US3207947A

US3207947A - Triggered spark gap

Info

Publication number: US3207947A
Application number: US176047A
Authority: US
Inventors: John H Goncz
Original assignee: Edgerton Germeshausen and Grier Inc
Current assignee: PerkinElmer Inc
Priority date: 1962-02-27
Filing date: 1962-02-27
Publication date: 1965-09-21
Anticipated expiration: 1982-09-21

Description

Sept. 21, 1965 J. H. GONCZ 3,

TRIGGERED SPARK GAP Filed Feb. 27, 1962 FIG.4

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JOHN H. GONCZ INVENTOR.

ATTORNEYS United States Patent 3,207,947 TRIGGERED SPARK GAP John H. Goncz, Waltharn, Mass, assignor to Edger-ton,

Germeshausen & Grier, Inc., Boston, Mass., a corporation of Massachusetts Filed Feb. 27, 1962, Ser. No. 176,047 4 Claims. (Cl. 315109) The invention described herein relates to electric discharge devices and more particularly to electric discharge devices having an evacuated discharge chamber.

Electric discharge devices are commonly used as highspeed electronic switches in a wide variety of applications. Most electronic switches today are gas-filled devices such as spark gaps, hydrogen thyratrons, ignitrons, etc. These devices are, of course, superior to mechanical relays and other such switches because of their higher power-handling capabilities, shorter delay time and longer life. But even with these advantages, gas-filled discharge devices are limited in their operating ranges, and are subject to certain disadvantages. One such limitation is the small range of anode voltages at which these devices are operative. The range of anode voltages is the difference between the maximum voltage that can be applied to the device without causing a discharge between the principal electrodes, and the minimum voltage at which the device may be triggered. The gas in the device is one of the factors which limits this range because the maximum voltage is restricted by the characteristics of the fill gas and its pressure. This maximum voltage, usually referred to as the static breakdown voltage (SBV), is normally of the order of 5 kv. for spark gaps and 15 kv. for thyratrons. Another way of referring to this anode voltage range is in terms of the anode voltage range factor which is the maximum anode voltage (SBV) divided by the minimum. Triggered spark gaps have a range factor of only about 4 while in the thyratrons, the figure is about 10.

A further disadvantage is found in the fact that the maximum current capability of hydrogen thyratrons is approximately 1,000 amperes.

It should also be noted that the presence of gas in these prior art devices cause an energy loss due to the fact that a portion of the discharge energy is used to ionize the gas in the device. Furthermore, discharges in gas-filled devices are accompanied by shock waves and noise which are injurious to the electrodes thereby shortening the life of the device.

The subject invention overcomes the limitations of these prior are devices by triggering an arc discharge in an evacuated chamber. The static breakdown voltage can be raised to more than 200 kv. in a device which can also be triggered with only 400 volts across the main electrodes. This results in an anode voltage range factor in excess of 500. Such a device is capable of passing currents of the order of 200,000 amperes. There is, of course, no energy loss in the device due to gas ionization since the discharge chamber is evacuated. The life of the device is greater than that of a gas-filled device because there is no noise or shock waves produced and electrode erosion is substantially less than is found in gaseous switches. Moreover, the repetition rate at which a vacuum device may fire is two to three times faster because its recovery time is that much shorter for the reason that there is no gas to deionize between pulses.

It is, therefore, an object of the present invention to provide an electric discharge device capable of operating as an electronic switch without the aforementioned disadvantages of the prior art.

Another object of the invention is to provide a novel high-speed, high-power switching device, small in size and simple in construction.

3,207,947 Patented Sept. 21, 1965 A further object of the invention is to provide a new and novel spark gap having operating capabilities substantially in excess of those found in gas-filled devices.

Still another object is to provide an electric discharge device having a long life and in which there is created no damaging shock waves or noise when the discharge takes place.

In the past, electric discharge devices having an evacuated chamber have been used for only very specialized purposes or in complicated apparatus. The subject invention, however, is a simple device having wide application as a fast switch for variable current and voltage levels. In summary, this invention consists of a pair of principal electrodes, spaced from each other in a small evacuated chamber, a trigger electrode disposed adjacent one of the principal electrodes, and means for causing a triggering are at a point that is under the influence of an intense electric field between the principal electrodes. Other and further objects of this invention will be hereinafter pointed out in both the specifications and the appended claims.

