams illustrating the operation of the spark source of Fig. 2, in which the shunting diode is forward biased.
As just indicated, Fig. 1 illustrates a spark source 10 which employs an analytical spark gap 12 having electrodes 13 -and 14. A spark is produced across the gap 12 by charging a capacitor 16 to a high voltage, and then discharging the capa-citor acros~ the gap 12. It is prefered to ground one electrode of the spark gap, in this case the electrode 13. -Thus the spark gap 12 and the capacitor are connected ~ " "

1~39346 in a series discharge circuit 18 which preferably also includes first and second inductance coils or other inductive elements Ll and L2, connected in series between the capacitor 16 and the spark gap 12. The inductive elements Ll and L2 are preferably variable or adjustable, so that the inductive elements may be employed to control the waveform;! and the periodicity or frequency of the discharge current. It is also prefered to provide a shunting diode 20, in a shunting relationship to the series combination of the spark gap 12 and the second inductive element L2. A third inductance coil or other inductive element L3 is preferably connected in series with the shunting diode 20. Here .
aga n, the third inductance element L3 is preferably variable or adjustable, to provide a further control over the waveform~ of the current across the spark gap 12.
A high voltage power supply 22 is provided to charge - :
the capacitor 16. The high voltage power supply 22 may be of : . -any known or suitable construction. As shown, the power supply 22 ~- -includes a high voltage transformer 24 having a low voltage pri-mary winding 26 and a high voltage secondary winding 28. A com-mercial alternating source may be employed to energize the ~:
primary winding 26. As shown, the primary winding 26 is energized ;-- ~
through a variable autotransformer 30 and a pair of current ~ -limiting resistors 32a and 32b, connected in parallel with each other and in series between the autotransformer 30 and the pri-mary winding 26. It will be understood that the variable auto-transformer 30 makes it possible to adjust the high voltage output -~39346 of the power supply 22~
The high alternating voltage from the secondary winding 28 is rectified by a bridge rectifier circuit 34 which delivers its direct current output to the capacitor 16 through current limiting resistors 36a, b and c, connected in parallel with one another, and in series between the negative terminal of the bridge rectifier 34 and one side of the capacitor 16. The other side of the capacitor is connected to the positive output terminal of the bridge rectifier circuit 34. The charging resistors 36a, b and c, regulate the rate at which the capacitor 16 is charged by the high voltage power supply 22. ~
In accordance with one feature of the present invention, ~;
an electronic switching tube 38 is connected in series with the capacitor discharge circuit 18, so that the capacitor 16 can be discharged across the analytical spark gap 12 on command, when the tube 38 is rendered conductive. The switching tube 38 preferably has an anode 38a, a cathode 38k, a cathode heater 38h ~--and a grid or control electrode 38g. The cathode and the anode 38a and 38k are preferably connected directly into the capacitor discharge circuit 18 between the capacitor 16 and the analytical spark gap 12. The anode 38a is connected to the positive terminal -of the bridge rectifier circuit 34, and also to the corresponding terminal of the capacitor 16, such capacitor terminal being positively charged by the power supply 22.
The electronic switching tube 38 is preferably of the gaseous type, such as a thyratron, containing an ionizable gas or 1~39346 vapor, so that an arc discharge will be established between the anode and the cathode when the tube becomes conductive.
The electronic switching tube 38 can be rendered conductive on command by applying a positive trigger pulse or other signal between the control electrode 38g and the cathode 38k. Such pulse or signal may be supplied by a trigger source 40 which may supply positive pulses of approximately 50 to 100 volts.
