GB2088123A

GB2088123A - Thyratrons

Info

Publication number: GB2088123A
Application number: GB8037068A
Authority: GB
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1980-11-19
Filing date: 1980-11-19
Publication date: 1982-06-03

Abstract

A thyratron is provided in which the gap a between anode 1 and triggering grid 3 is chosen such that in the anode/grid region the thyratron operates on the left hand side of the characteristic minimum of the Paschen breakdown curve for the gas with which the envelope is filled and the separation c between the grid 3 and cathode 2 is chosen such that in that region the thyratron operates substantially at the characteristic minimum of the Paschen breakdown curve for the gas. As described, hollow cold cathode 2 contains a layer 9 of wire gauze onto which is painted an emissive coating, and the filling may comprise dentrium at 1 torr. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the thyratrons This invention relates to thyratrons.

As is well known, thyratrons are commonly used in protective or crowbar circuits.

The present invention seeks to provide an improved thyratron suitable for use in such circuits particularly where power is limited as, for example, with a portable equipment.

According to this invention, a thyratron is provided which comprises within an envelope an anode electrode structure, a cathode electrode structure and a triggering grid electrode arrangement therebetween, said cathode being a cold cathode, the separation between the anode electrode structure and said triggering grid electrode arrangement being such that, in that region, said thyratron operates on the left hand side of the characteristic minimum of the Paschen breakdown curve for a gas with which said envelope is filled with the separation between the triggering grid electrode arrangement and the cathode electrode structure being such that, in that region, said thyratron operates substantially at said characteristic minimum of the Paschen breakdown curve for said gas.

Preferably said gas is deuterium and the separations above referred to are such as to achieve hold off at approximately 20 kV with a gas pressure of approximately 1 TORR whilst at the same time triggering may be accomplished by a trigger voltage of approximately 300 volts.

Preferably said cathode is a hollow cathode and comprises a cylinder open towards said triggering grid electrode arrangement and having on the inner wall thereof a layer of metallic gauze bearing emissive material.

Preferably said cathode cylinder is of nickel.

In one embodiment of the invention said triggering grid electrode arrangement consists of a trigger grid alone.

In another embodiment of the invention, said triggering grid electrode arrangement comprises a trigger grid towards said anode structure and a primer or "keep-alive" electrode towards said cathode electrode structure.

Preferably said trigger grid comprises two plates mechanically and electrically united with one plate being nearer said anode than the other, said two plates having apertures therein which are staggered about the centre line of said thyratron whereby to control the length of the breakdown path from the anode, whilst permitting the anode field to take over from a triggering discharge initiated between said cathode and said trigger grid.

According to a feature of this invention, a circuit arrangement including a thyratron in accordance with said one embodiment of the invention above described, includes means for applying positive voltage pulses to said trigger grid to effect triggering. Said last mentioned positive voltage pulses may have a value of the order of 300 volts, or more.

According to another feature of this invention, a circuit arrangement including a thyratron in accordance with said other embodiment of the invention above described, includes means for applying positive voltage pulses to said trigger grid to effect triggering and means for maintaining a positive d.c. voltage on said primer or "keep-alive" electrode.

Preferably said last mentioned means for applying positive voltage pulses to said trigger grid is such that between said positive voltage pulses said trigger grid is biased negatively.

Preferably the value of said last mentioned positive voltage pulses is from 200 volts to 1000 volts and the value of the negative voltage to which said trigger grid is biased between said positive pulses is from -50 volts to -300 volts.

Preferably said means for maintaining a positive d.c. voltage on said primer of "keep-alive" electrode is such that said last mentioned voltage is of the order of 300 to 400 volts.

The invention is illustrated in and further described with reference to the accompanying drawings in which Figure 1 shows semischematically a section through one thyratron in accordance with the present invention.

Figure 2 is a plan view of the trigger grid 3 of the thyratron of Figure 1, Figure 3 illustrates the Paschen curve for the gas with which the envelope of the thyratron of Figures 1 and 2 is filled, and Figure 4 shows semi-schematically a section through another embodiment in accordance with the present invention.