The invention will now be discussed in conjunction with the drawings, FIGURE 1 of which is a longitudinal, parallel-perspective view of a cross section of one embodiment of the invention; and

FIGURE 2 is a circuit used with the device of FIG- URE 1.

FIGURES 3, 4 and 5 show sectional views of other electrode arrangements for use as modifications of the electrodes in the device of FIGURE 1.

Referring now to FIGURE 1, the invention is shown as a triggered spark gap having insulative envelope 1 of ceramic, glass, or the like, which consists of a plurality of sections including a base section 1', a cylindrical center section 1", and a top section 1". Ceramic is preferred be cause of its rugged nature, and its high insulating abilities. Within the envelope 1 is disposed a pair of principal electrodes 3 and 5 having a large active surface of, for example, a refractory material such as tungsten or molybdenum. Electrodes 3 and 5 are held in position by flange connectors 4 and 6 respectively, extending between the envelope sections to form rings on the exterior of the envelope. These rings 4 and 6 also provide electrical contact means to the electrodes 3 and 5.

An insulating sleeve 9 of ceramic or the like is disposed concentric with the cup-shaped electrode 3. Within this sleeve 9 is placed a trigger electrode 7 which is slightly smaller in diameter than the internal opening of the sleeve 9 at the end closer to electrode 3. Trigger electrode 7 passes through the upper wall of envelope 1. An anxiliary trigger electrode 8 is shown in the form of a cylinder enclosing a portion of insulating sleeve 9. A shield 2 is disposed between the principal electrodes 3 and 5, adjacent electrode 3 near the center portion thereof where trigger electrode 7, auxiliary trigger 8 and the insulating sleeve 9 terminate.

A means of evacuating the envelope such as a pump or the like, not shown, may be used by means of a connection through the envelope wall. It is preferable, however, to place the components in a bell jar or other housing capable of being evacuated, and to stack the elements in their correct position with soldering materials interposed between adjoining parts to effect a firm bond and seal therebetween. Such a method is disclosed in US Letters Patent No. 2,992,874, issued July 18, 1961 to Kenneth J. Germeshausen. The bell jar is then evacuated to the desired degree of Vacuum, at least to a pressure of 1O mm. of Hg, and preferably to 10* mm. of Hg. The device is then heated to a point where the solder will melt and, when cooled, will produce an effective seal of the device.

The principal electrodes 3 and 5 are separated by a distance d. This distance is critical in determining the maximum SBV. As point out above, a vacuum spark gap has a greater SBV than a gas-filled device for the same electrode separation distance because there is no gas present which can be ionized by the anode voltage to cause an arc discharge between said electrodes. The distance d can, therefore, be made quite small and still hold off high voltages. This is an important factor in the overall size and operating characteristics of a vacuum device. It has been found that SBV increases more or less as the square of the distance d. It is also affected by electrode material and surface finish; rough surfaces tend to break down at lower voltages than polished surfaces.

Referring now to FIGURE 2, which is a schematic diagram of a typical circuit for operating the device of FIGURE 1, a source of high voltage potential 11 is used to charge capacitor C through a charging resistor R. One side of capacitor C is connected to electrode 5. Electrode 3 and the other side of capacitor C are both connected to ground, the latter connection being made through a load L. When capacitor C is charged, a high voltage is maintained across electrodes 3 and 5, thereby creating an intense electric field within the discharge chamber. To cause the device to break down and an arc to discharge across electrodes 3 and 5, a trigger impulse is fed from a source of triggering impulses 12 through a step-up transformer T to trigger electrode 7. Since there is substantially no gas in the discharge chamber, no ionization takes place within the device, and therefore, a high-voltage trigger pulse is required to effect a discharge within the device. When the trigger impulse is applied to trigger electrode 7, breakdown takes place across the surface of ceramic sleeve 9 between the trigger 7 and the auxiliary trigger electrode 8 which is at a floating potential. The ionized particles of the trigger spark are accelerated by the intense electric field which exists between the principal electrodes 3 and 5. These particles strike electrode 5 with sufficient energy to liberate gas and metal vapor, which in turn are readily ionized to form a low impedance discharge path between the electrodes 3 and 5. This discharge path is capable of passing extremely high currents depending upon the charge given to capacitor C. Although only one capacitor C is shown, it is understood that a plurality of capacitors may be used.