The pulses preferably have sharp positive-going timing edges to produce highly precise triggering of the gaseous switching tube 38.
As shown, a selenium surge suppressor 42 is connected between the control electrode 38g and the cathode 38k. Such surge suppressor may be General Electric type number GRs-2l-s All-Dll-9H, or the equivalent. -The electronic switching tube 38 is preferably of the type containing hydrogen, but may be of other types containing other gases or vapors, such as argon and mercury vapor. One -~-suitable type is the commercial type number 6279/5C22, containing ~ -hydrogen. ~ -The tube 38 conducts current in one direction, between its anode 38a and its cathode 38k. To conduct current in the opposite direction during the oscillatory capacitor discharge ;
current, a diode rectifier 44 is connected between the anode 38a and the cathode 38k. As illustrated, such rectifier 44 takes ~-the form of a diode stack, polarized so as to be back-biased -by the initial positive voltage between the anode 38a and the -~ -cathode 38k , ~039346 The heater 38h of the gaseous switching tube 38 may be energized by a filament transformer 46, which may be connected to a commercial source of alternating current.
The circuit of the spark source 10 is arranged so that the cathode 38k of the gaseous switching tube 38 is connected to ground. Thus, in Fig. 1, the cathode 38k is connected to a grounded lead or conductor 48. With this arrangement, there is no need to provide special high voltage insulation between the primary and secondary windings of the filament transformer 46.
Moreover, no high voltage can be developed between the cathode 38k and the heater 38h. In addition, it is easy to supply pulses or triggering signals between the control electrode 38g and the cathode 38k, because no high voltage can be de~eloped on the cathode. Thus, it is not necessary to utilize an isolating trans-former or any other device to protect the trigger source 40 from the high voltage.
Due to the grounding of the cathode 38k, the anode 38a of the switching tube 38 is connected directly to the positive ~
terminal of the capacitor 16. Thus, the tube 38 is polarized ~- ' to carry the initial discharge current between the capacitor 16 and the analytical spark gap 12.
It is also highly desirable to ground one electrode of the analytical spark gap 12, for reasons of safety and convenience.
In this case, the electrode 13 is grounded and thus is connected directly to the grounded cathode 38k of the electronic switching tube 38. With this polarization, the grounded electrode 13 1~39346 becomes the anode during the initial half cycle of the oscillatory discharge current produced between the capacitor 16 and the spark gap 12.
In the operation of the spark source 10 of Fig. 1, the capacitor 16 is charged through the resistors 36a, b and c by ~-the high voltage power supply 22. As long as the gaseous switching tube 38 is non-conductive, no spark is produced. The diode 44 is back-biased and is non-conductive.
A spark discharge can be produced on command by applying -~
a positive triggering pulse or voltage between the control ~-electrode 38g and the cathode 38k of the tube 38, to trigger the tube into conduction. The triggering pulses can be supplied by a computer, some other programmed control system, or any other control device.
when the tube 38 becomes conductive, the high voltage ~ -across the capacitor 16 is applied across the analytical spark gap 12 causing it to break down into conduction so that a spark discharge is produced. Due to the presence of both capacitance and inductance in the discharge circuit, the discharge current is oscillatory. The first half cycle of the oscillatory spark discharge current is carried by the tube 38. The reversely -polarized current during the second half cycle is carried by the diode rectifier 44. During the remainder of the damped oscillatory discharge, the forwardly polarized current is carried -by the conductive tube 38, while the reverse current is carried by the diode rectifier 44, during alternate half cycles. -_ 9 _ . .

