US2979640A - Spark gap - Google Patents

Description

A ril 11, 1961 R. B. EDMONSON 2,979,640

SPARK GAP Filed July 28, 1958 TO CONTROL ELEMENT TO PRESSURIZE D TO ECHG- TRIGGER PULSES INVENTOR. ROBERT B. EDMONSON ATTORNEY United States Patent Office 2,979,640 Patented Apr. 11, 196i SPARK GAP Robert B. Edmonson, Upland, Calif., assignor to Aerojet- General Corporation, Azusa, Califi, a corporation of Ghio Filed July 28, 1958, Ser. No. 751,289

13 Claims. (Cl. 315-208) This invention relates to an improved spark gap and a control circuit therefor.

Both photomicrography of living specimens and shadowgraph photography of high speed phenomena, such as compressible-flow in wind tunnels and in high speed projectile flight,require as an indispensable element a high intensity light source of extremely short durationand small effective area. The spark gap light source has come to be favored over alternative means as possessing a number of distinct advantages, among them being the extremely short duration and high intensity of the light flash. The spark gap has also come to be the favored ignition means in power plants such as the internal combustion engine of the automobile and airplane, and the rocket engine. But reliable spark gap ignition is difficult to obtain at highaltitudes.

Spark gaps employed have been primarily of the two electrode type where an electric discharge is initiated and maintained between two electrodes by application of a voltage thereto, the value of voltage required being dependent uponthe distance between the electrodes, their geometry, and the pressure and nature of the gas between them. This configuration, however, is not entirely satisfactory as the gap breakdown, hence the ignition of the fuel or light flash, will not occur with the same applied voltage upon each occasion nor the same time in terval after application of the voltage to the electrodes.

To overcome these disadvantages, various types of control means have been employed in conjunction with the spark gap. Among these are thyratron circuits and auxiliary gaps. Both, however, are usually connected in series with the controlled gap and, in addition to other resultant disadvantages, thereby provide positive control of breakdown time and voltage at the expense of circuit energy dissipation in other than the discharge. i

, It is, therefore, the principal object of the present invention to provide a high efficiency controllable spark discharge system wherein practically all f the circuit energy is impressed across the spark gap.

Another object of this invention is to provide a multielectrode spark gap assembly that will reliably discharge at the same time interval after application of "each identical'triggering'voltage.

A further objectof the present invention is toprovide an improved spark gap light source of high intensity and short duration. 7 I

Stillanother object ofthis invention isrto provide a novel meansfor and an improved method of starting a rocket engine without altitudelimitations. v

In its principal aspect, the present invention comprises a novel multi-electrode spark gap' assembly and an improved control circuit for use in conjunction therewith. By spark is "meant that electric dischargewhich takes place between electrodes inre's'ponse to a high voltage applied thereon. Essentially, the gap assembly includes threev electrodes situated within a' common housing charge. A feature of the invention is provision of a hole through the second or trigger electrode to allow propagation of the spark to the third or cathode electrode which also has an opening therethrough to allow exit of spark light and/or spark gas from the assembly. Another feature of the invention is provision of means for maintain ing pressure within the confined spark channel irrespective of the pressure environmental to the housing, aflording use of the gap for rocket engine ignition irrespective of altitude.

The control circuit includes an energy storage capacitor shunted across the gap cathode electrode and anode electrode. A voltage division network, such as a pair of small serially connected capacitors are also connected across the storage capacitor. The trigger electrode of the gap is connected to a common point of connection between the two capacitors, thereby placing the trigger electrode at a potential intermediate that of the gap cathode and anode. A thyratron or other control element is placed across the gap trigger and cathode electrodes. When an actuating pulse is fed its control electrode, the thyratron begins to conduct, thereby placing the gap trigger electrode at the same potential as the gap cathode. Although the potential across the entire gap assembly remains unchanged, the potential gradient between the gap anode and trigger electrodes sharply increases, resulting in a voltage breakdown. Because of the hole through the trigger gap, the breakdown propagates throughout the entire channel, completing the discharge to the cathode electrode. As the cathode electrode also has an opening therethrough, the resulting high pressure gas exits to providea high intensity, high temperature light and ignition source.