The ceramic insulator 9 serves an important function in separating the trigger electrode from the auxiliary trigger electrode 8. It has been found that by using an insulative material, such as ceramic, for the sleeve 9 and maintaining the sleeve close to, but out of contact with the tip of the trigger electrode 7, the trigger voltage required to cause a discharge within the tube may be materially reduced, in some cases by as much as 40%. The reason for this is that the electric field created by the trigger pulse is made very intense by the small spacing between'trigger electrode 7 and sleeve 9, thereby decreasing the minimum voltage of the trigger pulse.

The minimum trigger voltage required to initiate a discharge within the device is substantiallyindependent of .the difierence in potential between the principal electrodes 3 and 5. A constant trigger voltage may be used for all values of the voltage across the principal electrodes 3 and 5 from the minimum breakdown voltage to the static breakdown voltage. This is considerably different from gas-filled devices where the minimum trigger voltage increases as the voltage across the principal electrodes increases. This feature increases the versitility of the vacuum device since the main discharge voltage can be varied within the aforementioned very wide range and still be triggered by the same trigger pulse.

Another factor which influences the minimum trigger voltage is the polarity of the voltages used. If the trigger pulse and the main discharge voltage are both positive or the former is positive and the latter negative, a higher voltage trigger pulse is necessary. If they are both negative, the smallest trigger voltage may be utilized. If the trigger pulse is negative with the main voltage positive, then a trigger voltage intermediate the previous two cases, may be employed.

The trigger pulse applied to the trigger electrode 7 creates an arc discharge to the auxiliary trigger electrode 8. This is a low-energy arc and, therefore, little or no erosion takes place at the auxiliary trigger electrode. This means that the minimum trigger voltage remains substantially constant during the life of the device.

When the main discharge takes place between the principal electrodes 3 and 5, gas and metal vapor are emitted from electrode 5. To prevent these materials from being deposited upon the ceramic sleeve 9, the trigger electrode 7 and the auxiliary trigger electrode 8, a shield 2 is disposed near these elements in the path such materials would follow from the electrode 5. If this shield 2 were not used, the material deposited upon these elements would eventually cause a short circuit between the trigger electrode 7 and the auxiliary trigger electrode 8 thereby decreasing the life of the device.

External arcing between flanges 4 and 6 is prevented by spacing them a considerable distance apart. In order to obtain this wide separation while electrodes 3 and 5 are maintained a much shorter distance apart, electrodes 3 and 5 are formed in dome-shaped configurations with vertical side walls terminating in horizontal flanges 4 and 6. External arcing is prevented between the trigger electrode 7 and flange 4 because each of them is located on different walls of the device.

A further advantage of the configuration of the principal electrode is the elimination of corona effects at the external junction of the envelope wall, and the terminal flange 6 of the high voltage electrode 5. By this electrode configuration, the electric field on the external surface of the envelope is spread over a wider space, thereby preventing corona at the flange 6.

The large active electrode surfaces of the electrodes 3 and 5 insure discharge when the triggering are is produced. By using smooth, polished surfaces, the static breakdown voltage is increased. The life of the device is extended by using very hard materials for the principal electrodes.

The SBV of the device is further increase if a plurality of connections are made to flange rings 4 and 6. If a single lead is used to connect electrode 5 to the load L and another single lead from electrode 3 to ground, then the plasma produced during discharge is driven to the side of the discharge chamber opposite the points at which the single leads are connected to the flanges 4 and 6. When this occurs, metal evaporated during each discharge is deposited on the envelope walls in the area of the plasma. The deposits of sputtered metal builds up on the envelope walls, thereby causing breakdown at lower SBV and also, decreasing the life of the device.

