GB2312322A

GB2312322A - Electron guns

Info

Publication number: GB2312322A
Application number: GB9608235A
Authority: GB
Inventors: Kevin Dennis Ward; Alan Griggs
Original assignee: EEV Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1996-04-20
Filing date: 1996-04-20
Publication date: 1997-10-22
Anticipated expiration: 2016-04-20
Also published as: GB2312322B; GB9608235D0; WO1998054744A1

Abstract

An electron gun includes a cathode 4 and an anode 7 and a grid 6 located between them to control the electron beam. The grid 6 is formed of CVD diamond, giving good thermal conduction properties and permitting the tube to operate at higher power levels than would otherwise be the case with a single grid design. The gird 6 has a coating of electrically conductive material such as molybdenum.

Description

Electron Guns This invention relates to electron guns and more particularly to electron guns which include a grid for controlling the electron beam produced by the gun.

An electron gun is used in devices such as travelling wave tubes and other vacuum electron devices to generate a beam of electrons. In a travelling wave tube (TWT) the electrons of the beam interact with an applied high frequency signal to provide amplification of the signal.

An electron gun comprises a cathode at which electrons are generated and an anode. A voltage is applied between the cathode and anode to accelerate the electrons. In some applications this voltage may be of the order of kilovolts. In rnany devices, a metal grid is included between the cathode and the anode to control the emission characteristics of the cathode. In a typical electron gun, the control grid may be biased slightly positive with respect to the cathode, say 100 V where the cathode is at high potential, to switch on the electron beam such that electrons generated at the cathode are accelerated towards the anode. To switch the beam off, the grid would be biased to say -200 V.

This arrangement is satisfactory for low mean power devices. However, at higher powers excessive heating of the grid can occur. Electrons are accelerated towards and intercept the grid causing it to become hot. This may lead to distortion of the grid affecting the beam configuration and focussing of the beam, and reducing efficiency. In extreme cases, the grid could become severely damaged or even melted causing the device to fail completely. The mean power must therefore to be limited, reducing the duty ratio period for which the gun can be used.

Another difficulty is that cathode material may be deposited on the grid and hence become heated up during use as the grid temperature rises. This may lead to electron emission from the grid itself, distorting the electron beam.

A gridded electron gun such as that as described above is suitable for low mean power use but for higher mean powers a two grid arrangement is required. One grid acts as a control grid and a second grid, known as the shadow grid, is located between the control grid and the cathode. The apertures in the two grids are of the same configuration and the shadow grid is aligned so that it is in register with the control grid. The shadow grid is maintained at or near cathode potential so that there is effectively a free field between the cathode and the shadow grid. Thus, electrons generated at the cathode are not accelerated onto the shadow grid which is heated mainly by convection from the cathode surface. The shadow grid does not reach excessively high temperatures and any cathode material which reaches it tends not to reach temperatures at which it becomes electron emissive to any significant extent. The shadow grid also protects the control grid, physically masking it from the cathode so that the current intercepted by the control grid (or switching grid) is greatly reduced and hence its temperature tends to be lower than that of a single grid. Typically, an increase in power compared to a single grid arrangement of the order of a hundred times is possible using a shadow grid.

The present invention seeks to provide an improved electron gun which is particularly suitable for high mean power use.

According to the invention, there is provided an electron gun comprising a cathode, an anode and a grid located between them, the grid comprising diamond coated with electrically conductive material.

By employing the invention, it is possible to provide an electron gun capable of operating at relatively high power levels dispensing with the need for the complex constructional arrangement of a double grid which is presently used for high power operation. Use of the invention offers the constructional simplicity of a single grid but with a greatly enhanced power dissipation. Also diamond is a mechanically strong material. Diamond is a good conductor of thermal energy and by providing a grid which comprises diamond, heat is conducted away from the centre of the grid to its outer periphery where it can be removed from the vicinity of the electron gun via a heat sink. Thus, although only a single grid is employed, it does not reach such high temperatures that significant distortion occurs to significantly impair the operational efficiency of the device in which the gun is used. Also, cathode material arriving at the grid does not reach temperatures sufficiently high to cause it to become electron emissive to a detrimental degree.

The diamond may be fabricated to the required configuration using chemical vapour deposition techniques and typically has a thermal conductivity which is approximately 10 to 20 times greater than that of molybdenum which is commonly used as a grid material in conventional arrangements.

The diamond is coated with electrically conductive material, and preferably, this layer is of the order 1 micron in thickness. In one advantageous embodiment of the invention the part or all of the diamond.

According to a feature of the invention, an electron beam tube includes an electron gun in accordance with the invention and advantageously, the electron beam tube is a travelling wave tube.

One way in which the invention may be performed is now described by way of example with reference to the accompanying drawings in which: Figure 1 schematically illustrated in longitudinal cross-section a travelling wave tube in accordance with the invention; and Figures 2 and 3 schematically illustrated in plan and sectional views a grid used in the electron gun of the TWT shown in Figure 1.

With reference to Figure 1, a TWT includes an electron gun indicated generally at 1, a slow wave structure 2 and a collector 3. The electron gun 1 includes a thermionic cathode 4 of electron emissive material having a concave front surface which is located in front of a heater shown schematically at 5. A control grid 6 is located in front of the cathode 4 and is followed by a focusing anode 7 having a central aperture. In this arrangement, the slow wave structure 2 includes a helix 8. Means are included for applying a high frequency signal to be amplified to the TWT at 9 and means for coupling it from the arrangement at 10. The collector 3 is arranged to receive electrons of the beam after they have travelled through the slow wave structure 2 and is typically of copper.

The potential difference between the cathode 4 and anode 7 is of the order of 10 kV, the cathode in this arrangement being at ground potential. The control grid is connected to a circuit (not shown) which switches its potential from +100 V to -200 V to switch the electron beam on and off.

The grid 6, as shown in Figures 2 and 3, is of a vaned configuration having radial and circumferential parts and an outer annular region. The grid is formed of CVD diamond 11 and coated with a typically 1 micron layer 12 of molybdenum. In this arrangement, the molybdenum covers both major faces of the grid 6 but in other embodiments, only one face is coated with electrically conductive material.

Claims

1. An electron gun comprising a cathode, an anode and a grid located between them, the grid comprising diamond coated with electrically conductive material.

2. An electron gun as claimed in claim 1 wherein the electrically conductive material is molybdenum.

3. An electron gun as claimed in claim 1 or 2 wherein the electrically conductive material is of the order of 1 micron thick.

4. An electron beam tube including an electron gun as claimed in any preceding claim.

5. A travelling wave tube including an electron gun as claimed in any preceding claim.

6. An electron gun substantially as illustrated in and described with reference to the accompanying drawings.

7. A travelling wave tube substantially as illustrated in and described with reference to Figure 1 of the accompanying drawings.