Thyratrons

Abstract

 0 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 conduct- j ing.

Description

[0001] This invention relates to thyratrons, and more particularly to thyratrons which are able to conduct in a reverse direction.

[0002] 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 somewhat and this can cause problems in the circuitry associated with the main discharge of the thyratron. Also a reverse current may be damaging to the thyratron itself, particularly to its anode surfaces.

[0003] 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 Specification No. 1568506, and is - schematically illustrated in Figure 1. The anode is formed as a box-like structure 1 enclosing a volume 2, and having a control grid 3 and cathode 4.

[0004] When the thyratron is triggered on application of a positive pulse on control grid 3 and becomes conducting, a plasma is formed between the front surface 5 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 together with the plasma enclosed in the volume 2 assume the function of a cathode when the current reverses in the thyratron, since on reversal of the thyratron current, any plasma still persisting within 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.

[0005] 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 current the plasma charge density achieved and the duration for which the plasma persists within the volume 2 are not controllable, and in some circumstances may be insufficient.

[0006] The present invention seeks to provide an improved thyratron.

[0007] According to this invention there is provided a thyratron 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 provided for applying different potentials to said grid and said anode whereby in operation plasma is contained within said enclosure to act as a cathodic medium for reverse conduction. The grid may form all or part of the enclosure. Plasma may be produced within the enclosed volume before, during or after the thyratron has been triggered into normal conduction by application of suitable potentials to the anode and grid.

[0008] If the thyratron is already in a forward conducting state the plasma which is present may be made controllably 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 sufficient to enable a glow discharge to be obtained. Thus the plasma persists for appreciably longer than in a hollow anode thyratron, a much faster reverse current rise time can be achieved and jitter characteristics improved.

[0009] 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 for the 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 density to persist within the volume for some time. Then if the anode voltage reverses immediately or sometime after plasma has been established in the volume, the thyratron is able to conduct rapidly in the reverse direction.

[0010] 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 greater flexibility in choosing the operation of the thyratron.

[0011] Preferably the 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 the thyratron, and this is termed active biasing. The grid or grids may be actively biased, passively biased, or biased both actively and passively.

[0012] 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 by the 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 enclosure to act as a cathodic medium. Thus each structure can act as an anode or a cathode for the main thyratron discharge depending on the direction of the current through the thyratron. Each structure may include more than one enclosure for plasma.

[0013] The invention is further described by way of example with reference to Figures 2 to 5 of the accompanying drawings, in which:

Figure 2 schematically illustrates a thyratron in accordance with the invention;

Figure 3 schematically illustrates another thyratron in accordance with the invention;

Figure 4 schematically illustrates yet another thyratron in accordance with the invention; and

Figure 5 schematically illustrates a further thyratron in accordance with the invention, with like references being used for like parts throughout.

[0014] With reference to Figure 2, a thyratron contains a hydrogen gas filling at about 0.5 torr and an anode 7, a cathode 8, and located between them an anode grid 9 and a control grid 10. The anode 7 and anode grid 9 are configured to form an enclosure enclosing a volume 11 between them. The anode grid 9 is actively biased by applying a desired potential on conductor 12.

[0015] In one mode of operation the thyratron is initially triggered into its forward conducting state by applying a suitable positive potential to control grid 10. When the thyratron is conducting in the forward direction, a suitable potential is applied at 12 to produce 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 the volume 11 is of a great enough charge density to permit reverse conduction without harming the surface of the anode 7, the plasma acting as a cathodic medium.

[0016] In another mode, external control means are 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 prior to it becoming conducting in either direction.

[0017] 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 anode 7 via resistors 18 such that a potential difference of a few kilovolts is produced between each pair of adjacent anode grids. Thus when the thyratron 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.

[0018] With reference to Figure 4, a thyratron having an alternative enclosure arrangement to that of the embodiment illustrated in Figure 2 has an anode 7 which is located within an enclosure formed completely by an anode grid 22.

