GB2176934A

GB2176934A - Thyratrons

Info

Publication number: GB2176934A
Application number: GB08611964A
Authority: GB
Inventors: Hugh Menown; Leslie John Kettle
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1985-06-07
Filing date: 1986-05-16
Publication date: 1987-01-07
Also published as: EP0204457A2; EP0204457A3; GB8611964D0; JPS622429A; GB8514487D0

Abstract

A thyratron which enables reverse current conduction has an anode 7 and an adjacent anode grid 9 which together enclose a volume 11. A potential difference is applied between the anode 7 and anode grid 9 such that on forward conduction of the thyratron the charge density of plasma in the volume 11 is increased and when forward conduction of the thyratron stops, persists for some time in that space. On reverse conduction, the plasma contained within the volume 11 acts as a cathodic medium. In another mode plasma may be created in volume 11 prior to the thyratron becoming conducting. In other embodiments (Figures 3 and 5) a plurality of enclosures for containing plasma to act as a cathodic medium are provided. <IMAGE>

Description

1 GB 2 176 934 A 1

SPECIFICATION

Thyratrons A This invention relates to thyratrons, and more particularlytothyratrons which are ableto conduct in a reverse direction.

Thyratrons have an anode and cathode with one or more control grids arranged between them which are enclosed by an envelope which also contains a gas filling. The thyratron is arranged to conduct electrons in the forward direction, that is from the cathode to the anode. Reverse conduction may occur however, either intentionally, or because of irregularities in the voltage applied across it. In a conventional thyratron development of the reverse current is inhibited some what and this can cause problems in the circuitry associated with the main discharge of the thyratron.

Also a reverse current may be damaging to thethyrat ron itself, particularly to its anode surfaces.

A previous thyratron which is capable of reverse current conduction is that known as a "hollow anode" thyratron, such as is disclosed in our UK Patent Speci fication No. 1568506, and is schematically illustrated in Figure 1. The anode is formed as a box-like struc ture 1 enclosing a volume 2, and having a control grid 3 and cathode 4.

When the thyratron is triggered on application of a positive pulse on control grid 3 and becomes con ducting, a plasma is formed between the front surface 95 of the anode facing the cathode 4, and the cathode 4.

Some of the plasma penetrates into the enclosed volume 2 by apertures 6 in the front surface 5. Thus the anode 1 togetherwith the plasma enclosed in the volume 2 assume the function of a cathode when the 100 current reverses in the thyratron, since on reversal of the thyratron current, any plasma still persisting with in the volume 2 acts as a cathodic medium and hence reduces the damage done to the anode surfaces and deleterious effects in any associated circuitry.

Penetration of plasma into the enclosed volume 2, which is believed to be principally by diffusion, is effectively beyond external control. Thus, for a given currentthe plasma charge density achieved and the duration forwhich the plasma persists within the volume 2 are not controllable, and in some circum stances may be insufficient.

The present invention seeks to provide an im proved thyratron.

According to this invention there is provided a thy ratron comprising an anode, a grid and an enclosure for plasma, part of said enclosure being formed by said grid and part of the volurnewithin said enclosure being between said anode and said grid, means being provided for applying different potentials to said grid and said anode whereby in operation plasma is con tained within said enclosure to act as a cathodic medium for reverse conduction. The grid mayform all or part of the enclosure. Plasma may be produced within the enclosed volume before, during or afterthe thyratron has been triggered into normal conduction by application of suitable potentials to the anode and grid.

If the thyratron is already in a forward conducting state the plasma which is present maybe made con- trollably to penetrate into the enclosed volume on application of suitable potentials to the anode and grid. If the thyratron is not in a forward conducting state plasma may be produced in the enclosure by applying an appropriately large potential difference between the grid and the anode. A greater charge density of the resulting plasma is achievable than would be possible with a hollow anode thyratron and may be sufficientto enable a glow discharge to be obtained. Thus the plasma persists for appreciably longerthan in a hollow anode thyratron, a much faster reverse current rise time can be achieved and jitter characteristics improved.