Like references are used for like parts in all of the Figures.

Referring to Figures 1 and 2 of the drawings, the thyratron consists of an anode electrode structure 1 in the form of a closed cylinder, a cathode electrode structure 2 in the form of an open cylinder, and between the anode 1 and the cathode 2, a triggering grid electrode arrangement consisting, in this example, only of trigger grid 3.

The envelope of the thyratron is formed of two cylindrical ceramic insulators 4 and 5. The interior of the envelope is filled with deuterium gas to a pressure of approximately 1 TORR. The Paschen curve for this gas is represented in Figure 3, but without particular regard to scale or accuracy, since it is provided purely for explanatory purposes. As well known, this relates breakdown voltage in kV to the product of pressure p and breakdown gap d.

The trigger grid 3 consists of two closely spaced plates 3 a and 3b mechanically and electrically connected to each other. The plate 3a has on one side of the centre line 6, an oblong slot 7, whilst the plate 3b has on the opposite side of the centre line 6 an oblong slot 8. The slot 7 is shown in dotted outline in Figure 2 to illustrate its relation with slot 8. The construction of the trigger grid 3 is such as to control the length of the breakdown path of the anode, whilst allowing the anode field to take over from a triggering discharge initiated between the cathode structure 2 and the trigger grid 3.

The cathode structure 2 is in the form of a hollow cathode consisting of a cylinder of nickel open towards the trigger grid 3 and within which is a layer 9 of wire gauze on to which is painted an emissive coating.

The dimension a between the end face of the anode structure 1 and the upper plate 3b of the trigger grid 3 is chosen, having regard to the Paschen breakdown characteristic for deuterium, in order to achieve voltage hold off at 20 kV with 1 TORR pressure. Due regard is also given in this connection to the dimension b between the anode structure 1 and the inside wall of the cylinder 4. In other words the thyratron is designed to operate in the grid/anode region of the left hand side of the characteristic minimum of the Paschen breakdown curve for deuterium -- as represented in Figure 3 at V.

In addition, the dimension c separating the cathode structure 2 from the trigger grid electrode arrangement (i.e. trigger grid 3) is chosen having regard to the Paschen breakdown characteristic in order to achieve a voltage hold off equal to 300 volts, the value at the characteristic minimum in the Paschen breakdown curve so that, as represented at V2 in Figure 3, in the grid/cathode region the thyratron operates at said characteristic minimum.

The thyratron described above is particularly suitable for use in a protective or crowbar circuit for portable equipments, where space and power are limited. As described the thyratron should be capable of holding off high tension voltages in the region of 20 kV, whilst being capable of being triggered by a low power trigger signal of voltage above 300 volts (e.g. 500 volts) applied to triggering grid structure 3. It has also been found that the thyratron has been capable of sustaining currents in a glow discharge mode down to a few milliamperes.

Referring to Figure 4, again the thyratron consists of an anode electrode structure 1 in the form of a closed cylinder, a cathode electrode structure 2 in the form of an open cylinder, and between the two a triggering grid electrode arrangement in this case referenced generally 10.

The envelope of the thyratron consists of three ceramic insulators 11, 12 and 1 3. As with the thyratron shown in Figure 1, the interior of the envelop is filled with deuterium gas to a pressure of approximately 1 TORR, and the anode 1 and cathode 2 are similar to those of the thyratron shown in Figure 1.

Unlike the thyratron shown in Figure 1, the triggering grid electrode arrangement consists of a trigger grid 3 towards the anode 1 and a primer or "keep-alive" electrode 14 towards the cathode 2.

The trigger grid 3 again consists of two closely spaced plates 3a and 3b mechanically and electrically connected to each other. The plates 3a and 3b again have relatively staggered apertures therein but in this case the apertures are in the form of concentric circular slots 1 5, 1 6. Slots like those in the plates 3a and 3b of Figure 1 could have been used however.