[0019] Referring to Figure 5, another thyratron in accordance with the invention includes a first structure comprising a first electrode 23 at one end thereof and includes two grids 24 and 25 connected to the electrode 23 through resistors 26 to give passive biasing. The thyratron also includes a second structure identical to the first, having a second electrode 27, which has two associated grids 28 and 29 which are passively 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.

[0020] In operation, if the thyratron is required to conduct electrons in a direction from the second anode 27 to the first anode 23, a cathode potential is applied to the second electrode 27. By suitably choosing the values of the resistors 30 the potential differences between the grids 28 and 29, and grid 29 and electrode 27 may be large enough, at approximately a few kilovolts to produce plasma in the volumes 31 and 32 between them. The plasma then acts as a cathodic medium when the thyratron is triggered into conduction. 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 prior to the thyratron becoming conducting may similarly generate plasma in volumes 33 and 34 between the electrode 23 and grid 24, and grids 24 and 25 respectively, thus reducing the current rise time.

[0021] Whilst the 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. A thyratron comprising an anode (7, 23, 27) and a grid (9, 13-17, 24, 25, 28, 29) and characterised by an enclosure (11, 19-21, 31-34) for plasma, part of said enclosure (11, 19-21, 31-34) being formed by said grid (9, 13-17, 24, 25, 28, 29) and part of the volume within said enclosure (11, 19-21, 31-34) being between said anode (7, 23, 27) and said grid (9, 13-17, 24, 25, 28, 29) and means (12, 18, 26, 30) for applying different potentials to said grid (9, 13-17, 24, 25, 28, 29) and said anode (7, 23, 27) whereby in operation plasma is contained within said enclosure (11, 19-21, 31-34) to act as a cathodic medium for reverse conduction.

2. A thyratron as claimed in Claim 1 and including a plurality of grids (13-17, 24, 25, 28, 29) arranged to form a plurality of enclosures (19-21, 31-34) and means (18, 26, 30) for applying respective different potentials to the anode (7, 23, 27) and each of the grids (13-17, 24, 25, 28, 29) whereby in operation plasma is contained within each of said enclosures (19-21, 31-34) to act as a cathodic medium for reverse conduction.

3. A thyratron as claimed in Claim 1 or 2 and wherein the grid (9) or at least one of the plurality of grids (13-17, 24, 25, 28, 29) is electrically connected to the anode (7, 23, 27).

4. A thyratron as claimed in Claim 1 or 2 and wherein the potential of the grid (9) or at least one of the plurality of grids (13-17, 24, 25, 28, 29) is controlled external to the thyratron.

5. A thyratron comprising two structures spaced apart from one another, each structure including an electrode (23, 27), a grid (24, 25, 28, 29), an enclosure (33, 34, 31, 32) for plasma, at least part of the enclosure (33, 34, 31, 32) being formed by the grid (24, 25, 28, 29) and part of the volume within the enclosure (33, 34, 31, 32) being between the electrode (23, 27) and the grid (24, 25, 28, 29) and means (18, 26, 30) for applying different potentials to the grid (24, 25, 28, 29) and the electrode (23, 27) whereby in operation plasma is contained within the enclosure (33, 34, 31, 32) to act as a cathodic medium.

6. A thyratron as claimed in claim 5 and wherein at least one structure comprises a plurality of grids (24, 25, 28, 29) arranged to form a plurality of enclosures (31, 32, 33, 34), means being provided for applying respective different potentials to the electrode (23, 27) and each of the grids (24, 25, 28, 29) such that in operation plasma is contained within each of said enclosures (31, 32, 33, 34) to act as a cathodic medium.

7. A thyratron as claimed in any preceding claim and including control means (12) external to the thyratron for changing the potential of the or a grid (9).

8. A thyratron as claimed in claim 7 and wherein the control means (12) is arranged to produce a potential difference between the grid (9) and the anode (7) or electrode before the thyratron is triggered such that plasma is created within said enclosure (11) prior to the thyratron becoming conducting.

Drawing