US3087091A - Spark gap switch - Google Patents

Description

April 23, 1963 c, MCFARLAND SPARK GAP SWITCH Filed May 1, 1958 United States Patent 3,087,091 SPARK GAP SWITCH George C. McFarland, Berkeley, Calif., assignor to High Voltage Engineering Corporation, Burlington, Mass, a corporation of Massachusetts Filed May 1, 1958, Ser. No. 732,359 4 Claims. (Cl. 315241) This invention relates to high-power, high-duty-cycle spark-gap switches for use in the production of substantially square-wave pulses. Such spark gap switches are useful in a variety of applications, including providing the DO pulsed power for klystrons and other high-power microwave tubes. In klystrons and other microwave tubes microwave power is produced by means of a modulated beam of electrons which are accelerated to rather high energies by the application of DC. voltage. Since this DC. voltage is rather high, the tube cannot be operated continuously at the high powers required, and so the DC. voltage is supplied in substantially squarewave pulses. In general the square-wave D.C. pulses are obtained by means of a pulse-forming network, in which a high-voltage power supply charges condensers which are connected in parallel in the pulse-forming network. These condensers are then discharged through other elements in the network which restrain current flow, so that the condensers are discharged in a steady manner, thus forming the required square-wave pulse of high power. In order to initiate the discharge of the condensers, some sort of switch mechanism must be used, and in highpower applications this switch mechanism must be capable of handling large currents, on the order of hundreds of amperes. A common switch mechanism is a hydrogen thyratron, but this is an expensive piece of apparatus at high power. An alternative switch mechanism is a sparkgap switch, which is relatively inexpensive and adapted to high currents. However, there are various difficulties and problems associated with conventional spark-gap switches when operated at high power and high duty cycle. These problems include point erosion, geometry criticalness, turn-on jitter, changing conditions with time, triggering difliculties, de-ionization, and RF broadcasting. Moreover, many spark-gap switches do not function over a wide range of voltages. Such wide-range operation is desirable in the operation of klystrons, which require a lengthy warm-up time in which the tube is gradually brought up to full voltage.

Briefly stated, the invention comprises a coaxial spark gap with novel triggering means. Advantages resulting from the coaxial system include the provision of cathode areas of large dimensions so as to lessen deterioration, and the minimizing of RF broadcast. In a preferred embodiment of the invention, a multi-gap switch is provided which has exhibited a larger range of switching voltages than conventional two-electrode-type spark gaps. The multi-gap switch has a voltage-dividing resistor string to equalize the voltage gradient at each gap. The trigger pulse transformer, by various coupling means one of which is shown, causes eventual breakdown in each gap of the multi-gap system, so that the total gap acts as a high-current switch operating in a range of voltages whose lower limit is 3,000 to 6,000 volts, depending on design dimensions, and whose upper limit is determined by the number of series gap sections.

The invention has many useful applications, and is particularly useful for rapid or slow command switching of high-voltage circuits where hundreds to thousands or more amperes flow through the switch, such as in radar modulators, particle accelerator modulators, and other devices having high-voltage high-current switching requirements, such as safety or protecting shorting devicesand the like.

The invention may best be understood from the followice ing detailed description thereof, having reference to the accompanying drawing in which:

FIG. 1 is a diagrammatic view partly in vertical central section of one embodiment of the invention; and

FIG. 2 is a diagram partly in vertical central section of a preferred embodiment of the invention.

Referring to the drawing and first to FIG. 1 thereof, a high-voltage pulse-forming network 1 is connected to ground through a load 2, and the high-voltage end thereof is connected to the outer electrode 3 of a coaxial spark gap by a high-voltage lead 4. The coaxial spark gap comprises the outer tubular electrode 3 and an inner cylindrical electrode 5. The inner electrode 5 is the one which is triggered; that is to say, it is the potential of the inner electrode 5 which is suddenly changed to produce the trigger action. The triggered or inner electrode 5 is connected through an inductance 6 to the output of the trigger transformer 7, which is activated by the trigger pulse generator 8. The purpose of the inductance 6 is to limit the current surge. In addition to the protection afforded by the inductance 6, the secondary of the trigger pulse transformer 7 is protected from the current surge by the spark transfer gap, formed by and between the cup 9 and the flanged disk 10, across which the discharge is maintained after being initiated.

