US3653766A

US3653766A - Current-injection spark source for emission spectroscopy

Info

Publication number: US3653766A
Application number: US8462A
Authority: US
Inventors: John P Walters; Thomas V Bruhns
Original assignee: Wisconsin Alumni Research Foundation
Current assignee: Wisconsin Alumni Research Foundation
Priority date: 1970-02-04
Filing date: 1970-02-04
Publication date: 1972-04-04
Anticipated expiration: 1989-04-04
Also published as: CA928772A

Abstract

An analytical spark gap is provided between the conductors of a resonant line at one end thereof. The other end of the line is connected to a radio frequency power source which can be electronically pulsed. The conductors of the line are preferably in the form of inner and outer coaxial cylinders. The spark gap is preferably between a first electrode at one end of the inner conductor and a second axial electrode connected by means of an end wall to the outer electrode. A current injection input lead is connected to an intermediate point along the line, so as to provide an effective length of one-quarter wavelength between the injection point and the spark gap. In this way, a node is produced at the injection point. Around the spark gap, the outer conductor forms a cylindrical chamber, in which acoustical resonances are set up, with resulting stabilization of the spark in that the spark is held stationary in an axial position. Optical resonance can also be produced within the chamber by providing a cylindrical mirror on the inside of the outer cylindrical conductor around the spark gap.

Description

United States Patent Walters et al.

[ 51 Apr. 4, 1972 [54] CURRENT-INJECTION SPARK SOURCE FOR EMISSION SPECTROSCOPY [72] Inventors: John P. Walters, Madison, Wis.; Thomas V. Bruhns, Lacey, Wash.

Wisconsin Alumni Research Foundation, Madison, Wis.

22 Filed: Feb. 4, 1970 21 Appl.No.: 8,462

[73] Assignee:

[52] U.S. Cl. ..356/86, 313/231, 313/325,

2,929,953 3/1960 Mitteldorf et al. ...356/86 UX 3,513,516 5/1970 Oddo et al. ..3l3/325 X OTHER PUBLICATIONS Walters, Analytical Chemistry, Vol. 40, No. 11, September 1968, pages 1672- 1681 Primary Examiner-Ronald L. Wibert Assistant Examiner-F. L. Evans AttorneyBurmeister, Palmatier & Hamby [5 7] ABSTRACT An analytical spark gap is provided between the conductors of a resonant line at one end thereof. The other end of the line is connected to a radio frequency power source which can be electronically pulsed. The conductors of the line are preferably in the form of inner and outer coaxial cylinders. The spark gap is preferably between a first electrode at one end of the inner conductor and a second axial electrode connected by means of an end wall to the outer electrode. A current injection input lead is connected to an intermediate point along the line, so as to provide an effective length of onequarter wavelength between the injection point and the spark gap. In this way, a node is produced at the injection point. Around the spark gap, the outer conductor forms a cylindrical chamber, in which acoustical resonances are set up, with resulting stabilization of the spark in that the spark is held stationary in an axial position. Optical resonance can also be produced within the chamber by providing a cylindrical mirror on the inside of the outer cylindrical conductor around the spark gap.

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CURRENT-INJECTION SPARK SOURCE FOR EMISSION SPECTROSCOPY The invention described herein was made in the course of or under a grant from the National Science Foundation, an agency of the United States Government.

This invention relates to the production of spark discharges for analytical and other purposes in connection with optical emission spectroscopy.

One object of the present invention is to provide new and improved apparatus whereby spark discharges are initiated by high voltages at radio frequencies, but the major spark currents are injected into the spark gap from an auxiliary source.

A further object is to provide new and improved apparatus whereby the radio frequency source and the injection current source are effectively isolated from each other, despite the connection of both to the spark gap.

Another object is to provide such a new and improved apparatus in which the radio frequency source can be pulsed electronically and at a lower power level.

It is a further object to provide such a new and improved apparatus in which the high radio frequency voltage is produced by a radio frequency source working into a resonant output line.

Another object is to providenew and improved apparatus whereby an atmospheric spark discharge is stabilized so that it is held stationary on the spark gap electrodes.

Accordingly, the present invention preferably comprisesan axial spark gap in a coaxial configuration, whereby the spark gap is disposed axially in a cylindrical chamber. The spark is stabilized. and held stationary by acoustical resonances in the chamber, due to acoustical reflections from the cylindrical wall of the chamber.

The spark is preferably initiated by a radio frequency source, connected to one end of a resonant line, the spark gap being connected to the opposite end. The line conductors are preferably in the form of coaxial cylinders. One of the axial spark gap electrodes is connected to the inner conductor of the line, while the other electrode is preferably connected to an end well, and then to the outer cylindrical conductor.

A current injection lead is connected to the axial line conductor at an intermediate point along the line. Preferably, the electrical length of the line, between the injection point and the spark gap, is substantially one-fourth wavelength. In this way, a voltage node or minimum is produced at the injection point, so that the injection current source is effectively isolated from the radio frequency source.