~()39346 The provision of the gaseous switching tube 38 makes it possible to produce sparks on command, and to time the sparks with a high degree of precision. With respect to the precise timing of the sparks, the present invention constitutes a significant improvement over the control gap arrangements used heretofore. The precise firing of the gaseous switching tube 38 is not affected by atmospheric variations and other physical factors which do affect the breakdown of an atmospheric control gap. The ability to time the sparks with greatly improved precision is an important advantage of the present invention.
In addition, the provision of the gaseous switching tube 38 substantially eliminates the loud noises produced by the sparks jumping across the control gap, as used heretofore.
The reduction in noise is quite dramatic and is an important environmental advantage of the present invention.
In Fig. 1, the shunting diode 20 is polarized so as to be back-biased during the first half cycle of the capacitor discharge current. The effect of this polarization will be discussed presently.
Fig. 2 illustrates a modified spark source 50, in which the shunting diode 20 is reversed in polarity so as to be forward biased during the initial half cycle of the oscillatory discharge current. Otherwise, the spark source 50 of Fig. 2 may be the same as the spark source 10 of Fig. 1. The significance of reversing the polarization of the shunting diode 20 will be discussed presently.

1~39346 Fig. 3 illustrates the waveform of the spark current across the analytical spark gap 12 for the spark source 52 of Fig. 1, in which the shunting diode 20 is reverse biased during the first current half cycle of the capacitor discharge, but with the inductance of L3 reduced to zero or omitted so that only the stray inductance of the circuit is in series with the shunting diode 20. Due to the back bias, the diode 20 is non-conductive initially so that the entire current during the first half cycle passes through L2 and across the spark gap 12. The buildup of current in the inductive element L2 results in the ~ -storage of energy in L2. The stored energy produces a relaxation ~ -current across the spark gap 12 during the second half cycle.
In Fig. 3a, the current across the spark gap during the first half cycle is indicated at 57. The current in the spark gap 12 due to the relaxation of L2 during the second half - -cycle is indicated at 58. This current flows during the interval indicated as iL2 , when the shunting diode 20 is forward biased. -~In Fig. 3a, the broken line 60 represents the current that would flow across the spark gap 12 during the second half cycle in the --absence of the relaxation current produced by L2.
The waveform of the current across the spark gap during -the interval iL2 is completely independent of the main oscillatory capacitor discharge current. Such current waveform, in direction and in magnitude, depends on the value of L2. The effect of changing the magnitude of L2 is illustrated by Figs. 3a, b and c.
If L2 is large, as it i~ for Figs. 3a and b, the relaxation 1~)39346 current may be high enough to drive the discharge uni-directiona Thus, the uni-directional discharge current across the spark gap is indicated at 58 in Fig. 3a and at 62 in Fig. 3b. It will be evident that Fig. 3b represents a case in which L2 is larger than for the case represented by Fig. 3a, with the result that the relaxation current produced by L2 is greater, as will be evident from a comparison of the currents represented at 62 and 58.
Fig. 3c represents the case in which L2 is relatively small, so that the relaxation current is insufficient to produce a uni-directional discharge current across the spark gap.
Instead, the relaxation current reduces the oppositely polarized current during the second half cycle, as indicated at 64, but does not change the polarity of such current. It will be evident that the waveform of the discharge current across the spark gap 12 can be changed to a great extent by changing the ~alue of L2.
The examples illustrated by Figs. 3a, b and c presuppose that the impedance of the shunting diode 20, when it is forward biased, is high enough to keep the analytical spark gap 12 in conduction during all phases of the discharge. If not, there may not be any relaxation current from L2. Instead, the action of the diode 20 may be simply to clip the discharge current, parti-cularly when L2 is small in value. This clipping action is illustrated in Fig. 3d at 66. At such point 66, the spark gap 12 goes out of conduction, due to the very low impedance of the shunting diode when it is forward biased. The clipping action continues throughout the half cycle, as long as the diode is 1~39346 forward biased. This clipping action depends upon the specific characteristics of the diode which is employed, and upon the residual impedance in series with the diode, due to the resistance and distributed inductances of the connecting leads, for example.
The provision of the third inductance element L3 in series with the shunting diode 20 has the advantage of increasing the impedance of the diode circuit when the diode is forward biased. In this way, the discharge across the spark gap 12 will be maintained during all phases of the capacitor discharge. This situation is represented by the spark source 10 of Fig. 1, in which the third inductive element L3 is connected in series with the shunting diode 20, which is reverse biased~during the first -- -half cycle and is forward biased during the second half cycle. ---The waveforms of the spark currents across the spark gap 12 may be represented by the waveform diagrams of Figs. 3a, b and c, - -depending upon the value of the second inductive element L2.
Thus, the inductance L3 acts as a forward-unit .
impedance, inserted into the diode loop, to control the breakdown and continued ionization of the analytical gap 12. when L3 is adjustable, it may be called a programmed forward-unit impedance.
The action of L3 is to control the splitting of the current between the diode 20 and the gap 12. The inductance L3 will allow the gap 12 to stay in conduction wh~n L2 is small and the diode 20 is forward biased and in full conduction.
In the case of the spark source 10 of Fig. 1, the grounded electrode 13 is the anode during the first half cycle ', ' "

1~39346 of the oscillatory discharge. If the value of L2 is sufficiently large to produce a uni-directional spark current, as illustrated by Figs. 3a and b, the grounded electrode 13 is the anode for the entire discharge. This has the disadvantage that it is convenient and otherwise desirable to mount the sample to be analyzed on the grounded electrode, so that the sparks will be produced between the sample and the ungrounded electrode. For the purposes of spectroscopic analysis, it is generally preferable for the sample to be used as the cathode rather than the anode.
Of course, this disadvantage can be overcome by mounting the sample on the ungrounded electrode 14.
Another way of dealing with this problem is to reverse the polarity of the shunting diode 20, as illustrated in Fig. 2.
With this polarity,: the diode 20 is forward biased during the first half cycle and is reverse biased during the second half cycle. As a result of reversing the polarity of the diode in this manner, an entirely new class of discharge waveforms can be generated. These new waveforms constitute a substantial addition to the class of waveforms that are available to practical spectro-chemistry.
As shown in Fig. 2, the third inductive element L3 is connected in series with the shunting diode 20. L3 is preferably variable or adjustable and may range in value from zero to 100 microhenries, for example. The diode is arranged so that it is initially forward biased, during the initial portion of the first current half cycle.