These and other objects, aspects, features and advantages of the invention will be apparent to those skilled in the art from the following more detailed description taken in conjunction with the. appended drawings, wherein; 1

Figure l isa central longitudinal vertical section of the triple electrode gap assembly of the present invention; and,

.Figure 2 is a'sche'm atic circuitdiagram of the entire multi-electrode gap assembly and the control circuit therefor. V v

Figure 1 shows in detail the construction of a preferred embodiment of the triple electrode spark gap assembly 10,. the principal components of which are mountedwithin an electrically non-conductingv housing 26. The anode electrode 18 threads within a metallic mounting disc 12 which is recessed within the housing 26. .Electrical connection from the anode electrode 18 to associated circuitry is made by contact with a screw 19' placed through the mounting disc 12 into the housing 26.. A first cylindrical ceramic spacer 24 having a central coaxial cavity separates the anode 18 from the trigger electrode 20,'itself a cylindrical disc of a conducting material with a central, cavity therethrough. A trigger electrode plug 22 is arranged in' the housing 26 in direct physical and electrical contact with the trigger electrode .20to provide easy access for electrical connection to associated circuitryfrom the trigger electrode 29 which is la rranged with the first ceramic spacer 24 within a hol low excavation in said housing 26. A second cylindrical ceramic spacer 28, also having a coaxial central cavity separates the trigger electrode 20 from the cathode electrode 16 which isplaced as an insert within 'a second metallic mounting disc 14 arrfangedvwithin the electrode housing 26. Alternatively, the cathode electrode 16 could be fabricated as an integral part ofthe second disc 14. Electricalconnection of the cathode electrode 16 with associated circuitry is effected by connection to a screw 21 inserted within the second metallic mount 14 through the high value ohmic resistance 50. In the absence of a discharge between the trigger electrode 20 and the cathode 16, this current flow through the limiting resistor 50 provides sufiicient potential drop such that electrode 20 is brought to the same nominal potential as the anode 18. This causes breakdown between the cathode 16 and trigger electrode 20, completing the spark discharge throughout the entire channel. Upon discharge of the capacitor 36, the cycle described begins again and the light source is ready for firing once the storage capacitor 36 is recharged. The repetition rate is determined by the charging rate and the thyratron trigger rate.

By way of example only, a system as shown in Figure 2 was constructed yielding excellent results. High intensity light flashes of a duration less than one microsecond were obtained. The circuit elements of the system were as follows:

These values are by way ofexample only and are not to be considered limiting as other values may be used. Where control elements other than the thyratron are employed, it may be possible to reverse the polarity of the charging voltage.

It is to be understood that the form of the invention herewith shown and described is to be taken as a preferred example of the same and that various changes in the shape, size and arrangement of the parts may be resorted to without departing from the spirit of the invention and of the scope of the appended claims.

What is claimed is:

1. A spark discharge system comprising, a storage capacitor connected to be charged from a high voltage current source, a spark gap having a first, second, and third electrode and having a confined spark channel, a unilateral current conducting device, said capacitor being also connected in parallel with the first and second gap electrodes, and said device being arranged in parallel with the second and third electrodes, whereby the third electrode assumes the electrical potential of the second electrode thereby inducing a voltage breakdown through the entire channel when said device is made conductive.

2. A system for the generation of high intensity, short duration spark discharges comprising, spark gap including in the order named, an anode, trigger and cathode electrodes arranged to form a confined spark channel, said trigger electrode having a passage therethrough, means for placing a difference of potential between said anode and cathode electrodes, and means for placing a potential on said trigger electrode which changes from a value intermediate said anode and cathode electrode potentials to the cathode potential, whereby the potential gradient between said anode and trigger electrodes is sharply increased to cause a spark discharge throughout the entire channel and means for introducing a gas into said confined channel at any desired pressure.