The geometry and spacing of the anode grid may be chosen so that the enclosed volume is large and the areas of the surfaces bounding the enclosed volume are small, thus giving a large value forthe ratio of volume to surface area. Thus the contribution of surface recombination to the decay of any plasma within the volume will be minimised, enabling plasma of substantial charge densityto persistwithin the volume for sometime. Then if the anodevoltage reverses immediately or sometime after plasma has been established in thevolume, thethyratron is able to conduct rapidly in the reverse direction.

Advantageously there may be included a plurality of grids arranged to form a plurality of enclosures, means being provided for applying respective different potentials to the anode and each of the grids whereby in operation plasma is contained within each of said enclosures to act as a cathodic medium for reverse conduction. This enables a large amount of plasma to be contained and gives greaterflexibility in choosing the operation of the thyratron.

Preferablythe grid, or at least some of the grids, are electrically connected to the anode. Connection of the grids to the anode by combinations of passive components such as resistors, inductors, and capacitors is termed passive biasing. Alternatively the grid or at least some of the grids are at potentials controlled external to the thyratron. Grid potentials may be set and controlled by bias sources of current at relatively low impedance external to thethyratron, and this is termed active biasing. The grid or grids may be ac- tively biased, passively biased, or biased both actively and passively.

According to an aspect of this invention there is provided a thyratron comprising two structures spaced apart, each structure including an electrode, a grid and an enclosure for plasma, part of the enclosure being formed bythe grid and part of the volume within the enclosure being between the electrode and the grid, means being provided for applying different potentials to the grid and the electrode whereby, in operation plasma is contained within the enclosureto act as a cathodic medium. Thus each structure can act as an anode or a cathode forthe main thyratron discharge depending on the direction of the current through thethyratron. Each structure may include morethan one enclosurefor plasma.

The invention isfurther described byway of examplewith referenceto Figures 2to 5 of the accompanying drawings, in which:

Figure2 schematically illustrates a thyratron in accordance with the invention; 2 GB 2 176 934 A 2 Figure 3 schematically illustrates anotherthyratron in accordance with the invention; Figure 4schematically illustrates yet anotherthyratron in accordancewith the invention; and Figure 5schematically illustrates afurtherthyrat- ron in accordance with the invention,with like refer ences being usedfor like parts throughout.

With referenceto Figure 2, a thyratron containsa hydrogen gasfilling at aboutO.5torrand an anode7, a cathode8, and located between them an anodegrid 9and a control grid 10. The anode 7 and anode grid 9 are configuredto form an enclosure enclosing a volume 11 between them. The anode grid 9 is actively biased by applying a desired potential on conductor 12.

In one mode of operation the thyratron is initially triggered into its forward conducting state by ap plying a suitable positive potential to control grid 10.

When the thyratron is conducting in the forward direction, a suitable potential is applied at 12to pro duce a potential difference of a few kilovolts between the anode 7 and anode grid 9. This increases the charge density of the plasma in the volume 11. When forward conduction ceases, and if the thyratron is made to conduct in the reverse direction, the plasma persisting within thevolume 11 is of a greatenough charge densityto permit reverse conduction without harming the surface of the anode 7, the plasma acting as a cathodic medium.

In another mode, external control means are 95 arranged to produce a potential difference of a few kilovolts between anode 7 and the anode grid 9 to produce a plasma within the volume 11 before the thyratron conducts in its forward direction. Thus the thyratron is already prepared for any reverse current priorto it becoming conducting in either direction.

With reference to Figure 3, another thyratron has a plurality of anode grids, 13,14,15,16 and 17. These are passively biased and are interconnected with the apode 7 via resistors 18 such that a potential differ ence of a few kilovolts is produced between each pair of adjacent anode grids. Thus when thethyratron is in its forward conducting mode, plasma is collected within the volumes 19,20 and 21 between the anode 7 and its adjacent anode grid 13 and between the other pairs of anode grids.