1'039346 If the inductance L3 is not too small, it increases the impedance in the diode path to such an extent that the spark gap 12 will be broken down into conduction even though the diode 20 is forward biased and is conductive during the initial half ' cycle. In this case, the current will split between the spark gap and the shunting diode during the first current half cycle, so that both L2 and L3 will be charged with energy.
when the main oscillatory discharge current in the capacitor circuit is reversed in direction, during the second -half cycle, the diode 20 is reverse biased, with the result that the diode becomes non-conductive, so that the diode path -~
is opened. The full parent oscillatory capacitor discharge current ' is then conducted through the air gap 12. During this half j' cycle, only the second inductance L2 is charged. If L2 is sufficiently large, the remainder of the spark discharge current is uni-directional.
Fig. 4a illustrates the waveforms involved in this sequence of events. The split current in the spark gap during the first half cycle is indicated at 68. At the point 70, the ~ ~-polarity is reversed, so that the diode 20 is rendered non-conductive.
It thus is effectively out of the circuit. The passage of the '~
entire reversely polarized capacitor discharge current across the spark gap during the second half cycle is indica~ed'-at''72.- At approximately the point indicated at 74, the diode 20 again becomes comductive, to carry the relaxation current produced by the second inductance L2. This relaxation current prevents the ~(~39346 reversal of the spark gap current, as indicated at 76, so that the spark current is uni-directional during the remainder o the `~ discharge. It will be recognized that the portion 76 of the waveform corresponds to the portion 58 in Fig. 3a, and also to the portion 62 in Fig. 3b, except that the polarity of the spark current is reversed. In Fig. 4a, as in Figs. 3a and b, the relaxation current produced by the second inductance L2 causes the spark current to be uni-directional during the remainder of the discharge.
Thus, reversing the polarity of the shunting diode 20, as shown in Fig. 2, while providing the third inductance L3 in series with the diode, makes it possible to produce a class of waveforms, as illustrated in Fig. 4a, in which the current is of one polarity during the first half cycle, and is of the opposite polarity during the remainder of the discharge.
This situation is illustrated in Fig. 4b, in which 78 designates the spark current of one polarity produced during the first half cycle, while 80 designates the spark current of the opposite polarity during the remainder of the discharge. waveforms of this class can be utilized very advantageously, in that the current 78 of one polarity during the first half cycle can be employed to vaporize sample material from the member to be analyzed, mounted on one of the electrodes.
If the inductance L3 is reduced to a residual value, and the diode itself has a sufficiently low forward bias impedance, the sp~rk gap will not go into conduction during the first half ' , - cycle of the capacitor discharge current. As a result, the waveform of the spark current, as represented in Fig. 4b, will ~e modified in the manner shown in Fig. 4c. During the first half cycle, the spark gap current is essentially zero, as indi-cated at 81. The current that would normally exist in the spark gap during this portion of the discharge is diverted entirely through the diode, which has such a low impedance that the voltage drop across the diode is insufficient to break down the air gap.
The provision of the electronic switching tube makes it possible to control the timing of the analytical sparks with a high degree of precision, to much better advantage than hereto-fore. Repetitive sparks can readily be produced under the precise ~
control of externally generated pulses or other timing signals, -derived from a computer or any other source.
The ability to produce the spark current waveforms illustrated in Figs. 3 and 4 constitutes another important advan-tage of the present invention, resulting from the arrangements ~ -- involving the shunting diode 20 and the inductive elements L2 and L3.
In Figs. 1 and 2, a leak resistor 82 of a high value is preferably connected across the air gap 12 to discharge the - capacitor 16 when the spark source is shut down.
Those skilled in the art will understand that the values of the various components of the spark sources may be varied to suit a variety of conditions. However, it may be helpful, by way of example, to give the following set of values which have been found to be suitable, with the understanding - -- 1~39346 that these values are subject to considerable variation:
16 Glass and oil capacitor bank, 0.01 microfarad.
Diode stack.
24 Transformer, 110 volts AC to 16-23 kilovolts RMS.
Variac, 8-15 ampere.
32a and 32b Resistors, 5 ohms each, 700 watts, wire-wound.
36a, 36b and 36c Resistors, 100 k ohms each, 200 watts, wire-wound.
38 Thyratron type 6279/5C22.
42 selenium surge suppressor, G.E. type #GRS-21-S-All-Dll-9H.
44 Diode stack 48 Transformer, 110 to~6.3 volts at 10 amps.
82 1 megohm.
The provision of the electronic switching tube has the particular advantage of making it possible to eliminate the control gap heretofore used. The loud noise produced by such control gap, when it breaks down, is thereby eliminated, so that a dramatic reduction in noise is achieved. The resulting improvement in the working environment around the spark source 20 is an important advantage of the present invention.
As previously indicated, the ability to control the timing of the sparks with a high degree of precision is another important advantage of the present invention, resulting from the control circuits involving the gaseous switching tube. The triggering characteristics of the gaseous switching tube are highly precise and are not appreciably affected by atmospheric .' ;

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conditions or other environmental factors. when a positive triggering pulse or signal of sufficient magnitude is applied to the control electrode of the gaseous switching tube, the tube is rendered conductive so that a spark is produced with minimal delay. Thus, the timing of the spark is precisely controlled.