The radio frequency source is arranged so that it can be pulsed electronically, with pulses of low power. In this way, sparks can readily be produced with a high repetition rate. Because of the low power requirements, the triggering pulses can be derived directly from a computer.

The present invention has the advantage of making it possible to time the breakdown of the spark gap with a high degree of precision. Such precise timing makes it easy to synchronize the spark with other devices which must be properly phased with respect to the spark. Moreover, the accurate spark ignition, possible with the spark source of the present invention, may be useful in studying the mechanism by which a spark gap is broken down.

The present invention makes it easy to deliver a wide variety of current pulses to the formed spark. The current injection system affords low impedance to current delivery. Moreover, the voltage level at the injection point is low. A wide variety of current sources can be connected to the injection point. Low voltage components can be employed in the current injection devices. The inductance between the injection current source and the spark gap is low. It is not necessary to use high inductance filters between the injection current source and the spark gap.

The cylindrical outer conductor of the coaxial line provides complete electrical shielding around the spark gap. Accordingly, the spark gap is not appreciably affected by external electric or magnetic fields. Moreover, the radiation of radio frequency energy from the spark gap is minimized.

The cylindrical outer conductor of the resonant line produces acoustical resonances around the spark gap, which has the effect of stabilizing the spark so that it stands still, particularly when the spark repetition rate is fairly high. The stabilization of the spark makes it possible to achieve optical resonance in the cylindrical chamber around the spark gap. In this way, it becomes possible to generate laser beams emanating from the spark.

The system operates at atmospheric pressure, yet produces a stable, unwavering spark across the spark gap. The acoustical resonance, whereby the spark is stabilized, involves the action of the shock waves from the spark. If desired, the system can be employed as a source of such shock waves.

The system of the present invention is especially useful for making electrochemical analyses, in which the materials to be analyzed are vaporized by the action of the spark. The light emitted by the spark can be used for optical emission spectroscopy. In addition, the spark can be used as a vapor or ion source in connection with mass spectroscopy.

Solid materials to be analyzed can be placed on the spark gap electrodes. Gases to be analyzed can be introduced into the spark through one or more openings in one of the electrodes.

The system of the present invention is also useful for the controlled erosion of various materials by the action of the spark. The spark repetition rate can be high, yet extremely high currents can be injected from a low voltage source.

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 block diagram of a spark source to be described as an illustrative embodiment of the present invention.

FIG. 2 is a longitudinal section of the quarter-wave line and associated components, employed in the spark source of FIG. 1.

FIG. 3 is an enlarged elevational section, showing the spark gap electrodes and the associated adjusting mechanism.

FIG. 4 is a fragmentary enlarged section of one of the spark gap electrodes.

FIGS. 5a and 5b are approximate equivalent circuit diagrams of the quarter-wave line and associated components.

FIG. 6 is a circuit diagram of the output stage of the radio frequency source.

FIG. 7 is a circuit diagram of the pulsed driver stage of the radio frequency source.

FIG. 8 is an oscillogram of the radio frequency voltage as the spark gap is broken down.

FIG. 9 is an oscillogram of the injection current across the spark gap with a fully oscillatory discharge.

FIG. 10 is an oscillogram representing an enlargement of the initial portion of the discharge shown in FIG. 9.

FIG. 11 is an oscillogram of the injection current due to a unidirectional discharge pulse.

FIG. 12 is an oscillogram of the injection current for a half sinusoidal discharge current.

FIG. 13 is another oscillogram of discharge current representing a unidirectional pulse from a low impedance line with distributed parameters.

FIG. 14 is an oscillogram representing the parallel combination of the systems which produced the pulses of FIGS. 12 and 13.

FIG. 15 is an oscillogram of a low current, non-reversing pulse.

FIG. 16 is an oscillogram of a medium current, reversing pulse.

FIG. 17 is an oscillogram representing an enlargement of the leading portion of the reversing pulse of FIG. 16.

It will be seen that FIGS. 1 and 2 illustrate a system 20 for producing repetitive sparks. Thus, a spark gap 22 is provided between upper and

lower electrodes

24 and 26. In this case, the upper electrode 24 is in the fonn of a rod with a flat lower end 28. However, the upper electrode may assume a variety of other forms, such as a flat disc, for example. The lower electrode 26 is illustrated as being sharply pointed but here again the form of electrode can be varied.

Either or both of the electrodes can be made of a material to be analyzed. Alternatively, a quantity of such material can be placed on one or both of the electrodes.

FIG. 4 illustrates means for introducing gases into the spark. It will be seen that the lower electrode 26 is formed with an axial passage 30. The electrode 26 is cylindrical in form with a tapered upper end portion 31. In this case, the electrode 26 is mounted for sliding adjustment within a sleeve 32. The gas to be analyzed is introduced into the sleeve 32 through a side tube 34. The gas passes upwardly through the axial opening 30 and around the outside of a needle or pin 36 mounted in the opening 30. The pin 36 is smaller in diameter than the opening 30. As shown, the pin 36 is secured within the opening by means of a plurality of set screws 38. The pin 36 has a sharp point 40 at its upper end to facilitate the ignition of the spark.