With reference to Figure 4, a thyratron having an alternative enclosure arrangementto that of the embodiment illustrated in Figure 2 has an anode 7 which is located within an enclosure formed com pletely by an anode grid 22.

Referring to Figure 5, anotherthyratron in accord ance with the invention includes a first structure com prising a first electrode 23 at one end thereof and includes two grids 24 and 25 connected to the elec trode 23 through resistors 26 to give passive biasing.

The thyratron also includes a second structure iden tical to the first, having a second electrode 27,which has two associated grids 28 and 29 which are passive ly biased and connected to the second electrode 27 via resistors 30. A control grid 10 is located between the grid 25 which is connected to the first electrode 23 and the grid 28 which is connected to the second electrode 27.

In operation, if thethyratron is required to conduct electrons in a direction from the second anode 27to thefirst anode 23, a cathode potential is applied to the second electrode 27. By suitably choosing thevalues of the resistors 30the potential differences between the grids 28 and 29, and grid 29 and electrode 27 may be made large enough, at approximately a few kilovolts to produce plasma in the volumes 31 and32 between them. The plasma then acts as a cathodic medium when the thyratron is triggered into conduc- tion. Thus the second structure then acts as a cathode, and the first structure acts as an anode. The application of an anode potential to the first electrode 23 priorto the thyratron becoming conducting may similarly generate plasma in volumes 33 and 34 be- tween the electrode 23 and grid 24, and grids 24 and 25 respectively, thus reducing the current risetime.

Whilstthe thyratron is conducting, the charge density of the plasma in the volumes 33 and 34 increases, thus enabling conduction to be rapidly established in the other direction.

Claims

1. Athyratron comprising an anode, a grid and an enclosure for plasma, part of said enclosure being formed by said grid and part of the volume within said enclosure being between said anode and said grid, means being providedfor applying different potentialsto said grid and said anodewhereby in operation plasma is contained within said enclosureto act as a cathodic medium for reverse conduction.

2. Athyratron as claimed in Claim land including a plurality of grids arranged to form a plurality of enclosures, means being provided for applying re- spective different potentialsto the anode and each of the grids whereby in operation plasma is contained within each of said enclosuresto act as a cathodic medium for reverse conduction.

3. Athyratron as claimed in Claim 1 or 2 and wherein the grid or at least one of the plurality of grids are electrically connected to the anode.

4. Athyratron as claimed in Claim 1 or2 and wherein the potential of the grid or at least one of the plurality of the grids is controlled external to the thy- ratron.

5. Athyratron substantially as described and illustrated with reference to Figure 2 of the accompanying drawings.

6. A thyratron substantially as described and ill us- trated with reference to Figure 3 of the accompanying drawings.

7. Athyratron substantially as described and illustrated with reference to Figure 4 of the accompanying drawings.

8. Athyratron comprising, two structures spaced apart from one another, each structure including an electrode, a grid and an enclosure for plasma, at least part of the enclosure being formed by the grid and part of the volume within the enclosure being be- tween the electrode and the grid, and means for applying different potentials to the grid and the electrode whereby, in operation plasma is contained within the enclosure to act as cathodic medium.

9. Athyratron as claimed in claim 8 and wherein at least one structure comprises a plurality of grids c 3 GB 2 176 934 A 3 arranged to forma plurality of enclosures, means being provided for applying respective different potentials to the electrode and each of the grids such that in operation plasma is contained within each of 5 said enclosuresto act as a cathodic medium.

10. Athyratron as claimed in any preceding claim and including control means external to the thyratron for changing the potential of the or a grid.

11. Athyratron as claimed in claim 10 and where- in the control means is arranged to produce a potential difference between the grid and the anode or electrode before the thyratron is triggered such that plasma is created within said enclosure prior to the thyratron becoming conducting.

12. Athyratron substantially as described and illustrated with reference to Figure 5 of the accompanying drawings.

Printed in the U K for HMSO, D8818935,11186,7102. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies maybe obtained.