US3317776A - Triggered arc discharge device having a liquid cathode - Google Patents

Description

y 2, 11967 R. E. BRITTAIN ETAL TRIGGERED ARC DISCHARGE DEVICE HAVING A LIQUID CATHODE Filed May 18, 1964 1 4 2 l 1| I i /n N a o v m 5 g 5 ML I INVENTO'RS QTTOENEYQ I United States Patent 3,317,776 TRIGGERED ARC DISCHARGE DEVICE HAVING A LIQUID CATHODE Raymond Ernest Brittain, Sutton, and William Robert Mayberry, Hounslow, England, assignors to The M-O Valve Company Limited, London, England Filed May 18, 1964, Ser. No. 368,201 Claims priority, application Great Britain, May 20, 1963, 19,949/63 7 Claims. (Cl. 313-170) This invention relates to electric discharge devices.

The invention is concerned in particular with electric discharge devices of the kind having a sealed envelope within which are disposed a cathode in the form of a pool of conducting liquid, an anode and a trigger electrode interposed between the anode and cathode, the envelope having a low pressure gas filling and the arrangement being such that, with the anode maintained positive with respect to the cathode by a voltage lying Within a range below the anode breakdown voltage, a discharge between the anode and cathode may be initiated by applying a suitable triggering voltage between the trigger electrode and the cathode. By the anode breakdown voltage is meant the lowest value of voltage applied between the anode and cathode, in a sense which makes the anode more positive, for which a discharge would occur with the trigger electrode maintained at the same potential as the cathode; by a low pressure gas filling is meant a gas filling at a pressure less than that pressure of gas filling for which the anode breakdown voltage would have its minimum value in respect of devices of identical construction and having gas fillings of the same composition.

One possible application of an electric discharge device of the kind specified is as a surge diverter. For such an application the device may be required to have a relatively high anode breakdown voltage (say of at least five kilovolts), while at the same time it should be possible reliably to initiate a discharge between the anode and cathode using a relatively small trigger power when the anode-cathode voltage is considerably less than the anode breakdown voltage. The device should be capable of passing relatively high peak anode currents, but Will not normally be required to have nearly such a high ratin in respect of mean anode cur-rent, since anode current will normally only be passed for short periods which are not rapidly repeated.

In practice it has been found some-what diflicult to meet these requirements in a device which is relatively inexpensive to manufacture, and it is therefore an object of the invention to provide an electric discharge device of the kind specified which is improved in this respect.

According to one aspect of the invention, in an electric discharge device of the kind specified the anode is effectively in the form of an end portion of a metal rod which is disposed coaxially within a tubular insulator, the insulator being spaced from the rod over a length of the rod contiguous With said end portion; the trigger electrode is effectively in the form of a tubular metal member which is closed at one end by a metal closure member, the tubular metal member being supported at its open end from the insulator so as to be disposed coaxial with the rod and with its closed end spaced from said end portion of the rod, the closure member being apertured to permit the passage of a discharge between the anode and cathode, the spacing between the closure memher and the adjacent end of the rod lying in the range O.52 millimetres and the spacing between the rod and the tubular metal member lying in the range 1-4 millimetres; and the pressure of the gas filling has a value within the range 0.3-1.0 times P, where P is that prespensive techniques.

3,317,776 Patented May 2, 1967 sure of gas filling for which the value of the anode breakdown voltage would be five kilovolts in a device of identical construction and having a low presure gas filling of the same composition.

It may be noted that in discharge devices of the kind specified, for a given construction and composition of the gas filling the value of the anode breakdown voltage decreases with an increase of the pressure of the gas filling, while at the same time it becomes easier to meet the requirement concerning reliable triggering of the device. Moreover, for a given composition and pressure of the gas filling, the value of the anode breakdown voltage is largely dependent on the length of the electric lines of force between the anode and the trigger electrode, the breakdown voltage increasing with a decrease of this length at least to the point at which it becomes so small that breakdown is possible by Virtue of effects associated with the occurrence of a high value of the electric field strength.

Thus the use of relatively small values for the spacings between the anode and the trigger electrode, :as specified above, makes it possible, with a given composition of the gas filling, to obtain a relatively high value for P, and hence to achieve a satisfactory compromise between the requirements concerning the anode breakdown voltage and the ease of reliable triggering over a wider range of pressures of the gas filling than would be possible when using larger values for these spacings. The manufacture of the device is thus not rendered diflicult and expensive by virtue of a requirement to control the pressure of the gas filling within very narrow limits. It will be appreciated that with such an arrangement it is necessary to control the values of thespacings between the anode and the trigger electrode relatively closely during manufacture; the simple and rigid form specified above for the anode-trigger electrode assembly, which is in itself of a relatively inexpensive nature, enables this requirement to be met without involving the use of complicated and ex- Further, the form of this assembly is such that it is readily able to withstand the mechanical stresses which may be associated with the passage of relatively high peak anode currents.