In order to provide for ignition of the spark by radio frequency voltages, the spark gap 22 is effectively connected between the conductors of a radio frequency transmission line 42. The spark ignition voltage is built up by producing standing waves along the line. While the line.42 may be of various forms, it is shown as being of the coaxial type, having inner and outer

cylindrical conductors

44 and 46. It will be seen that the inner conductor 44 is in the form of a rod or tube of relatively small diameter, while the outer conductor 46 cpmprises a tube or cylinder of large diameter. The inner conductor 44 is made as small as possible, consistent with the ability to carry substantial currents, so that the characteristic impedance of the line will be high. In this way, less energy will be required to build up standing waves of sufficient voltage to break down the spark gap. To provide for adjustment of the line 42, the inner conductor 44 may comprise telescopically joined sections 44a and b. In the illustrated construction, the upper section 44a is slidably received within the lower end of the sleeve 32. Support for the sleeve 32 is provided by a disc or partition 48, preferably made of an insulating material with a very low loss factor. The partition 48 is mounted within the outer cylindrical conductor 46. The sleeve 32 is slidable through an axial opening 50 in the partition 48.

It will be understood that the

line conductors

44 and 46 are made of copper or other highly conductive materials. Support for the upper electrode 24 is provided by an end wall or disc 52, closing the upper end of the outer cylindrical electrode 46. The end wall 52 is also made of a highly conductive material and is preferablymade so that it can be removed from the outer cylinder 46. The electroderod 24 is connected electrically to the end wall 52, which in turn is connected to the outer cylindrical conductor 46.

To provide for precise adjustment of the electrode 24, it is connected to a micrometer screw 54 having its counterpart threaded element 56 mounted on the end wall 52 by means of a mounting plate 58. The electrode 24 can be advanced or retracted by rotating the screw 54 in opposite directions.

Additional support for the lower member 44b of the axial conductor 44 is preferably provided by another partition 60, similar to the partition 48. The partition 60 is made of insulating material and is mounted within the outer cylindrical conductor 46. The member 44b extends through an axial opening 62 in the partition 60.

It will be recalled that the spark gap 22 is connected to one end of the resonant transmission line 42. The other end of the line is connected to a radio frequency source 64, indicated diagrammatically in FIG. 2. The source 64 may include an output anode terminal 66, adapted to supply a radio frequency voltage to the axial conductor 44. It is preferred to connect one or more blocking capacitors 68 between the anode terminal and the axial conductor 44 so that the direct voltage on the anode terminal 66 will not be applied to the axial conductor 44. Three such capacitors 68, connected in parallel, are shown in FIG. 2. The capacitors 68 are connected between the anode terminal 66 and a disc or plate 70, welded or otherwise secured to the lower end of the axial conductor 44.

A current injection lead 72 is connected to an intermediate point along the inner conductor 44 of the coaxial transmission line 42. The lead 72 extends radially out of the outer cylindrical conductor 46 through an insulator 74.

The effective length of the line between the spark gap 22 and the injection lead 72 is one-fourth of a wavelength. Due to the reflection of traveling waves along the line from the spark gap, a standing wave exists along the transmission line 42, with a node at the injection point, where the injection lead 72 is connected to the axial conductor 44. Thus, the radio frequency voltage is at a minimum and is very low at the injection point. Accordingly, the injection lead 72 is effectively isolated from the radio frequency source 64.

The length of the coaxial transmission line 42 between the anode terminal 66 and the spark gap 22 is illustrated as somewhat greater than one-quarter wavelength. Thus, the line 42 presents an inductive impedance at the anode terminal 66.

One or more tuning devices are preferably provided to tune the coaxial line 42 to a resonant condition. As illustrated, this tuning adjustment is provided by a variable capacitor 76, connected between the inner and

outer conductors

44 and 46, near the radio frequency source 64. In this case, the variable capacitor comprises a stationary plate 78, connected to the axial conductor 44, and a movable plate-80, mounted on the outer cylindrical conductor 46. Support for the plate is provided by an adjusting screw 82, threaded through a sleeve 84 on the outer cylindrical conductor 46. The screw 82 may be provided with a manually rotatable adjusting knob 86.

The telescopically adjustable sections 440 and b of the axial conductor 44 make is possible to adjust the effective length of the coaxial transmission line 42. This adjustment affects the location of the node, and. also the resonant input impedance of the line 42. I

Additional details of the apparatus will be evident from FIG. 1, which is a block diagram of the system. In this case, the radio frequency source 64 provides an output to the resonant line 42 at 162 MHz. However, the particular frequency is merely a matter of convenience and can be varied over a wide range. Moreover, the details of the radio frequency source 64 are subject to wide variations.

