GB2148048A

GB2148048A - Photocathode for a crossed-field discharge tube

Info

Publication number: GB2148048A
Application number: GB08327338A
Authority: GB
Inventors: Nicholas Armstrong
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1983-10-12
Filing date: 1983-10-12
Publication date: 1985-05-22

Abstract

In a magnetron, emission of primary electrons from a platinum cathode 1 is caused photoelectrically by directing ultraviolet light onto it from an ultraviolet source 7. The primary electrons, returning to the cathode, cause the release of secondary electrons causing resonance and the generation of microwave energy in the usual way. Once the emission of secondary electrons has begun, source 7 may be switched off. <IMAGE>

Description

SPECIFICATION Photocathode for a crossed-field discharge tube This invention relates to a crossed field electron tube which is an electron tube in which an electric field between an anode and a cathode is crossed by a magnetic field. The magnetic field causes primary electrons emitted by the cathode to be deflected along an arcuate path. This arcuate path brings some of the electrons back to the cathode, resulting in the emission of secondary electrons.

The invention arose when considering problems in the design of a magnetron, which is a type of crossed field electron tube designed to generate microwave energy. Conventionaliy, magnetron cathodes are heated to cause therm ionic emission of primary electrons. In some designs only part of the cathode is heated, the other part being unheated but of a material chosen to give good emission of secondary electrons. In either case the need for a heater causes four problems as follows: 1. The time required to heat the cathode means that the magnetron cannot be switched on instantaneously. This is a very serious problem in some circumstances.

2. The power requirements of the heater can be a problem.

3. Parts of the magnetron which are required to be kept cool or at a constant temperature may become hot.

4. The electron-emissive material of the cathode eventually evaporates, because of the heat, thus limiting the life of the magnetron.

The problems listed 1 to 3 above are particularly severe in magnetrons designed for operation at very high frequencies, e.g. at millimetric wavelengths.

This is because such magnetrons need to be very small. Consequently there is insufficient space to locate a heater in the immediate proximity of the cathode surface. The heater thus needs to be located elsewhere, the heat being transferred to the cathode surface by conduction. This means that the warm-up time is increased, more heating power is required because of the loss of heat between the heater and the cathode surface, and this loss of heat accentuates problems associated with the heating of parts not required to be heated.

The invention provides a crossed field electron tube comprising a cathode and a source of electromagnetic radiation, which, in use, causes photo emission of electrons from the cathode.

By employing the photoelectric effect in this way it is possible to avoid the need to heat the cathode.

Consequently all the problems previously listed are eliminated whilst at the time simplifying the cathode structure.

The type of electromagnetic energy used and the material of the cathode surface must be suitably chosen to ensure that photo-emission of electrons takes place. A combination of ultraviolet energy and a platinum cathode surface is an example of one possibility.

It is also desirable to select the type of electromagnetic energy, the material of the cathode surface and the strength of the magnetic field so as to result in most of the primary electrons being emitted with a kinetic energy such that they will be returned to the cathode and cause the emission of secondary electrons. It is expected that the use of ultraviolet energy and a platinum cathode surface will be suitable in most circumstances, platinum being generally suitable for its photoemissive properties and for its ability to emit secondary electrons. It may be beneficial to use a cathode having a first surface of a material which is illuminated by the electromagnetic energy and is chosen for its photoemissive properties: and a second surface of a material which is chosen for its ability to emit secondary electrons.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying schematic drawing which shows an axial cross-section through a magnetron constructed in accordance with the invention and design to generate microwave energy of millimetric wavelengths.

Referring to the drawing the illustrated magnetron comprises a pure platinum cathode 1 mounted on a support 2 in a cavity defined by an annular anode 3. The simplicity of the cathode should be noted, there being no heating facilities included in it. The anode is generally conventional in structure and has a number of vanes extending radially towards the cathode. An output waveguide 4 extends radially through the anode and through a conical recess 5 in the anode, (this recess serving to locate mechanical parts which are not shown since they are not relevant to the invention). Opposite the output waveguide 4 is a light guide 6 also extending through the anode and arranged to conduct ultraviolet energy from a continuous wave source 7 in the form of a mercury lamp.

In an alternative embodiment of the invention a pulsed source of ultraviolet energy, e.g. a flash tube, could be used. The pulses of ultravoilet energy could then be synchronised to a modulator driving the magnetron. Another alternative would be to position the ultraviolet source on the axis of the magnetron anode e.g. as shown at 7A. Another possibility would be to replace the light guide 6 with a window.

In operation, ultaviolet energy from the source 7 is incident on the cathode and causes the emission of primary electrons initially towards the anode. A magnetic field extending in a direction parailel to the axis of the anode causes these electrons to move along an arcuate path which eventually leads them back to the cathode causing the release of secondary electrons. The emission of secondary electrons toward the anode creates electrical resonance within the anode structure resulting in the generation of microwave energy which is propagated along the output waveguide 4. Once the emission of secondary electrons has begun the source of ultraviolet energy can be switched off.

The Applicant is optimistic that a magnetron constructed as illustrated will benefit from requiring only a negligible start-up time, will have a long life and will be relatively simple to construct because of the simple nature of the cathode. It will be appreciated however, that the illustrated magnetron has been described only as an example of one way of using the invention and that many modifications are possible. For example, although ultraviolet energy is incident on a platinum cathode in the illustrated embodiment, other embodiments could employ other forms of electromagnetic energy and other cathode surfaces.

Claims

CLAIMS 1. A crossed field electron tube comprising a cathode and a source of electromagnetic radiation which, in use, caused photoemission of the electrons from the cathode. 2. A magnetron constructed in accordance to Claim 1. 3. A crossed field electron tube according to Claim 1 or 2 in which the source of electromagnetic radiation is a source of ultraviolet radiation. 4. A crossed field electron tube according to any preceding claim in which the cathode includes platinum metal. 5. A magnetron accordingly to Claim 1 and substantially as described with reference to the accompanying drawing. Superseded claims 1 to 5 New or amended claims:

1. A crossed field electron tube for generating electromagnetic radiation of millimetric wavelengths comprising a cathode, an anode surrounding the cathode, and a source of electromagnetic radiation which, in use, initiates emission of primary electrons from the cathode, the arrangement being such that primary electrons are returned to the cathode and cause emission of secondary electrons.

2. A magnetron constructed in accordance with Claim 1.

3. A crossed field electron tube according to Claim 1 or 2 in which the source of electromagnetic radiation is a source of ultraviolet radiation.

4. A crossed field electron tube according to any preceding claim in which the cathode includes platinum metal.

5. A magnetron for generating electromagnetic radiation of millimetric wavelengths and comprising a central cathode, means for causing emission of primary electrons from the cathode, an anode surrounding the cathode and means for applying crossed electric and magnetic fields between the anode and cathode thereby constraining most of the electrons emitted from the cathode to return thereto and cause the emission of secondary electrons; characterised in that at least a part of the cathode is photoemissive and in that a light guide is included for directing light or other electromagnetic radiation onto the said photoemissive part to cause the release of the primary electrons.

6. A magnetron according to claim 4 including a light guide for directing the light or other electromagnetic radiation onto the cathode.

7. A magnetron according to claim 5 in which the light guide extends through the anode.

8. A magnetron according to claim 1 and substantially as described with reference to the accompanying drawing.