WO1998054744A1

WO1998054744A1 - Electron gun with a diamond grid

Info

Publication number: WO1998054744A1
Application number: PCT/GB1997/001440
Authority: WO
Inventors: Kevin Dennis Ward; Alan Griggs
Original assignee: Eev Limited
Priority date: 1996-04-20
Filing date: 1997-05-27
Publication date: 1998-12-03
Also published as: GB9608235D0; GB2312322B; GB2312322A

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 single grid design. The grid (6) has a coating of electrically conductive material such as molybdenum.

Description

ELECTRON GUN WITH A DIAMOND GRID

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 many 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

electrically conductive layer is of molybdenum. The electrically conductive material is of

molybdenum. The electrically conductive material may coat part or all of the diamond. The

electrically conductive coating may be applied using conventional deposition techniques. 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. The invention may be advantageously applied to any device in which electrons from

a cathode are accelerated towards a grid.

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 illustrates in longitudinal cross-section a travelling wave tube in

accordance with the invention; and

Figures 2 and 3 schematically illustrate 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 schematically 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. The grid configuration may be more complicated depending on the particular operational requirements.

Although the benefits of the invention are particularly appUcable where a single grid is located

between the cathode and anode, in some applications additional grids, which may be of

conventional construction or of coated diamond, may also be included.

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 gun as claimed in claim 1, 2 or 3 wherein the grid is a control grid.

5. An electron gun as claimed in any preceding claim wherein the electrically

conductive material wholly coats the surface of the diamond.

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

7. A travelling wave tube including an electron gun as claimed in any one of claims

1 to 5.