GB2197750A

GB2197750A - Discharge tube devices

Info

Publication number: GB2197750A
Application number: GB08621024A
Authority: GB
Inventors: Clifford Robert Weatherup
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1986-08-30
Filing date: 1986-08-30
Publication date: 1988-05-25
Also published as: GB8621024D0

Abstract

A discharge tube device, such as a thyratron, may be able to conduct current for only a limited time because of migration of gas within its envelope to one end of the device. Postive ions passing from the anode to the cathode via apertures in the grid structure tend to cause non- ionised gas molecules to travel in that direction also. By providing a gas return path and means for conducting heat from the grid and anode region, current may be conducted for relatively long periods, to give higher power operation. As described, gas travels along the dotted lines by virtue of apertures 11 in cathode heat shield 10. Cooling may be achieved by providing a generally cylindrical metal portion 8 in the envelope which may either be attached directly to grid structure 4 (figure 2, not shown) or by metal rods 9. The anode stem may be hollow for receipt of cooling air or oil. <IMAGE>

Description

Discharge Tube Devices This invention relates to discharge tube devices and more particularly, but not exclusively, to thyratrons.

A thyratron is a gas discharge device in which an anode and cathode are contained within a gas-filled envelope with one or more control grids located between them. When it is desired to trigger the thyratron into conduction, typically a positive potential pulse is applied to the control grids to produce breakdown of the gas between the anode and cathode.

The present invention arose in an attempt to provide an improved discharge tube device.

According to this invention there is provided a discharge tube device comprising an envelope which contains an anode, a cathode, a grid structure and gas, part of the envelope being of metal and arranged to act as a heat conduction path, and there being a gas return path from the cathode region to the anode region during conduction between the anode and cathode. When there is conduction between the anode and cathode, positive ions attracted to the cathode tend to draw non-ionised gas molecules with them through apertures in the grid structure. As conduction through the device continues, a pressure difference arises between the anode and cathode regions. Eventually conduction becomes erratic because there are insufficient gas molecules in the anode region, and hence possible current carriers, to sustain a current.By including a gas return path, the pressure within the envelope may be kept substantially uniform, and thus currents may be conducted for longer periods than would otherwise be the case. It is believed that the inclusion of a gas return path may enable currents in the region of 1,000 amps to be conducted between the anode and cathode for a duration of about 300us. However, full utilization of this power conducting capability could lead to problems because of the large amount of energy which would then be dissipated within the tube, which may result in undesirable heating effects such as distortion of the grid structure. By including a metal part in the tube envelope and arranging that this acts as a heat conduction path, this limitation is reduced or eliminated, thus enabling high power operation to be achieved.

Advantageously, part of the envelope is glass. The open structure presented by a glass thyratron is such that gas return path from the higher pressure cathode region to the lower pressure region at the anode can easily be included.

It is preferred that the metal part of the envelope has a surface which is in contact with a surface of the grid structure, enabling good transfer of heat between them to be achieved. The metal part may be, for example, a cylinder which is arranged co-axially about and in contact with the grid structure, or it could be connected via metal rods to the grid structure.

Advantageously, the envelope includes a ceramic portion arranged co-axially about a lead to the anode.

The inclusion of such a ceramic portion enables a larger diameter anode lead to be successfully used than if that part of the envelope surrounding the lead were of glass.

This is because satisfactory tolerances on the distance between the anode lead to the envelope over the length required for a desired voltage hold-off may be much more easily achieved with ceramic than if glass were employed. It is preferred that the anode lead is hollow. This is practicable where the portion of the envelope surrounding the anode lead is of ceramic enabling a larger diameter anode lead to be used. Such a construction allows coolant, for example, air, oil or water, to be passed through the lead to provide cooling to the anode. Thus, in addition to the cooling made available by the inclusion of a metal part in the envelope, further cooling, and thus higher power operation may be obtained.

Some ways in which the invention may be performed are now described by way of example with reference to the accompanying drawings, in which: Figures 1 and 2 schematically illustrate, partially in section, respective thyratrons in accordance with the invention.

With reference to Figure 1, a thyratron includes an envelope 1 containing an anode 2, a cathode 3 and a grid structure 4 located between them. The grid structure 4 comprises several apertured grids surrounded by a cylindrical metal mesh 5 which supports them. The envelope 1 also contains hydrogen at a few torr pressure, and comprises a first glass portion 6 at the anode end of the device, a second glass portion 7 near the cathode 3 and a cylindrical piece 8 of metal located between them to which they are sealed. In this case the metal is Kovar (registered trademark) but other metals having a similar thermal expansion co-efficient to that of the glass may be used. Metal rods 9 connect the mesh 5 and the metal piece 8 of the envelope.

During conduction of a current through the thyratron, positive ions travel from the anode region towards the cathode 3 and neutral gas molecules tend to migrate with them through the apertures in the grid structure 4. The path of the gas flow is shown as broken lines. Gas molecules move between the cathode 3 and its heat shield 10 and pass through apertures 11.

They then reach the low pressure region via a passage 12 between the cathode heat shield 8 and the second glass portion 7 of the envelope 1. At the grid structure 4 the molecules pass through the mesh 5 and into the low pressure region at the anode, giving continuous circulation within the envelope 1. Heat produced in the anode region when the thyratron is conducting a current is conducted, by means of the rods 9, from the grid structure 4 to the metal piece 7 by means of the rods 9, where it is radiated or where external cooling is provided.

With reference to Figure 2, another thyratron in accordance with the invention includes an anode 13, a cathode 14 and an intermediate grid structure 15 which comprises a plurality of grids and a surrounding mesh 16. In this thyratron, the envelope consists of a glass portion 17 at the cathode end of the tube, a metal port 18 surrounding the grid structure 15 and fixed to a metal disc 19, and a ceramic tube 20 which surrounds the lead or stem 21 of the anode 13. The metal part 18 is cylindrical and has a greater diameter at one end 22 than the other to provide clearance between the grid structure 15 and the glass portion 17 of the envelope, the smaller diameter end being in contact with the grid structure 15. The use of a ceramic tube 20 around the anode stem 21 enables a large diameter lead 21 to be used, which is hollow. Coolant, such as air or oil, is arranged to flow through it to provide cooling of the anode 13. During conduction, the neutral gas molecules travel along a similar path to those of the thyratron described with reference to Figure 1, the gas flow being illustrated by the broken lines. Heat is conducted from the anode region via the surrounding mesh 16 of the grid structure 15 and the metal part 18 of the envelope.

Claims

1. A discharge tube device comprising an envelope which contains an anode, a cathode, a grid structure and gas, part of the envelope being of metal and arranged to act as a heat conduction path, and there being a gas return path from the cathode region to the anode region during conduction between the anode and cathode.

2. A device as claimed in claim 1 and wherein part of the envelope is glass.

3. A device as claimed in claim 1 or 2 and wherein the metal part of the envelope has a surface which is in contact with a grid structure surface.

4. A device as claimed in claim 1, 2 or 3 and wherein the envelope includes a ceramic portion arranged coaxially about a lead to the anode.

5. A device as claimed in claim 4 and wherein the lead is hollow.

6. A thyratron substantially as illustrated in and described with reference to Figure 1 of the accompanying drawings.

7. A thyratron substantially as illustrated in and described with reference to Figure 2 of the accompanying drawings.