In the illustrated system, to be described by way of example, the output at 162 MHz. is derived by starting with a relatively low frequency signal and utilizing a series of frequency multipliers. Thus, as shown in FIG. 1, the system comprises a master crystal oscillator which provides an output at 6.75 MHz. This signal is amplified by a tuned amplifier 92 operating at 6.75 MHz. The amplifier 92 drives a frequency doubling amplifier 94 which delivers an output at 13.5 MHz. This output drives another frequency doubling amplifier 96, providing an output at 27 MHz.

The signal at 27 MHz. drives still another frequency doubling amplifier 98 providing an output at 54 MHz. This output is fed to a frequency tripling amplifier 100, developing an output at 162 MHz.

The 162 MHz. signal drives a push-pull gated power amplifier 102 which is adapted to be pulsed, as will be described in detail presently. The-pulsed output drives the final power amplifier 104 having its output connected to the anode terminal 66, shown in FIG. 2. Thus, the power amplifier 104 supplies its pulsed output to the quarter-wave resonant line 42, which develops a high radio frequency voltage across spark gap 22.

The arrangement for pulsing the gated power amplifier 102 is also subject to wide variations. As shown, the gated power amplifier 102 is controlled by a pulser 106 utilizing an electronic switching element, such as a silicon controlled rectifier, as will be described in detail presently. The pulser 106 is driven by a trigger generator 108, which is adapted to respond to an external source 110 of triggering signals. Such external source may take the form of a computer, inasmuch as the required signal level is very low.

As previously indicated, an injection current source 112 is connected to the resonant line 42 by means of the input lead 72. The source 112 is energized by a power supply 114. The injection current source 112 may assume a wide variety of forms. For example, it may comprise a capacitor which is f charged by the power supply 114 and is discharged into the resonant line 42 and thence across the spark gap 22. Various elements may be connected to the capacitor to change the wave form of the discharge current.

It will be recalled that the current injection lead 72 is connected to the resonant line 42 at a point where the radio frequency voltage is at a minimum. Thus, only a minimum of filtering is required to isolate the injection current source 112 from the radio frequency source 64. As shown in FIG. 6, such filtering is provided by

small inductances

116, 118 and 120, together with a small bypass capacitor 122. The inductance 116 represents the distributed inductance of the current injection lead 72. Each of the

inductances

118 and 120 may com-' prise a coil having one turn or at most a few turns of wire. The capacitor 122 is connected to ground from the junction 124 between the

inductances

118 and 120. The injection current from the source 112 may be delivered to a jack or connector 126, as shown in FIG. 6.

The details of the final power amplifier 104 are shown in FIG. 6, but it will be understood that such details are subject to wide variation. As shown, the power amplifier 104 utilizes a beam power tube 130 having its anode connected to the anode terminal 66, and thence through the blocking capacitors 68 to the axial conductor 44 of the resonant line 42. The energizing voltage may be supplied to the anode by a lead 132 running along the axial conductor 44 to a point adjacent the current injection lead 72. The lead 132 is then connected through

small inductances

134 and 136 to the high voltage anode supply lead 138. The

inductances

134 and 136 have a filtering action which is aided by bypass capacitors 138 and 140, connected between ground and the opposite ends and the inductance 136.

The pulsed input power at 162 MHz. is supplied to the grid of the tube 130 by a transformer 144 having primary and

secondary windings

146 and 148. A variable tuning capacitor 150 is connected in series with the primary winding 146. The secondary winding 148 has a tap 152 which is connected to a bias supply lead 154 through filtering

inductances

156 and 158.

Bypass capacitors

160, 162 and 164 are connected to the opposite ends of the inductance 158.

One end of the secondary winding 148 is connected to the control grid of the tube 130. The other end is connected to a neutralizing probe 166 extending into the outer cylindrical conductor 46, to a point near the anode terminal 66. A variable neutralizing capacitor 168 is connected to ground from the last mentioned end of the secondary winding 148.

The screen grid of the tube 130 is connected to a power supply lead 170 through filtering

inductances

172 and 174, aided by

bypass capacitors

176, 178, 180 and 182.

The cathode and one side of the filament of the tube 130 are grounded. The other side of the filament is connected to a power supply lead 184 through filtering

inductances

186 and 188, aided by bypass capacitors 190, 192 and 194.

FIG. 7 shows details of the push-pull gated power amplifier 102 and the SCR pulser 106. It will be understood that the details of these circuits are subject to wide variations. The illustrated amplifier 102 utilizes a dual beam power tube 200 having its control grids connected to the center tapped secondary winding 202 of an input transformer 204, which also has a center tapped primary winding 206. It will be understood that input power at 162 MHz. is supplied to the primary winding 206. The center tap 208 of the secondary winding 202 is connected to ground through biasing

resistors

210 and 212. A bypass capacitor 214 is connected across the resistor 212.