In an electric discharge device in accordance with the invention the cathode suitable consists of mercury containing a small proportion of at least one of the alkaline earth metals barium, strontium, calcium and magnesium, the total amount of the additive being such that if the mercury is rolled over a glass surface it tends to stick to that surface. Preferably the additive consists solely of barium, in which case the amount incorporated in the mercury may suitably lie in the range 1-10 milligrams per cc. of mercury.

The use of an alkaline earth metal additive in such an amount results in an improvement in the triggering characteristics of the device as compared with a case in which a plain mercury cathode is used. In particular, for a given device and a given trigger power, it makes it possible to obtain reliable triggering down to lower values of applied anode-cathode voltage. Alternatively, it makes it possible, with a device of given construction and composition of the gas filling, to obtain reliable triggering with a given trigger power and applied anode-cathode voltage while using a lower pressure for the gas filling. This feature may thus contribute further to the possibility of obtaining a satisfactory compromise between the requirements concerning the anode breakdown voltage and the ease of reliable triggering over a relatively Wide range of pressures of the gas filling.

One electric discharge device will now be described by Way of example with reference to the accompanying drawing, which is a sectional view of the device.

The device has a sealed glass envelope 1 within which are disposed a cathode 2, and anode 3 and a trigger electrode 4. The envelope 1 is generally of circular cylindrical form, and in operation of the device is disposed with its longitudinal axis vertical, the cathode 2, which is in the form of a pool of mercury having a volume of about two cc., being disposed at the lower end of the envelope 1 in direct contact with the inside of the envelope 1. At this end, the envelope 1 is narrowed down to form a hollow projecting pip 5, the mercury filling the space within the pip 5 and extending into the space above it; a connector for the cathode 2 is provided in the form of a metal rod 6 which is sealed through the end of the pip 5 so that one end of the rod 6 is immersed in the mercury.

At its other end the envelope 1 is formed with a reentrant portion 7, through which is sealed an assembly including a glass tube 8 and a tungsten rod 9 which are disposed coaxial with each other, the rod 9 having a circular cross-section of diameter four millimetres and the tube 8 having a circular cross-section with internal and external diameters respectively of 6.5 millimetres and one centimetre. The outer end of the tube 8 is sealed on to the rod 9 at a point near the outer end of the rod 9, while the tube 8 is sealed through the portion 7 of the envelope 1 at a point about midway along its length. The inner end of the tube 8 projects for a distance of one millimetre beyond the inner end of the rod 9, the portion of the rod 9 adjacent this end effectively constituting the anode 3 of the device. It will be appreciated that the remainder of the rod 9, which has a total length of about eight centimetres, constitutes a connector for the anode 3.

The trigger electrode 4 includes a nickel tube 10 having a length of 1.5 centimetres and an internal diameter of one centimetre, the tube being provided at one end with an outwardly projecting radial flange 11 and being disposed so that it fits around the tube 8 with its flanged end in register with the inner end of the tube 8; the tube 10 may for example be formed by bending round a flat sheet of nickel and clamping the free edges together. The flanged end of the tube 10 is closed by a closure member in the form of an iron disc 12 having a diameter substantially equal to the outer diameter of the flange 11, the disc 12 being welded at its periphery to the flange 11 and having one main face abutting against the end of the tube 8 so that the trigger electrode 4 is accurately located in a longitudinal direction. The disc 12 has formed at its centre a circular aperture 13 of diameter one millimetre, and has projecting from its periphery a bent tab 14 whose purpose is explained below. It will be appreciated from the dimensions given above that the spacing between the disc 12 and the adjacent end of the rod 9 is one millimetre and the spacing between the rod 9 and the tube 10 is three millimetres. It will be further appreciated that the spacing of the tube 8 from the rod 9 over a length of the rod 9 contiguous with the anode 3 reduces the risk of breakdown occurring between the anode 3 and the trigger electrode 4 through or along the surface of the tube 8. The total length of the envelope 1 is such that the spacing between the cathode 2 and the trigger electrode 4, which is not critical, has a value of a few centimetres, and a connector for the trigger electrode 4 is provided in the form of a metal rod 15 sealed through the side wall of the envelope 1 at a position which is more dis- :tant from the cathode 2 than is the trigger electrode 4, the trigger electrode 4 being connected to the rod 15 by a suitable lead 16.

