GB2280539A

GB2280539A - A thyratron

Info

Publication number: GB2280539A
Application number: GB9411443A
Authority: GB
Inventors: Jonathan David Barry; Todd Alan Clymer
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1993-07-29
Filing date: 1994-06-08
Publication date: 1995-02-01
Also published as: JPH0785802A; GB9411443D0; FR2708784A1

Abstract

A thyratron, has a gas reservoir heater 22 and a cathode heater 24 powered by a common power supply, the voltages to the heaters being optimized internally by one or more fixed or variable impedance elements ZR, ZC, ZCR. In a preferred embodiment, the impedance elements comprise a variable resistance associated with the reservoir heater. The impedance elements may be incorporated in a separate adaptor. <IMAGE>

Description

THYRATRON WITH INTERNALLY OPTIMIZED RESERVOIR VOLTAGE BACKGROUND OF THE INVENTION The present invention relates to a gas discharge closing switch and, more particularly, to a hydrogen thyratron with internally optimized reservoir voltage.

Gas discharge closing switches, such as thyratrons, are used for rapid switching of high power signals with very low power consumption. A typical thyratron has an anode connected to high voltage, a cathode held at ground potential and a control electrode or "grid" placed between the anode and the cathode. The control electrode is used to apply a positive control pulse which closes the switch by drawing electrons from the cathode to transform gas within the device into a dense, conducting plasma in an avalanche process.

The gas within the thyratron, which is typically hydrogen, is stored chemically in the form of titanium hydride (TiH2) in a reservoir within the housing of the device. When the reservoir is heated, hydrogen gas is liberated from the TiH2 and the equilibrium gas pressure is determined by the reservoir temperature.

Satisfactory operation of a thyratron is strongly dependent on the pressure of the gas within the device.

Abnormally high pressures can cause a thyratron to "latch up" in a continuous conductive mode, whereas low gas pressure can damage the device due to overheating of the anode.

In order to control the pressure of the gas in a thyratron, it is essential to control both the amount of gas in the system and the temperature of the gas reservoir.

Thyratrons are normally evacuated and then "back-filled" with a precise amount of gas at the nominal operating reservoir heater voltage. When hydrogen is used, the reservoir is typically either a hollow titanium cylinder or a nickel cylinder filled with titanium hydride. The hydrogen injected into the device is taken up by the titanium or titanium hydride in solid form. The amount of hydrogen stored in the device is many times the amount ultimately required in gaseous form because of the need to set up a gas/solid equilibrium condition. Equilibrium is achieved during operation by heating the reservoir to a temperature between 4000 and 8000 C using a tungsten filament heater. The voltage applied to the filament heater is therefore critical in controlling the gas pressure within the device.

Thyratrons are typically designed to operate at an optimum gas pressure, which can be achieved by applying a specific voltage to the gas reservoir. However, the reservoir voltages required for actual devices can vary significantly due to manufacturing tolerances and other variables. Accordingly, thyratron manufacturers often test individual devices as part of the manufacturing process to determine the reservoir heater voltage, and therefore the gas pressure, at which the tube operates in an optimum manner. This voltage is then printed on the device itself for use by end users.

A separate power supply is often used to power the reservoir heater of a hydrogen thyratron inasmuch as the optimum operating voltage can vary from device to device and even change over time. For cost reasons, however, and sometimes because of the requirements of a specific application, it is often desirable to power a reservoir heater with the power supply used to heat the thyratron cathode. This is more compact and less expensive, but also eliminates the flexibility to control reservoir voltage independently of cathode heater voltage. Because the voltage applied to the cathode heater normally cannot be changed, systems with a single power supply tend to operate the reservoir heater at the cathode heater voltage. This can lead to unreliable and inefficient operation. "Locking in" the reservoir voltage in this way also precludes its adjustment as the device ages and gas is depleted.

Therefore, it is desirable in many applications to provide an efficient thyratron structure in which both the cathode heater and the reservoir heater can be powered by the same power supply without impacting on efficiency or longevity of the device.