If, on the other hand, a plurality of leads is used to connect each electrode in series then the plasma is confined to the central portion of the device and very little, if any, sputtered material is deposited upon the side walls. Therefore, a minimum of 4 connectors between each flange and the circuit is recommended and the connectors should be of uniform length and connect to uniformly spaced points around each flange.

The electrode structure shown in FIGURE 1 is not the only such arrangement which will accomplish these results. FIGURES 3, 4 and 5 show modifications which embody the inventive concept. In fact, any electrode arrangementmay be used if it. produces a trigger are at a point that is within the influence of main electric field.

In the modification of FIGURE 3 the insulating sleeve 9' is in contact with electrode 3. No auxiliary trigger electrode is used and the trigger arc takes place between i the trigger electrode 7 and electrode 3. The; life of the,

device is increased by the use of a shield 2 as discussed above.

FIGURES 4 and 5 show electrode arrangements where shields are not necessary. In FIGURE 4, an insulating material 9 such as ceramic is attached to the surface of electrode 3 remote from the main discharge area. Trigger electrode 7 is terminated at a point above electrode 3 adjacent the insulating material 9. The trigger pulse arcs from the trigger electrode 7 to the electrode 3 across the surface of the insulator 9. There is little need for a shield with this arrangement since the wall of insulator 9 separating the trigger electrode 7 and electrode 3 is a vertical wall parallel to the direction that sputtered material would follow, and, therefore, little or no such material would be deposited on this wall to cause the aforementioned short circuit of the triggering impulse.

In FIGURE 5, the electrode arrangement is the same as that shown in FIGURE 4 except that the trigger electrode 7' is now ring-shaped and is disposed on the insulator 9'. The electrode 3, the insulator 9' and the trigger electrode 7 are shown all having substantially the same sized central opening. No shield is necessary with this arrangement for the reasons set forth with regard to FIGURE 4. In addition, gas and metal vapor entering the opening in electrode 3 would pass through and beyond the insulator 9' and the trigger electrode 7.

In the devices shown in FIGURES 3, 4 and 5, the trigger electrodes are again separated a slight distance from the adjacent insulator in order that an intense electric field may be produced in the vicinity of the trigger electrode as explained above in regard to FIGURE 1.

The operation of the devices in FIGURES 3, 4 and 5 is the same as that of FIGURE 1 and all of these devices may employ a circuit such as that of FIGURE 2.

As an example, an evacuated spark gap having a separation distance between the principal electrodes of 10.0 mm. was connected to a Z-microfarad capacitor charged from a high voltage source of 50 kv. No static breakdown was produced. The cut off or minimum voltage across the main electrodes at which the device would discharge was less than 100 volts. This indicated that the device had an anode voltage range factor of 500. The minimum trigger signal was a 11.7 kv., 5 microsecond pulse. Similar devices have held off 60 kv. without static breakdown. When triggered at these voltages, the gaps passed currents on the order of 200,000 amperes.

Although I have described my invention with a certain degree of particularity, further modifications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

I claim:

1. A triggered spark gap comprising:

a ceramic envelope having a high vacuum;

a pair of principal electrodes having large electrode surfaces disposed spaced from and facing each other within the envelope, the first of said pair of principal electrodes having an opening therein;

a normally ineffective trigger electrode disposed in the said opening in the first electrode;

a hollow insulating member enclosing the trigger electrode except at a point adjacent to the first principal electrode;

an auxiliary trigger electrode disposed on the outer surface of said insulating member adjacent said point;

means for connecting the trigger electrode to a source of triggering impulses of sufficient potential to render the trigger electrode effective to produce a triggering arc between said trigger electrode and said auxiliary trigger electrode; and

means for connecting the principal electrodes to a source of high voltage potential thereby creating a strong electric field across the principal electrodes, said high voltage potential being ineffective to cause a discharge to take place therebetween in the absence of a triggering are but being effective to produce a sudden discharge therebetween when said triggering arc is produced.