The primer or "keep-alive" electrode also has a circular slot aperture 1 7 therein, concentric with and staggered with respect to the slot 1 6 of plate 3a.

The separation a between the anode 1 and the triggering grid electrode arrangement 10 (i.e.

between the anode 1 and trigger grid 3) and the separation between the cathode 2 and the triggering grid electrode arrangement 10 (i.e.

between the cathode 2 and primer or "keep-alive" electrode 14) is chosen as previously described with reference to Figure 1.

In operation a steady positive d.c. potential of between 300 and 400 volts is maintained on the primer or "keep-alive" electrode 14, whilst a negative bias of between50 volts and -300 volts is applied to trigger grid 3. When triggering of the thyratron is required the potential applied to trigger grid 3 is pulsed in a positive direction to between +200 volts and +1000 volts.

Claims

1. A thyratron comprising within an envelope, an anode electrode structure, a cathode electrode structure and a triggering grid electrode arrangement therebetween, said cathode being a cold cathode, the separation between the anode electrode structure and said triggering grid electrode arrangement being such that, in that region, said thyratron operates on the left hand side of the characteristic minimum of the Paschen breakdown curve for a gas with which said envelope is filled and the separation between the triggering grid electrode arrangement and the cathode electrode structure being such that, in that region, said thyratron operates substantially at said characteristic minimum of the Paschen breakdown curve for said gas.

2. A thyratron as claimed in claim 1 and wherein said gas is deuterium and the separations above referred to are such as to achieve voltage hold off at approximately 20 kV with a gas pressure of approximately 1 TORR whilst at the same time triggering may be accomplished by a trigger voltage of approximately 300 volts.

3. A thyratron as claimed in claim 1 or 2 and wherein said cathode is a hollow cathode and comprises a cylinder open towards said triggering grid electrode arrangement and having on the inner wall thereof a layer of metallic gauze bearing emissive material.

4. A thyratron as claimed in claim 3 and wherein said cathode cylinder is of nickel.

5. A thyratron as claimed in any of the above claims and wherein said triggering grid electrode arrangement consists of a trigger grid alone.

6. A thyratron as claimed in any of the above claims 1 to 4 and wherein said triggering grid electrode arrangement comprises a trigger grid towards said anode structure and a primer or "keep-alive" electrode towards said cathode electrode structure.

7. A thyratron as claimed in any of the above claims and wherein said trigger grid comprises two plates mechanically and electrically united with one plate being nearer said anode than the other, said two plates having apertures therein which are staggered about the centre line of said thyratron whereby to control the length of the breakdown path from the anode, whilst permitting the anode field to take over from a triggering discharge initiated between said cathode and said trigger grid.

8. A circuit arrangement including a thyratron as claimed in claim 5 or in claim 7 as dependent upon claim 5 and including means for applying positive voltage pulses to said trigger grid to effect triggering. Said last mentioned positive voltage pulses may have a value of the order of 300 volts or more.

9. A circuit arrangement including a thyratron as claimed in claim 6 or in claim 7 as dependent upon claim 6 and including means for applying positive voltage pulses to said trigger grid to effect triggering and means for maintaining a positive d.c. voltage on said primer or "keep-alive" electrode.

10. A circuit arrangement as claimed in claim 9 and wherein said last mentioned means for applying positive voltage pulses to said trigger grid is such that between said positive voltage pulses said trigger grid is biased negatively.

1 A circuit arrangement as claimed in claim 10 and wherein the value of said last mentioned positive voltage pulses is from 200 volts to 1000 volts and the value of the negative voltage to which said trigger grid is biased between said positive pulses is from -50 volts to -300 volts.

12. A circuit arrangement as claimed in claim 10 or 11 and wherein said means for maintaining a positive d.c. voltage on said primer or "keepalive" electrode is such that said last mentioned voltage is of the order of 300 to 400 volts.

1 3. A thyratron substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.

1 4. A thyratron substantially as herein described with reference to Figure 4 of the accompanying drawings.