Immediately prior to the discharge of the spark gap 35, the pulse-forming network 1 is fully charged, so that the outer electrode 3 is at high positive voltage and the inner electrode 5 is at ground potential, since it is connected to ground through the flanged disk 10, the inductance 6 and the secondary of the trigger transformer 7. The trigger pulse generator 8 then produces a trigger pulse of the form shown at 11, thus applying a negative potential to the central electrode 5. This increases the voltage difference across the spark gap 3, 5 by an amount sufficient to cause breakdown, and the resultant current surge travels from ground through a second inductance 12, across the spark transfer gap 9, 10, across the spark gap 3, 5, and through the pulse forming network 1 and the load 2. The second inductance 12 comprises a few-turn loop to cause the potential of the cup 9 to follow the potential produced by the trigger pulse at the flanged disk 10 by capacity coupling across the spark transfer gap 9, 10. In experiments performed by me with apparatus such as that shown in FIG. 1 voltages of 30,000 volts were successfully switched at repetition rates of 330 p.p.s. Jitters less than 0.2 microsecond were achieved.

As pointed out hereinbefore, the invention minimizes point erosion by means of coaxial geometry, which provides large cathode-anode areas. In tests conducted by me the discharge occurred on random radii of the coaxial system and also occurred at random positions along the length of the coaxial system. The coaxial design is useful and can incorporate easily changed coaxial cylinders that can be constructed so as to align themselves on assembly.

- The coaxial system removes the critical geometry and gap adjustment problems and permits the gap to be preset.

Turn-on jitter is minimized by triggering the inner coaxial cylinder from ground to 50,000 volts negative. Since the outer coaxial electrode is at the 30,000 volts positive furnished by the pulse forming network, breakdown is readily accomplished. The faster the rise of the trigger pulse, the less jitter is encountered. In my experiments this appeared true for a voltage range from 10 to 30 kilovolts. Although the trigger pulse is capable of reaching 50,000 volts, breakdown usually occurs between 10,- 000 and 20,000 volts on the trigger. This is the only turn-on jitter noticed and is very small for fast-rising trigger pulses.

Under certain conditions corona may occur at the cathode electrode as a result of which the discharge may be- 3 come self-initiated and erratic. Such a situation may be rectified by using an air blast from a small & H.P. blower (not shown) to force air through the coaxial system. In my experiments with the blower turned on, although corona occurred at the cathode electrode, it did not initiate discharge prior to the trigger pulse.

As previously mentioned, the negative 50,000 volt trigger applied to the ground electrode caused immediate gap breakdown with nearly zero jitter. Moreover, this type of trigger was not voltage or spacing sensitive. Other auxiliary triggers, which cause breakdown of the main gap by either ultraviolet light or ionization, were found to be unsatisfactory. This type of trigger caused considerable jitter (5 to 30 microseconds) and required considerably more criticalness in gap spacing and geometry. This type of trigger made the main gap discharge very voltage sensitive also.