The envelope 1 is filled with argon at a nominal pressure of 0.25 torr, the tolerance allowed on this parameter during manufacture being about '10%. The value of the anode breakdown voltage for a device having the construction described above and with an argon filling at a pressure of 0.25 torr is about six kilovolts; by way of comparison it may be stated that the pressure of gas filling for which the anode breakdown voltage would have its minimum value in respect of devices of identical construction and having argon fillings is several times greater, and that the value of the pressure P in respect of such a device is about 0.35 torr.

The device is provided with a number of barium getter elements 17 of the type conventionally used in vacuum tubes, these elements being supported from the tab 14 So as to be disposed to one side of the trigger electrode 4 and the total amount of barium incorporated in them being about five milligrams. After the envelope 1 has been filled with gas the getters 17 are fired by eddy current heating to evaporate the barium, and the major part of it becomes incorporated in the mercury of the cathode 2. As a result, the state of the mercury is altered in such a manner that, if it is caused to roll over the inside of the envelope 1 by tilting the device, it exhibits a tendency to stick to the inside of the envelope 1. The improved triggering characteristics thereby obtained may be illustrated by considering a case in which the device is triggered by means of a pulse corresponding to one half cycle of a sinusoidal alternating voltage of frequency 50 kc./s. and amplitude two kilovolts, the pulse being applied between the trigger electrode 4 and the cathode 2 so as to drive the trigger electrode 4 more positive and the current flowing through the trigger electrode 4 being limited to a maximum value of 10 milliamperes. With such an arrangement it is found that a discharge between anode 3 and cathode 2 may be reliably initiated in the device described above with values of the applied anodecathode voltage down to less than one kilovolt, whereas in an otherwise similar device having a plain mercury cathode the corresponding figure would be four kilovolts.

In alternative arrangements, a similar improvement in triggering characteristics may be brought about if the barium incorporated in the mercury is replaced by an equivalent quantity of strontium, calcium or magnesium.

Finally, it may be noted that the device described above is capable of passing peak anode currents having a value of 2000 amperes.

We claim:

1. An electric discharge device having a sealed envelope within which are disposed: a cathode in the form of a pool of conducting liquid; an anode which is eifectively in the form of an end portion of a metal rod which is disposed coaxially within a tubular insulator, the insulator being spaced from the rod over a length of the rod contiguous with said end portion; and a trigger electrode interposed between the anode and cathode, the trigger electrode being efiectively in the form of a tubular metal member which is closed at one end by a metal closure member, the tubular metal member being supported at its open end from the insulator so as to be disposed coaxial with the rod and with its closed end spaced from said end portion of the rod, the closure member being apertured to permit the passage of a discharge between the anode and cathode, the spacing between the closure member and the adjacent end of the rod lying in the range 0.5-2 millimetres and the spacing between the rod and the tubular metal member lying in the range 1-4 millimetres; the envelope having a low pressure gas filling whose pressure has a value within the range 0.31.0 times P, where P is that pressure of gas filling for which the value of the anode breakdown voltage would be five kilovolts in a device of identical construction and having a low pressure gas filling of the same composition.

2. An electric discharge device according to claim 1, in which the tubular metal member fits closely around the tubular insulator.

3. An electric discharge device according to claim 1, in which the tubular insulator is sealed through the envelope at a point intermediate along the length of the insulator, the tubular metal member being supported from the inwardly projecting end of the insulator, and the outwardly projecting end of the insulator being sealed on to the metal rod at a point intermediate along the length of the rod.

4. An electric discharge device according to claim 1, in which the tubular insulator is of glass.

5. An electric discharge device according to claim 1, in which the gas filling is of argon at a pressure in the range 0.2-0.3 torr.