SUMMARY OF THE INVENTION The present invention provides an internally optimized reservoir heater circuit which operates from the same power supply as that used to heat the thyratron cathode. This is accomplished by providing at least one electrical impedance element, such as a resistance or a capacitive bridge, in the reservoir and/or cathode heater circuits. The impedance can be either fixed or variable and can be located either inside the thyratron cavity or in a adapter structure outside the thyratron. In either case, the structure and circuitry of the present invention permit the reservoir heater to be operated at precisely its optimum voltage level and allow the reservoir voltage to be fine tuned as the device ages.

In addition, the invention contemplates a method of manufacturing such a thyratron in which an impedance element is used to provide an optimum reservoir voltage which can be adjusted both during and after the manufacturing process.

Accordingly, a thyratron constructed according to the present invention includes: a fluid-tight housing containing a cathode and a gas reservoir; supply circuitry for providing an electrical supply potential; a cathode subcircuit connected across the supply circuitry, the cathode subcircuit including at least one electrical device for heating the cathode in response to a first preselected voltage; a reservoir subcircuit connected across the supply circuitry, the reservoir subcircuit including at least one electrical device for heating the gas reservoir to provide a preferred gas pressure within the housing in response to a second preselected voltage; and at least one electrical impedance element having a preselected impedance, each of the electrical impedance elements being connected within at least one of the subcircuits so that the first and second voltages are impressed across the respective electrical devices. In a preferred embodiment, the electrical impedance element comprises an element having a preselected impedance connected in series with the electrical device for heating the gas reservoir, the preselected impedance having a value such that the voltage across the electrical device is the second preselected voltage.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other features of the present invention may be more fully understood from the following detailed description, taken together with the accompanying drawings, wherein similar reference characters refer to similar elements throughout and in which: FIGURE 1 is a somewhat schematic cross-sectional view of a closing switch constructed according to one embodiment of the present invention; FIGURE 2A is a schematic diagram of the electrical circuit for the cathode heater and reservoir heater elements of the device of FIGURE 1; FIGURE 2B is a schematic diagram of a simplified circuit for powering the cathode heater and reservoir heater in accordance with another embodiment of the present invention; and FIGURE 3 is a partial sectional view of a closing switch constructed according to a further embodiment of the present invention wherein the impedance elements are located in a separate adapter structure outside the thyratron cavity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGURE 1, a thyratron or other gas discharge closing switch 10 constructed in accordance with the present invention has an anode 12, a control electrode 14, a cathode 16 and a gas reservoir 18, all of which are located within a gas-filled housing 20. The source of the gas within the housing 20 is the reservoir 18, which is heated by a reservoir heater filament 22 to establish a desired level of gas pressure. The cathode 16 also has a heater filament, identified as 24 in the drawing.

The reservoir heater filament 22 and the cathode heater filament 24 are both powered by an external power supply (not shown) applied across the leads 26 and 28 which extend outside the housing 20. The heater filaments 22 and 24 are essentially in parallel with each other across the power supply, except that each has a separate impedance in series with it. Thus, the reservoir heater filament 22 is in series with an impedance, ZR, and the cathode heater filament 24 is in series with an impedance, Zc. For the purposes of this discussion, each series combination of a filament and an impedance is considered a different "subcircuit", so that two such subcircuits are connected in parallel across the power supply. In addition, a third impedance, ZCR, is connected across the power supply itself.In the embodiment of FIGURE 1, all three of these impedances are located within the housing 20.

The generalized heater circuit of the thyratron 10 is shown in schematic form in FIGURE 2A. It will be appreciated from FIGURE 2A that the voltages across the reservoir heater 22 and the cathode heater 24 can be varied independently to any value at or below the voltage of the power supply itself by choosing appropriate values for the impedances ZR and Zc. Additional control is achieved with the impedance ZCR, which is connected directly across the power supply leads.

Although the impedances ZR, ZC and ZCR are preferably resistances, they can also comprise capacitances or other complex impedances depending on the requirements of the thyratron and its heater elements. One specific nonresistive embodiment is a capacitive bridge wherein Zc, ZR and ZCR are all capacitances. In addition, one or more of the impedances can be variable, if desired, in order to "fine tune" the operation of the reservoir and cathode heaters.