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Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A spark source, comprising a spark gap including a grounded electrode and an ungrounded electrode, a capacitor having positive and negative terminals adapted to be positively and negatively charged, a power supply for charging said capacitor and having positive and negative terminals, means connecting said positive and negative terminals of said power supply to said positive and negative terminals of said capacitor, a discharge circuit connected between said negative terminal of said capacitor and said ungrounded electrode of said spark gap, an electronic switching tube having an anode, a thermionic cathode and a control electrode, means connecting said anode to said positive terminal of said capacitor, means connecting said cathode to said grounded electrode of said spark gap, a heating circuit for supplying heating power to said thermionic cathode, and means for supplying control signals between said control electrode and said cathode of said electronic switching tube, said signals being of such magnitude and polarity as to render said tube conductive so as to cause the discharge of said capacitor across said spark gap, sparks thereby being produced across said gap in precisely timed relationship to said control signals.

2. A spark source according to claim 1, in which said discharge circuit includes first and second inductive elements connected in series with said spark gap, and a shunting diode connected across the series combination of said second inductive element and said spark gap for modifying the waveform of the spark current.

3. A spark source according to claim 2, in which said shunting diode is reversely polarized relative to the polarization of said electronic switching tube whereby said shunting diode is reverse biased and nonconductive during the first half cycle of the capacitor discharge current.

4. A spark source according to claim 2, in which said shunting diode is polarized the same as said electronic switching tube and thereby is forward biased and conductive during the first half cycle of the capacitor discharge current.

5. A spark source according to claim 1, in which said discharge circuit includes first and second inductive elements connected in series with said spark gap, and a shunting diode circuit connected across the series combination of said second inductive element and said spark gap, said shunting diode circuit including a shunting diode and a third inductive element connected in series with said shunting diode.

6. A spark source according to claim 5, in which said shunting diode is polarized the same as said electronic switching tube and thus is forward biased and conductive during the first half cycle of the capacitor discharge current.

7. A spark source according to claim 5, in which said shunting diode is reversely polarized relative to the polarization of said electronic switching tube and thus is reverse biased and nonconductive during the first half cycle of the capacitor discharge current.

8. A spark source according to claim 5, in which said shunting giode is polarized the same as said electronic switching tube and thus is forward biased and conductive during the first half cycle of the capacitor discharge current, said third inductive element being adjustable for controlling the waveform of the spark current across said spark gap.

9. A spark source according to claim 5, in which said shunting diode is reversely polarized relative to the polarization of said electronic switching tube so that said diode is reverse biased and nonconductive during the first half cycle of the capacitor discharge current, said third inductive element being adjustable for controlling the waveform of the spark current across said spark gap.

10. A spark source according to claim 1, including diode rectifier means connected between the anode and cathode of said electronic switching tube, said diode rectifier means being polarized oppositely with respect to the polarization of said electronic switching tube.

11. A spark source according to claim 1, in which said electronic switching tube is of the gaseous type containing an ionizable gas or vapor.

12. A spark source according to claim 1, in which said electronic switching tube is of the gaseous type containing hydrogen.

13. A spark source, comprising a spark gap including a grounded electrode and an ungrounded electrode, a capacitor having positive and negative terminals adapted to be positively and negatively charged, a power supply for charging said capacitor and having positive and negative terminals, means connecting said positive and negative terminals of said power supply to said positive and negative terminals of said capacitor, a discharge circuit connected between said negative terminal of said capacitor and said ungrounded electrode of said spark gap, an electronic switching tube having an anode, a thermionic cathode and a control electrode, means connecting said anode to said positive terminal of said capacitor, means connecting said cathode to said grounded electrode of said spark gap, a heating circuit for supplying heating power to said thermionic cathode, and means for supplying positive control pulses between said control electrode and said cathode of said electronic switching tube, said pulses being of sufficient magnitude to trigger said tube into a conductive state so as to cause the discharge of said capacitor across said spark gap, sparks thereby being produced across said gap under the precise timing control of said pulses.