6. An electric discharge device having a sealed envelope within which are disposed: a cathode in the form of a pool of conducting liquid; an anode which is effectively in the form of an end portion of a metal rod which is disposed coaxially within a tubular insulator, the insulator being spaced from the rod over a length of the rod contiguous with said end portion; and a trigger electrode interposed between the anode and cathode, the trigger electrode being efiectively in the form of a tubular metal member which is closed at one end by a metal closure member, the tubular metal member being supported at its open end from the insulator so as to be disposed coaxial with the rod and with its closed end spaced from said end portion of the rod, the closure member being apertured to permit the passage of a discharge between the anode and cathode, the spacing between the closure member and the adjacent end of the rod lying in the range 0.5-2 millimetres and the spacing between the rod and the tubular metal member lying in the range 1-4 millimetres; the envelope having a low pressure gas filling whose pressure has a value within the range 0.3- 1.0 times P, where P is that pressure of gas filling for which the value of the anode breakdown voltage would be five kilovolts in a device of identical construction and having a low pressure gas filling of the same composition, and the cathode consisting of mercury containing a small proportion of an additive selected from the group consisting of barium, strontium, calcium, magnesium, and mixtures of these metals, the total amount of the additive being such that if the mercury is rolled over a glass surface it tends to stick to that surface.

7. An electric discharge device having a sealed envelope filled with argon at a pressure in the range 0.2-0.3 torr, the envelope having disposed within it: a cathode in the form of a pool of mercury containing barium in an amount lying in the range 1-10 milligrams per cc. of mercury; an anode Which is efiectively in the form of an end portion of a metal rod which is disposed coaxially within a tubular glass insulator, the insulator being spaced from the rod over a length of the rod contiguous with said end portion and being sealed through the envelope at a point intermediate along the length of the insulator, and the outwardly projecting end of the insulator being sealed on to the rod at a point intermediate along the length of the rod; and a trigger electrode interposed between the anode and cathode, the trigger electrode being effectively in the form of a tubular metal member which is closed at one end by a metal closure member, the tubular metal member fitting closely around the inwardly projecting end of the insulator so as to be supported therefrom with the closure member spaced from said end portion of the rod, the closure member being apertured to permit the passage of a discharge between the anode and cathode, the spacing between the closure member and the adjacent end of the rod lying in the range 0.5-2 millimetres and the spacing between the rod and the tubular metal member lying in the range 1-4 millimetres.

References Cited by the Examiner UNITED STATES PATENTS 2,675,495 4/1954 DeLany 313328 X 2,976,451 3/1961 Reiling 313-328 X 2,985,786 5/1961 Humphrey 313-225 X JAMES W. LAWRENCE, Primary Examiner. S. D. SCHLOSSER, Assistant Examiner.

Claims (1)

1. AN ELECTRIC DISCHARGE DEVICE HAVING A SEALED ENVELOPE WITHIN WHICH ARE DISPOSED: A CATHODE IN THE FORM OF A POOL OF CONDUCTING LIQUID; AN ANODE WHICH IS EFFECTIVELY IN THE FORM OF AN END PORTION OF A METAL ROD WHICH IS DISPOSED COAXIALLY WITHIN A TUBULAR INSULATOR, THE INSULATOR BEING SPACED FROM THE ROD OVER A LENGTH OF THE ROD CONTIGUOUS WITH SAID END PORTION; AND A TRIGGER ELECTRODE INTERPOSED BETWEEN THE ANODE AND CATHODE, THE TRIGGER ELECTRODE BEING EFFECTIVELY IN THE FORM OF A TUBULAR METAL MEMBER WHICH IS CLOSED AT ONE END BY A METAL CLOSURE MEMBER, THE TUBULAR METAL MEMBER BEING SUPPORTED AT ITS OPEN END FROM THE INSULATOR SO AS TO BE DISPOSED COAXIAL WITH THE ROD AND WITH ITS CLOSED END SPACED FROM SAID END PORTION OF THE ROD, THE CLOSURE MEMBER BEING APERTURED TO PERMIT THE PASSAGE OF A DISCHARGE BETWEEN THE ANODE AND CATHODE; THE SPACING BETWEEN THE CLOSURE MEMBER AND THE ADJACENT END OF THE ROD LYING IN THE RANGE 0.5-2 MILLIMETRES AND THE SPACING BETWEEN THE ROD AND THE TUBULAR METAL MEMBER LYING IN THE RANGE 1-4 MILLIMETRES; THE ENVELOPE HAVING A LOW PRESSURE GAS FILLING WHOSE PRESSURE HAS A VALUE WITHIN THE RANGE 0.3-1.0 TIMES P, WHERE P IS THAT PRESSURE OF GAS FILLING FOR WHICH THE VALUE OF THE ANODE BREAKDOWN VOLTAGE WOULD BE FIVE KILOVOLTS IN A DEVICE OF IDENTICAL CONSTRUCTION AND HAVING A LOW PRESSURE GAS FILLING OF THE SAME COMPOSITION.

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