It will, of course, be understood that the diagram of FIGURE 2A is simply the most general depiction of the internally optimized structure of the present invention, and that some of the impedances may not be operational in a given commercial device. For example, either the impedance ZR or the impedance Zc will be zero whenever the power supply itself is designed to match the requirements of one of the two heaters. In most instances, it will be the cathode heater 24 which does not have a separate non-zero impedance associated with it. The impedance ZcR, which is connected directly across the power supply, also will not contribute to the heater voltages when the power supply itself matches one of the two heaters. In these instances, the impedance ZCR approaches infinity and preferably takes the form of an open circuit.

A more specific circuit for optimizing the voltage applied to the reservoir heater 22 is illustrated in FIGURE 2B, wherein the impedance in series with the reservoir heater 22 is a "pure" resistance R1 and there is no measurable impedance in series with the cathode heater 24.

The impedance across the power supply is also not a factor in this embodiment.

Turning now to FIGURE 3, a thyratron 30 constructed according to an alternative embodiment of the present invention has impedances ZR, ZC and ZCR, which are not located within the thyratron housing but instead are located in a separate plug-type adapter 32. This simplifies manufacture to some degree and permits any of the impedances which are variable to be adjusted more easily. Whereas adjustment of the impedances of the thyratron 10 would require airtight feed-through elements (not shown), the adapter 32 can simply have openings (not shown) in its outer wall for manual adjustment of the impedance values.

The thyratron 30 of FIGURE 3, it has an anode 34, a control electrode 36 and a cathode 38 within a gas-tight housing 40. The cathode 38 has a wound multi-filament heater 42 and is surrounded by a cylindrical heat shield 44.

The gas reservoir, depicted at 46, also has a filament-type heater (not shown). The two heaters can be connected as illustrated in FIGURES 2A and 2B with the impedances having appropriate values. As mentioned above, the impedances ZR, Zc and ZCR are all located within the adapter element 32.

They are connected to the heaters through a plurality of pins 48 which extend from the bottom of the housing 40 in the manner of prior thyratrons. Rather than being plugged directly into a power supply socket, however, these pins terminate within the adapter element 32 for connection by individual leads to the three impedances in the manner shown. Power to the heaters is provided by an external power supply 50 through additional pins 52 at the bottom of the adapter. Thus, the thyratron 30 is functionally equivalent to the thyratron 10 of FIGURE 1, except for placement of the impedances in a structure outside the thyratron housing.

In the manufacture of the thyratrons 10 and 30, the housings are evacuated and back-filled with controlled amounts of hydrogen gas in a conventional manner, with the gas reservoirs 22 and 46 taking the form of either hollow titanium cylinders or nickel cylinders filled with titanium hydride (TiH2). In either case, the hydrogen introduced into the system is taken up in a solid matrix from which it evolves when the reservoir is heated to between 400 and 8000 C.This temperature is achieved by application of power to both the reservoir heater filament and the cathode heater filament from a common power supply, with the precise voltages to each filament controlled by the impedances Za, ZC and ZCR. As far as the reservoir voltage is concerned, the optimum voltage level is preferably found through known operational tests, at which time the impedance values are set for optimal operation. This procedure can be repeated any time during the operational life of the thyratron in order to adjust for aging of the device. In addition to improving the behavior of the thyratron in the short term, such readjustment also promotes cooler operation of the device and thereby prolongs its useful life.

Although the parameters of the various electrical components vary widely between applications, a specific thyratron embodying the teachings of the present invention may have a cathode heater filament designed to operate at approximately 6.3 volts and a hydrogen loaded reservoir found by a separate manufacturing process to supply optimum working pressure at 5.8 volts. In such a case, the heater filaments would typically be powered by a 6.3 volt power supply. The impedance Zc of the cathode heater subcircuit would then be zero, as would the impedance ZCR across the power supply itself. Impedance Za in the gas reservoir heater subcircuit would be calculated to impress a voltage of exactly 5.8 volts across the reservoir heater filament, thereby optimizing gas pressure in the device.As indicated above, this can be accomplished either by installing a fixed resistance of appropriate value or by adjusting a variable resistor in line with the reservoir heater to the optimum operating point.

From the above, it can be seen that the optimized thyratron structure of the present invention permits the reservoir heater circuit and the cathode heater circuit of a thyratron to be powered by the same power supply without sacrificing reliability, efficiency or longevity of the device.