The anodes of the dual tube 200 are connected to the center tapped primary winding 216 of an output transformer 218, which also has a secondary winding 220. It will be understood that the output leads 222 and 224 are adapted to be connected to the input of the final power amplifier 104. A tuning capacitor 226 is connected in series with the secondary winding 220.

The primary winding 216 is tuned by dual variable capacitors 230, connected between the anodes and ground. The cathodes of the dual tube 200 are also grounded.

The anodes and the screen grids of the dual tube 200 are adapted to be pulsed by the pulsing circuit 106. It will be seen that the pulsing circuit 106 utilizes a silicon controlled rectifier (SCR) 234. The positive power supply lead 236 is connected to the anode of the SCR 234 through an inductor 238 and a resistor 240.

Bypass capacitors

242, 244 and 246 are connected between the power supply lead 236 and ground. A discharge capacitor 248 is connected between the anode of the SCR 234 and ground.

The cathode of the SCR 234 is connected to the center tap of the coil 216 through a radio frequency choke coil 250. The screen grids of the dual tube 200 are connected to the cathode of the SCR 234 through

parallel resistors

252 and 254, bypassed by a capacitor 256. A radio frequency bypass capacitor 258 is connected between the screen grids and ground.

When the SCR 234 is pulsed into conductivity, the anodes and screen grids of the dual tube 200 are energized momentarily by the discharge of the capacitor 248, following which the SCR becomes non-conductive, because the current through the resistor 240 is insufficient to maintain conduction in the SCR. The capacitor 248 is then recharged through the resistor 240.

The control circuit for the SCR 234 comprises a transformer 260 having primary and

secondary windings

262 and 264. The primary winding 262 is adapted to receive triggering signals from input leads 266 and 268. The secondary winding 264 is connected between the gate and the cathode of the SCR 234. A damping resistor 270 is connected across the secondary winding 264.

During normal operation, the SCR 234 is pulsed by the triggering signals at the input leads 266 and 268. However, when desired, the dual tube 200 can be energized continuously. This is accomplished by means of a switch 274 comprising a contactor 276 which is movable between

contacts

278 and 280. The contact 278 is grounded, while the contact 280 is connected to the positive power supply lead 236. The contactor 276 is connected to the cathode of the SCR 234 through a resistor 282, across which another resistor 284 and an inductance coil 286 are connected in series. A diode 288 is connected across the resistor 284. Bypass capacitors 290, 292, and 294 are connected between contactor 276 and ground.

For pulsed operation, the contactor 276 engages the grounded contact 278. The diode 288 is non-conductive and the network comprising the resistor 282, the resistor 284 and the coil 286 act as a load for the SCR 234. When continuous operation is desired, the contactor 276 is moved against the contact 280. The diode 288 and the coil 286 provide a low impedance bypassing circuit around the SCR 234.

The SCR pulsing system makes it possible to achieve very high spark repetition rates such as 5,000 Hz. or even higher. Only a low level of power is required for the triggering of the SCR 234. Thus, the pulsing can readily be computer controlled. Moreover, the pulsing can be synchronized with any other desired signal.

FIGS. 5a and 5b are approximate equivalent circuit diagrams of the resonant transmission line 42 and the associated components. Each circuit diagram comprises a resistance 298 which represents the internal impedance of the radio frequency source 64. The combination of the spark gap 22 and the quarter-wave resonant line 42 produces an automatic switching operation to change the impedance seen by the radio frequency source 64. In the diagram of FIG. 5a the switching action of the spark gap is represented by an equivalent switch 300 connected to the output end of the quarter-wave resonant line 42. In FIG. 5b, an equivalent switch 302 represents the automatic switching action of the spark gap, in combination with the quarter-wave line 42. Initially, before the spark is ignited, the spark gap 22 presents an open circuit, so that its impedance is virtually infinite. This condition is represented by the left-hand position of the switch 300, in which the open spark gap 22 is connected to the output end of the quarter-wave resonant line 42.

Due to the action of the resonant line 42, the impedance at the input end of the line is very low, represented by a low value equivalent resistor 304, which is connected to the line 42 by the switch 302, in its left-hand position. The value of this equivalent resistor 304 may be only a small fraction of 1 ohm, for example.

The quarter-wave resonance exists in the line 42 between the current injection point and the spark gap 22. This is the output portion of the line. The input portion, between the radio frequency generator 64 and the current injector point, is tuned to a parallel resonant condition by the variable capacitor 76. The reactance of the input portion of the line is represented by an inductance 306 in FIG. a. Due to the parallel resonant condition, the radio frequency generator 64 is presented with high impedance, so that it develops a high voltage.

When the spark is ignited, the spark gap presents a low impedance, represented by an equivalent resistance 308 in FIG. 5a. The value of this resistance may be only a few ohms, for example. The equivalent switch 300 is shifted to its right-hand position in which the low resistance 308 is connected to the output end of the resonant line 42.