2. A triggered spark gap comprising:

a cylindrical ceramic envelope having a high vacuum;

a pair of cup-shaped principal electrodes having large dome-shaped electrode surfaces disposed spaced from and facing each other within the envelope, and side walls extending away from the space between the electrodes, terminating in flanges passing through and beyond the side walls of the envelope, the first of said pair of principal electrodes having an opening in its electrode surface;

an insulating member disposed on the first electrode on the side thereof remote from the other principal electrode, and having an opening contiguous with the opening in the first electrode;

a normally ineffective trigger electrode disposed in the said opening in the insulating member having one end spaced from but adjacent to the first electrode and extending therefrom to and through the end wall of said envelope;

means for connecting the trigger electrode to a source of triggering impulses of sufiicient potential to render the trigger electrode effective to produce a triggering arc between the trigger electrode and said first electrode; and

3. A triggered spark gap comprising:

a cylindrical ceramic envelope having a high vacuum;

a normally ineffective trigger electrode disposed on said insulating member on the side thereof remote from the electrode, and having an opening therein contiguous with the opening in the insulating member;

means for connecting the trigger electrode to a source of triggering impulses of sufficient potential to render the trigger electrode effective to produce a triggering are between the trigger electrode and said first electrode; and

4. A triggered spark gap comprising:

a ceramic envelope having a high vacuum;

a normally ineffective trigger electrode disposed within the envelope adjacent to said opening in the first principal electrode;

a shield interposed between the second of said pair of principal electrodes and the opening in said first electrode, closer to the first of said electrodes;

means for connecting the trigger electrode to a source of triggering impulses of suflicient potential to render the trigger electrode efiective to produce a triggering arc; and

means for connecting the principal electrodes to a source of high voltage potential thereby creating a strong electric field across the principal electrodes, said high voltage potential being inffective to cause a discharge to take place therebetween in the absence of a triggering arc but being effective to produce a sudden discharge therebetween when said triggering arc is produced.

References Cited by the Examiner UNITED STATES PATENTS GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, Examiner.

Claims

1. A TRIGGERED SPARK GAP COMPRISING: A CERAMIC ENVELOPE HAVING A HIGH VACUUM; A PAIR OF PRINCIPAL ELECTRODES HAVING LARGE ELECTRODE SURFACES DISPOSED SPACED FROM AND FACING EACH OTHER WITHING THE ENVELOPE, THE FIRST OF SAID PAIR OF PRINCIPAL ELECTRODES HAVING AN OPENING THEREIN; A NORMALLY INEFFECTIVE TRIGGER ELECTRODE DISPOSED IN THE SAID OPENING IN THE FIRST ELECTRODE; A HOLLOW INSULATING MEMBER ENCLOSING THE TRIGGER ELECTRODE EXCEPT AT A POINT ADJACENT TO THE FIRST PRINCIPAL ELECTRODE; AN AUXILIARY TRIGGER ELECTRODE DISPOSED ON THE OUTER SURFACE OF SAID INSULATING MEMBER ADJACENT SAID POINT; MEANS FOR CONNECTING THE TRIGGER ELECTRODE TO A SOURCE OF TRIGGERING IMPULSES OF SUFFICIENT POTENTIAL TO RENDER THE TRIGGER ELECTRODE EFFECTIVE TO PRODUCE A TRIGGERING ARC BETWEEN SAID TRIGGER ELECTRODE AND SAID AUXILIARY TRIGGER ELECTRODE; AND MEANS FOR CONNECTING THE PRINCIPAL ELECTRODES TO A SOURCE OF HIGH VOLTAGE POTENTIAL THEREBY CREATING A STRONG ELECTRIC FIELD ACROSS THE PRINCIPAL ELECTRODES, SAID HIGH VOLTAGE POTENTIAL BEING INEFFECTIVE TO CAUSE A DISCHARGE TO TAKE PLACE THEREBETWEEN IN THE ABSENCE OF A TRIGGERING ARC BUT BEING EFFECTIVE TO PRODUCE A SUDDEN DISCHARGE THEREBETWEEN WHEN SAID TRIGGERING ARC IS PRODUCED.