While the preferred embodiment has been described as typical, the invention is not limited to these particular forms, but rather is applicably broadly to all such variations as fall within the scope of the appended claims.

For example, although the invention is disclosed specifically with respect to a closing switch in which the discharge gas is hydrogen, the teachings of the present invention are applicable, as well, to switches containing other suitable gases.

Claims

1. A thyratron comprising: a fluid-tight housing containing a cathode and a gas reservoir; supply circuitry for providing an electrical supply potential; a cathode subcircuit connected across the supply circuitry, said cathode subcircuit including at least one electrical device for heating the cathode in response to a first preselected voltage; a reservoir subcircuit connected across the supply circuitry, said reservoir subcircuit including at least one electrical device for heating the gas reservoir to provide a preferred gas pressure within the housing in response to a second preselected voltage; and at least one impedance element having a preselected impedance, each of said impedance elements connected within at least one of the subcircuits so that said first and second voltages are impressed across said electrical devices, respectively.

2. The thyratron of claim 1 wherein: the impedance of said at least one impedance element is variable.

3. The thyratron of claim 1 wherein: the impedance of said at least one impedance element comprises a resistance.

4. The thyratron of claim 1 wherein: the impedance of said at least one impedance element comprises a complex impedance.

5. The thyratron of claim 1 wherein: said at least one impedance element comprises an element having a first preselected impedance connected in series with the electrical device for heating the gas reservoir, said first preselected impedance having a value such that the voltage across the electrical device is said first preselected voltage.

6. The thyratron of claim 1 wherein said at least one impedance element comprises: a first impedance element having a first preselected impedance connected in series with said at least one electrical device for heating the cathode; and a second impedance element having a second preselected impedance connected in series with said at least one electrical device for heating the gas reservoir; said second preselected impedance being different than said first preselected impedance.

7. The thyratron of claim 6 wherein: said at least one impedance element comprises a third electrical impedance element having a third preselected impedance connected across the supply circuitry.

8. The thyratron of claim 1 wherein: said at least one impedance element is disposed within the fluid-tight housing.

9. The thyratron of claim 1 which further comprises: an adapting structure for mounting said housing and making electrical connection thereto; said at least one impedance element being disposed within said adapting structure.

10. A thyratron comprising: a fluid-tight housing containing a cathode and a gas reservoir; supply means for providing an electrical supply potential; a cathode subcircuit connected across the supply means, said cathode subcircuit including means for heating the cathode in response to a first preselected voltage; a reservoir subcircuit connected across the supply means, said reservoir subcircuit including means for heating the gas reservoir to provide a preferred gas pressure within the housing in response to a second preselected voltage different from said first preselected voltage; and at least one impedance means, each of said impedance means connected within at least one of the subcircuits so that said first and second voltages are impressed across said heating means, respectively.

11. A thyratron comprising: a fluid-tight housing containing a cathode and a gas reservoir; a first electrical device for heating the cathode; a second electrical device for heating the gas reservoir; a resistance in series with the second electrical device; circuitry for applying a preselected electrical potential across the cathode heating means and across the combination of the reservoir heating means and said series resistance.

12. The thyratron of claim 11 wherein: said series resistance is variable.

13. A method of manufacturing a thyratron comprising: placing a cathode and a gas reservoir in a fluid-tight housing; providing circuitry for supplying an electrical supply potential; connecting a cathode subcircuit across the supply circuitry, said cathode subcircuit including at least one electrical device for heating the cathode in response to a first preselected voltage; connecting a reservoir subcircuit across the supply circuitry, said reservoir subcircuit including at least one electrical device for heating the gas reservoir to provide a preferred gas pressure within the housing in response to a second preselected voltage; and connecting an impedance element having a preselected impedance within at least one of said subcircuits.

14. The manufacturing method of claim 13 which comprises the further step of: adjusting the impedance of said impedance element to adjust the preselected voltage applied to the corresponding electrical device.

15. The manufacturing method of claim 13 wherein the step of connecting an impedance element within at least one of said subcircuits comprises: connecting said element into the reservoir subcircuit in series with the electrical device for heating the gas reservoir.

16. The manufacturing method of claim 13 which comprises the further step of: adjusting the impedance of said impedance element to adjust gas pressure within the housing.