If the ground electrode is connected directly to ground through the secondary windings of the trigger transformer, an undesirable inductance is introduced thereby in the main discharge current path even under conditions where the trigger transformer core was saturated. Since the load impedance of the pulse-forming network being switched by the gap is low (on the order of 20 to 40 ohms) small inductances in the series discharge path have a very marked effect on the pulse shape. In order therefore to incorporate the advantages of non-criticalness of spacing and geometry and to minimize the turn-on jitter by using a negative trigger pulse on the ground electrode, the additional coaxial gap or spark transfer gap was provided at the ground electrode to provide a low inductance path to ground; and the additional inductance, which was an air core inductance of 20 turns, No. 12 wire, 1 inch in diameter and 3 inches long, was placed in the lead from the trigger transformer to the triggered ground electrode. With this arrangement the main gap was readily discharged and a second simultaneous discharge occurred between the triggered ground electrode and the second coaxial gap providing a low inductance path to ground. This system, with all the advantages of the negative trigger and low inductance to ground, is practical, cheap, and easily replaced.

The air blast and slow charging of the pulse-forming network, so as to keep voltage ed the gap immediately following the pulse, accomplishes deionization.

Indications are that the coaxial system spark gap tends to radiate less than the open gap itself, thereby minimizing RF- broadcasting.

Referring now to FIG. 2, therein is shown a preferred embodiment of the invention wherein an array of inner electrodes 13, 13', 13", 13" and outer electrodes 14, 14, 14" form several coaxial spark gaps connected in series. The cost of the device therein shown might be less than one-fifth that of an equivalent thyratron tube. A resistance 15 acts as a voltage divider to equalize gap voltage. Resistances 16 are current-limiting resistances which prevent the trigger pulse from being shorted out. Capacitors 17 prevent the main discharge from going through the trigger pulse transformer 7. A blower (not shown) may be used to remove residual ions, but this is not necessary. Addition of the blower would change the geometry requirements. In general the operation of the device shown in FIG. 2 is the same as that of the device shown in FIG. 1. The trigger pulse generator 8 activates the trigger pulse transformer 7, thus producing a negative trigger pulse 11, which reduces the potential of all the tubular outer electrodes 14, 14, 14". Prior to the pulse, the uppermost cylindrical electrode 13 is at the high positive voltage of the pulse-forming network, and the voltages of the other elements 13, 13", 13, 14, 14', 14" of the spark-gap series are controlled by the voltage divider 15. As a result of the voltage divider 15, the voltage across each gap 13-14, 14-13', 13'-14',

14-13", 13"14-, .14"13' before the pulse is just below the breakdown voltage. When the negative pulse is applied, at least one of the gaps will break down, usually the uppermost gap 1314, and this causes the others to break down. For instance, assume that the uppermost gap 13-14 is the one that breaks down. Imrnediately after breakdown the potential of the uppermost tubular outer electrode 14 will be raised to the potential of the uppermost inner electrode 13 thus increasing the voltage difference across the second gap 14-13, which then breaks down, and so on down the series until all the gaps are discharged. The device shown in FIG. 2 has a wide range since it may be used with pulse-forming networks ranging in voltage from a minimum voltage which is just below the breakdown voltage of one gap, to a maximum voltage which is the breakdown voltage across the entire series of spark gaps. For example, the device shown in FIG. 2 might have a range of between 6500 and 39,000 volts.

Having thus described the principles of the invention, together with illustrative embodiments thereof, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.

I claim:

1. A spark-gap switch comprising at least one central electrode, at least one outer tubular electrode coaxially surrounding said central electrode and spaced therefrom sufiiciently so that the gap therebetween supports a stored charge, said gap containing gas at a pressure sufficient to support an electric discharge after initiation thereof, means for electrically connecting said stored charge to one of said electrodes, and means for applying a trigger pulse to the other of said electrodes of polarity opposite that of said stored charge and of magnitude sufficient to initiate discharge, wherein said means for applying the trigger pulse comprises a pulse transformer actuated by a pulse generator, the triggered electrode being connected to the secondary of said pulse transformer through a currentlimiting inductance and to ground through a spark-transfer gap and a second inductance whose magnitude is such that capacity coupling across said spark-transfer gap prevents breakdown thereacross prior to initiation of discharge of said stored charge.