" 3. A system as described in claim 1, and in addition, a sonic flow venturi gas nozzle arranged to exhaust within said confined channel.

4. A system as described in claim 1, and inaddition, means-for maintaining the pressure within said confined channel above a predetermined value.

5. A system as described in claim 1, and in addition, means for maintaining the pressure within said confined channel above 5 to v6 pounds per square inch.

6. -A system as described in claim 2 wherein said cathode electrode has a passageway therethrough.

7. A system for generation of a spark comprising, a gas discharge means having in the order named cathode, trigger and anode electrodes arranged to form a confined channel, an energy storage means connected to elfect a difiference in electrical potential between said cathode and anode electrodes, when said storage means is charged with energy a voltage division means in parallel connection with said storage means, an intermediate point of which is connected to place said trigger electrode at an electrical potential intermediate to that of said anode and cathode electrodes, when said storage means is charged with energy and a pulse actuated unilateral current conducting device arranged between said trigger and cathode electrodes to place said trigger electrode at the potential of said cathode electrode in response to a triggering pulse whereby the potential gradient between said anode and trigger electrodes is sharply increased to cause a gas discharge throughout the entire arc channel.

8. A system as in claim 7 and, in addition, a critical fiow nozzle formed as an integral part of said anode electrode.

9. A high intensity, short duration spark light system comprising, a high potential current source, a voltage pulse source, a storage capacitor having two terminals, a first voltage dividing capacitor, a second voltage dividing capacitor, said first and second voltage dividing capacitors being joined in series, said storage capacitor being arranged in a parallel connection with said serially connected voltage dividing capacitors, said current source being connected to charge said storage capacitor, a spark gap having an anode, cathode and trigger electrode, an electron discharge device having an anode, cathode and control electrodes, a current limiting resistor, said gap trigger electrode being joined in electrical connection with the junction point between said first and second voltage dividing capacitors, said discharge device anode being connected to said gap trigger electrode through said current limiting resistor, said gap anode connecting to one terminal of said storage capacitor, said gap cathode connecting to the second terminal of said storage capacitor, said electron discharge device cathode electrode connecting to said gap cathode electrode, and said voltage pulse source connecting to said electron discharge device control electrode.

10. An assembly for the generation of a spark discharge within a confined channel, comprising, an anode electrode, a trigger electrode having an opening through the center thereof, a cathode electrode having an opening through the center thereof, a first-non-conductive spacer element having a central cylindrical opening, said first spacer element being arranged to separate said anode and trigger electrodes, and a second non-conductive spacer element having a central cylindrical opening, said second spacer element being arranged to separate said trigger and cathode electrodes.

11. An assembly as described in claim 10 and, in addition, a. nozzle through which gas is introduced and maintained at a desired pressure in contact with said electrodes.

12. A multi-electrode assembly for obtaining anelectrio discharge within a gas, comprising, in the order named, a first anode electrode of solid construction, a

first non-conductive spacer element having a central cylindrical cavity therein, a second trigger electrode having a central cylindrical cavity therein, a second non-conductive spacer element having a central cylindrical cavity therein, a third cathode electrode having a central conical cavity therein diverging to the ambient, and a non-conductive housing within which all said spacer elements and electrodes are arranged to have a common longitudinal axis, the central cavities of said spacer elements and said cathode and trigger electrodes together with said anode electrode forming a confined channel for the electric discharge.

to a multi-el'ectrode gas discharge device resting in its non-conductive state, passing a continuous pressurized gas flow past the electrodes of said multi-electrode gas discharge device, placing a switching device having infinitesimal energy consumption in an electrically parallel arrangement with at least two of the electrodes of said gas discharge device, and actuating said switching device to sharply increase the potential gradient within said gas discharge device whereby it changes to its conductive state. ileferences Cited in the file of this patent ''UNITED STATES PATENTS Plumm June 19, 1923 Lord May 23,1950 Sackett Apr. 8, 1952

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