Due to this change in output impedance, the input of the quarter-wave resonant line 42 appears as a fairly high equivalent resistance 310, connected into the circuit in the right-hand position of the switch 302 in FIG. 5b. When this high impedance is presented to the parallel resonant circuit, the input impedance is greatly reduced. The automatic switching action effectively decouples the radio frequency source 64 from the quarter-wave resonant line 42, so that the transfer of power to the line is greatly reduced.

Prior to spark ignition, the impedance presented at the current injection lead 72 is very low, so that virtually any current source can be connected to this point. When the spark is ignited, the current source dumps its current into the spark discharge.

The operation of the spark source is illustrated by the oscillograms of FIGS. 8-16, representing the voltage across the spark gap for various conditions. FIG. 8 shows an oscillogramv 320 of the 162 MHz. voltage as it builds up to ignite the spark gap. The point of ignition is approximately at 322, after which the magnitude of the radio frequency voltage decreases rapidly to a low level, due to the low impedance of the ionized spark gap. Due to the extremely high frequency of the radio frequency voltage, the spark discharge is initiated with a high degree of precision. Thus,'the ignition of the spark gap can readily be timed to coincide with other functions, such as the dumping of the injection current into the spark discharge.

FIG. 9 illustrates an oscillogram 324 representing a fully oscillatory discharge current across the spark gap. This wave form is produced by discharging a capacitor through the spark gap, with a low damping factor.

FIG. 10 shows an oscillogram 326, corresponding to the initial portion of the oscillogram 324, but with a much faster sweep, so as to magnify the oscillogram. It will be seen that the oscillogram 326 reveals stepped transients 328.

FIG. 11 illustrates an oscillogram 330 representing a unidirectional pulse of current, dumped into the spark discharge. Such a pulse can readily be produced by discharging a capacitor into the spark gap, with sufficient resistive damping to make the discharge unidirectional. In the situation represented by FIG. 11, a length of coaxial cable was connected across the current injection input, to produce reflections, represented by the successive saw-tooth peaks 332 of the oscillogram 330.

FIG. 12 illustrates an oscillogram 334 representing the dumping of a half sinusoid of current across the spark gap after the ignition of the spark. Such a current can readily be developed by a pulsed rectifier, deriving its input from an alternating current source.

FIG. 13 shows an oscillogram 336 representing a capacitor discharge through a low impedance line having distributed parameters.

FIG. 14 shows an oscillogram 338, representing the combined distributions of FIGS. 12 and 13. It will be evident that a wide variety of discharge wave forms can be produced by combining various discharges.

Another wave form of this type is represented by the oscillogram 340 of FIG. 15. This is a unidirectional wave form.

A reversing pulse wave form is represented by the oscillogram 342 of FIG. 16.

FIG. 17 shows an oscillogram 344 corresponding to the initial portion of the oscillogram 342 but with a much faster sweep, to produce magnification. The oscillogram 344 reveals stepped transients representing the precise ignition of the spark by the radio frequency voltage.

In summary, the present invention utilizes a radio frequency voltage, fed through a resonant transmission line, to ignite or form the spark. The line has an output section, tuned as a quarter-wavelength resonant line, and an input section, tuned to parallel resonance. The quarter-wavelength resonant line section develops the high voltage necessary to break down the spark gap.

Sustaining current is injected into the system at the nodal point between the parallel resonant section and the quarterwavelength resonant-section. This is a point of minimum radio frequency voltage so that very little reactive isolation is necessary between the radio frequency system and the injection current source.

Injection current pulses having a wide variety of waveforms can easily be delivered to the formed spark by way of the injection point. The system affords low impedance to. the delivery of such current pulses. Thus, a high current can be delivered at a low voltage. The current injection circuit can utilize low voltage components.

The ignition or formation of the spark by the high radio frequency voltage makes it possible to ignite the spark with a high degree of precision. Thus, thespark can be precisely synchronized with other apparatus, or a wide variety of signalsv and events.

The radio frequency voltage can be pulsed in response to signals at a low power level. Thus, computer control of the pulsing is readily possible. High spark repetition rates can readily be achieved. Moreover, the repetition rate can be varied continuously over a wide range, simply by'changing the frequency of the triggering signal.

The coaxial resonant line provides complete shielding around the spark gap, so that the spark discharge is not subject to outside radio frequency interference. Moreover, the radiation ofradio frequency energy is minimized, so as to prevent interference with external radio equipment.

Due to the utilization of the coaxial resonant line the spark gap is axially disposed in a cylindrical enclosure. It has been found that this configuration results in stabilization of the spark so that it stands still on the spark gap electrodes, particularly at high repetition rates. It is believed that this stabilization of the spark is due to acoustical resonance in the cylindrical chamber around the spark gap. The shock waves from the spark are reflected within the cylindrical enclosure so as to stabilize the spark.