2. A spark-gap for a spark gap switch which spark gap comprises: a number N of central electrodes spaced longitudinally along a common axis, and a number (N 1) of outer tubular electrodes spaced longitudinally along said axis so that each outer electrode coaxially surrounds adjacent extremities of a pair of central electrodes, said outer electrodes being spaced from said central electrodes surficiently so that the gaps therebetween support a stored charge, said gaps containing gas at a pressure sufiicient to support an electric discharge after initiation thereof.

3. A spark-gap for a spark gap switch which spark gap comprises: a number N of central electrodes spaced longitudinally along a common axis, a number (N -1) of outer tubular electrodes spaced longitudinally along said axis so that each outer electrode coaxially surrounds adjacent extremities of a pair of central electrodes, said outer electrodes being spaced from said central electrodes sufficiently so that the gaps therebetween support a stored charge, and said gaps containing gas at a pressure sufiicient to support an electric discharge after initiation thereof, a voltage divider adapted to equalize the voltage across said gaps.

4. A spark-gap switch comprising, in combination, a number N of central electrodes spaced longitudinally along a common axis, a number (N1) of outer tubular electrodes spaced longitudinally along said axis so that each outer electrode coaxially surrounds adjacent extremities of a pair of central electrodes, said outer electrodes being spaced from said central electrodes sufficiently so that the gaps therebetween support a stored charge, said gaps containing gas at a pressure sulricient to support an electric discharge after initiation thereof, a voltage divider adapted to equalize the voltage across said gaps, and means for applying a trigger pulse to at least one of said electrodes of magnitude suificient to initiate discharge, wherein said means for applying the trigger pulse comprises a pulse transformer actuated by a pulse generator, the triggered electrode being connected to the secondary of said pulse transformer through a current-limiting resistance and a capacitance adapted to pass said trigger pulse but not the main discharge.

References Cited in the file of this patent UNITED STATES PATENTS Campos Apr. 22, Jobst July 6, Brasch et a1 Nov. 16, Lindenblad June 7, Keller Apr. 16, Mayer June 7, Smith Apr. 18, Germeshausen Jan. 18, Pearson Dec. 31,

FOREIGN PATENTS Great Britain Mar. 23,

Claims (1)

1. A SPARK-GAP SWITCH COMPRISING AT LEAST ONE CENTRAL ELECTRODE, AT LEAST ONE OUTER TUBULAR ELECTRODE COAXIALLY SURROUNDING SAID CENTRAL ELECTRODE AND SPACED THEREFROM SUFFICIENTLY SO THAT THE GAP THEREBETWEEN SUPPORTS A STORED CHARGE, SAID GAP CONTAINING GAS AT A PRESSURE SUFFICIENT TO SUPPORT AN ELECTRIC DISCHARGE AFTER INITIATION THEREOF, MEANS FOR ELECTRICALLY CONNECTING SAID STORED CHARGE TO ONE OF SAID ELECTRODES, AND MEANS FOR APPLYING A TRIGGER PULSE TO THE OTHER OF SAID ELECTRODES OF POLARITY OPPOSITE THAT OF SAID STORED CHARGE AND OF MAGNITUDE SUFFICIENT TO INITIATE DISCHARGE, WHEREIN SAID MEANS FOR APPLYING THE TRIGGER PULSE COMPRISES A PULSE TRANSFORMER ACTUATED BY A PULSE GENERATOR, THE TRIGGERED ELECTRODE BEING CONNECTED TO THE SECONDARY OF SAID PULSE TRANSFORMER THROUGH A CURRENTLIMITING INDUCTANCE AND TO GROUND THROUGH A SPARK-TRANSFER GAP AND A SECOND INDUCTANCE WHOSE MAGNITUDE IS SUCH THAT CAPACITY COUPLING ACROSS SAID SPARK-TRANSFER GAP PREVENTS BREAKDOWN THEREACROSS PRIOR TO INITIATION OF DISCHARGE OF SAID STORED CHARGE.

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