The stabilization of the spark makes it possible to achieve optical resonance within the cylindrical enclosure, so that laser beams can be produced. Such optical resonance requires the provision of a cylindrical mirror or reflecting surface within the enclosure.

The primary utility of the spark system is to provide for the emission of light from the spark discharge, for use in optical emission spectroscopy. The materials to be analyzed can be placed on the spark gap electrodes, or introduced as gases into the spark discharge.

The spark system can also be used as a vapor or ion source for mass spectroscopy. For such use, the vapors or ions produced by the spark discharge are introduced into a mass spectrometer.

Due to the stabilization of the spark, the system can be used for the controlled erosion of materials placed on the spark gap electrodes. The system is also useful as a source of shock waves, which are produced by the spark discharge.

Various other advantages, modifications and equivalents will be evident to those skilled in the art.

The values of electrical components are not critical and can be assigned by those skilled in the art. However, it may be helpful to list suitable values of various components, as follows:

RESISTORS Ohms 210 lSK 212 114 240 120K 252 56K 254 56K 270 270 282 1K CAPACITORS 68 3x100 picofarads (pf.) 76 0.140 r. 122 I pf. 138 0.005 microfarad (mf) 140 0.005 mi. 150 -40 pf. 168 3-20 pf. 160 180 pf. 162 330 pf. 164 330 r. 176 1,500 pf. 178 180 pf. 180 330 pf. 182 330 pf. 190 180 pf. 192 330 pf. 194 330 pf, 214 330 pf. 226 2.3l4.2 pf. 230 l.8-5.l pf. per section 242 330 pf. 244 0.0] mf. 246 330 pf.

248 0.0l mf.

256 0.01 mf. 25a 56 pf. :90 0.01 ml 292 0.01 mf. 294 330 mi.

We claim:

1. A spark source, comprising a resonant line having a pair of conductors,

a spark gap connected across said conductors at one end of said line,

a radio frequency power source connected across said conductors at the opposite end of said line,

means for connecting a discharge current source across said conductors at an intermediate injection point along said line,

and means for producing a radio frequency node at said injection point to obviate interaction between said radio frequency power source and said discharge current source.

2. A spark source according to claim 1,

in which said line is coaxial,

one of said conductors being generally in the form of a cylinder while the other conductor is disposed axially within said cylinder.

3. A spark source according to claim 1,

in which said line has an electrical length of approximately one-fourth wave length between said spark gap and said injection point.

4. A spark source according to claim 3,

in which said line is coaxial,

one of said conductors being in the form of a cylinder while the other conductor is axially disposed therein.

5. A spark source according to claim 1, including a tuning device connected to said line.

6. A spark source according to claim 5,

in which said tuning device comprises a variable capacitor connected between said conductors of said line.

7. A spark source according to claim 6,

in which said variable capacitor is connected to said line adjacent said radio frequency power source.

8. A spark source according to claim 7,

in which said conductors of said line are substantially coaxi- 9. A spark source according to claim 1,

in which said conductors of said line are coaxial,

one of said conductors being generally in the form of a cylinder while the other conductor is axially disposed therein,

said spark gap comprising a first axial electrode connected to the end of said axial conductor,

and a second electrode disposed axially in said cylinder and spaced axially away from said first electrode,

said second electrode being connected to said cylinder.

10. A spark source according to claim 9,

in which said cylinder fonns a chamber around said spark gap and is acoustically resonant for stabilizing the spark discharge across said gap.

1 l. A spark source according to claim 10,

including an end wall closing said cylinder at the end thereof adjacent said spark gap.

12. A spark source according to claim 11,

in which said second electrode is mounted upon said end wall and projects therefrom into said cylinder,

13. A spark source according to claim 12,

including means for adjusting said second electrode toward and away from said first electrode.

' 14. A spark source according to claim 1,

including means for adjusting the length of said spark gap.

15. A spark source according to claim 9,

in which one of said electrodes is formed with a longitudinal passage for injecting materials into the spark gap.

16. A spark source according to claim 9,

in which said first electrode comprises an axial passage for injecting material into the spark gap 17 A spark source according to claim 16,

including a pointed electrode pin mounted axially in said passage,

said pin being smaller than said passage to provide for the flow of materials through said passage and along said pin.

18. A spark source,

comprising a hollow member having an internal cylindrical wall forming a cylindrical chamber in said member,

a first spark gap electrode disposed axially within said chamber,

a second spark gap electrode disposed axially in said chamber and spaced axially from said first electrode to form an axial spark gap therebetween,

a high voltage source connected to said first and second spark gap electrodes for producing sparks between said electrodes and across said gap,

each spark producing an acoustical shock wave which travels outwardly to said cylindrical wall and is reflected inwardly by said wall,

and pulsing means for pulsing said high voltage source to produce repetitive sparks between said electrodes at a repetition rate related to the internal size of said chamber such as to produce acoustical resonance in said chamber in response to the repetitive shock waves,

the repetitive sparks being stabilized by said acoustical resonance.

19. A spark source according to claim 18,

including an end wall closing one end of said cylindrical chamber,

said second electrode being mounted upon said end wall.

20. A spark source according to claim 18,

including means for axially adjusting one of said electrodes to change the length of said spark gap between said electrodes.

21. A spark source according to claim 18,

including an end wall across one end of said cylindrical chamber,

said second electrode being mounted upon said end wall,

and another end wall extending across said cylindrical chamber and spaced axially from said end wall,

said spark gap between said electrodes being disposed in said chamber and between said end walls.

22. A spark source according to claim 18,

said cylindrical wall being conductive and being connected to said second electrode,

said high voltage source being connected between said first electrode and said cylindrical wall,

the dimensions of said cylindrical wall and said first electrode being such as to produce electromagnetic resonance in said cylindrical chamber.

Claims

1. A spark source, comprising a resonant line having a pair of conductors, a spark gap connected across said conductors at one end of said line, a radio frequency power source connected across said conductors at the opposite end of said line, means for connecting a discharge current source across said conductors at an intermediate injection point along said line, and means for producing a radio frequency node at said injection point to obviate interaction between said radio frequency power source and said discharge current source.

2. A spark source according to claim 1, in which said line is coaxial, one of said conductors being generally in the form of a cylinder while the other conductor is disposed axially within said cylinder.

3. A spark source according to claim 1, in which said line has an electrical length of approximately one-fourth wave length between said spark gap and said injection point.

4. A spark source according to claim 3, in which said line is coaxial, one of said conductors being in the form of a cylinder while the other conductor is axially disposed therein.

6. A spark source according to claim 5, in which said tuning device comprises a variable capacitor connected between said conductors of said line.

7. A spark source according to claim 6, in which said variable capacitor is connected to said line adjacent said radio frequency power source.

8. A spark source according to claim 7, in which said conductors of said line are substantially coaxial.

9. A spark source according to claim 1, in which said conductors of said line are coaxial, one of said conductors being generally in the form of a cylinder while the other conductor is axially disposed therein, said spark gap comprising a first axial electrode connected to the end of said axial conductor, and a second electrode disposed axially in said cylinder and spaced axially away from said first electrode, said second electrode being connected to said cylinder.

10. A spark source according to claim 9, in which said cylinder forms a chamber around said spark gap and is acoustically resonant for stabilizing the spark discharge across said gap.

11. A spark source according to claim 10, including an end wall closing said cylinder at the end thereof adjacent said spark gap.

12. A spark source according to claim 11, in which said second electrode is mounted upon said end wall and projects therefrom into said cylinder,

13. A spark source according to claim 12, including means for adjusting said second electrode toward and away from said first electrode.

14. A spark source according to claim 1, including means for adjusting the length of said spark gap.

15. A spark source according to claim 9, in which one of said electrodes is formed with a longitudinal passage For injecting materials into the spark gap.

16. A spark source according to claim 9, in which said first electrode comprises an axial passage for injecting material into the spark gap

17. A spark source according to claim 16, including a pointed electrode pin mounted axially in said passage, said pin being smaller than said passage to provide for the flow of materials through said passage and along said pin.

18. A spark source, comprising a hollow member having an internal cylindrical wall forming a cylindrical chamber in said member, a first spark gap electrode disposed axially within said chamber, a second spark gap electrode disposed axially in said chamber and spaced axially from said first electrode to form an axial spark gap therebetween, a high voltage source connected to said first and second spark gap electrodes for producing sparks between said electrodes and across said gap, each spark producing an acoustical shock wave which travels outwardly to said cylindrical wall and is reflected inwardly by said wall, and pulsing means for pulsing said high voltage source to produce repetitive sparks between said electrodes at a repetition rate related to the internal size of said chamber such as to produce acoustical resonance in said chamber in response to the repetitive shock waves, the repetitive sparks being stabilized by said acoustical resonance.

19. A spark source according to claim 18, including an end wall closing one end of said cylindrical chamber, said second electrode being mounted upon said end wall.

20. A spark source according to claim 18, including means for axially adjusting one of said electrodes to change the length of said spark gap between said electrodes.

21. A spark source according to claim 18, including an end wall across one end of said cylindrical chamber, said second electrode being mounted upon said end wall, and another end wall extending across said cylindrical chamber and spaced axially from said end wall, said spark gap between said electrodes being disposed in said chamber and between said end walls.

22. A spark source according to claim 18, in which said high voltage source includes means for producing a high voltage at a high frequency.

23. A spark source according to claim 18, in which said cylindrical wall is conductive and is connected to one of said electrodes.

24. A spark source according to claim 18, in which said high voltage source comprises means for producing a high voltage at a high frequency, said cylindrical wall being conductive and being connected to said second electrode, said high voltage source being connected between said first electrode and said cylindrical wall, the dimensions of said cylindrical wall and said first electrode being such as to produce electromagnetic resonance in said cylindrical chamber.