GB1597693A - Electron gun cathode arrangements - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRON GUN CATHODE ARRANGEMENTS (71) We, EM I LIMITED, a British company of Blyth Road, Hayes, Middlesex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to electron gun cathode arrangements providing an electron beam suitable for focussing onto a target. It is especially, though not exclusively, applicable to gun arrangements for use with X-ray tubes in which the electron beam is to be scanned relative to the X-ray target.

In certain circumstances, for example X-ray tubes in which high power beams are to be scanned over relatively large areas, the design of suitable deflecting coils can present significant problems. To reduce such problems it is desirable to provide an electron gun which can either allow a suitably focussed spot to be formed on a target spaced a sufficient distance away from the electron gun or inject, into the beginning of the scanning region of the tube, a very finely focussed beam.

According to the invention there is provided a cathode arrangement for an electron gun, the arrangement including a focussing cup electrode and an electron emitter comprising two adjoining coil filaments each conforming substantially to part of a spherical surface so that the two coil filaments together form at least a hemispherical electron emitting surface disposed to face an open end of the cup electrode to project electrons out of said cup.

Scanning X-ray tubes may take different forms. In one example an electrostatically focussed electron beam is injected into an electric-field free drift space in which it is scanned by magnetic fields from a coil system to be incident on a target which may be, say, 100mm diameter. Because of space charge in the focussed beam, small spot dimensions which are desirable can only be realised if the cathode-target distance is short and/or the cathode area is very large. To achieve the former condition large deflection angles would be required and for typical scanning frequencies of the order of 10kHz it is difficult to design suitable scanning coils.

In an alternative scanning arrangement both focussing and deflection are magnetic. The beam diameter at the target (spot-size) is restricted by the axial magnetic field of a focussing solenoid. A relatively strong magnetic field is required to restrict sufficiently the, typically 1 amp, beam, which must also enter the magnetic field in a finely focussed state, and consequently strong fields are required to deflect it.

In this case also deflection coil design is not straightforward.

It is proposed to reduce these problems by using a cathode design which provides a large virtual cathode without the need for a correspondingly large electron source.

In order that the invention may be clearly understood and readily carried into effect it will now be described by way of example with reference to the accompanying drawings of which: Figures la and ib show in schematic form in selection and end elevation respectively a cathode arrangement incorporating the invention and Figure 2 shows a sectional view of the cathode arrangement with a practical form of electron emitter.

In Figure 1 there is illustrated schematically an electron gun arrangement suitable for applications such as the two scanning X-ray tube arrangements described hereinbefore.

Figure la shows a sectional side elevation and Figure ib shows an end elevation. There is provided a convex hemispherical electron emitter 1. The cathode assembly includes a metal block 2 which has a cylindrical focussing cup 3 set in it with the emitter 1 placed axially at the back face. Cup 2 is nominally at the same potential as emitter 1. The electrons emitted then follow trajectories 4, imposed in part by other field defining electrodes, where appropriate, such that there is formed a large diameter virtual cathode 5, of diameter similar to that of cup 3. This helps focussing to a fine focal spot 3. This helps focussing to a fine focal spot 6 from which the electrons can be injected into the drift tube of the X-ray tube.The use of a hemispherical emitter also decreases space charge at the emitter surface and yields the largest emitting area from the space occupied by the emitter. If desired there may also be included an aperature plate 7, which can be one of the field defining electrodes. Such a plate assists the setting of suitable focussing conditions and prevents unwanted effects which may result from magnetic fields in that region. After the aperture the resultant beam can be accelerated to its full potential.

Although the emitter 1 is preferably hemispherical as shown in full line it will be appreciated that there would be no essential difference if it were extended to a full sphere, as indicated at 1' by the broken line, set into an aperture 8.

It will be apparent that the hidden hemisphere will not contribute to the emission used for forming the electrons which are subsequently injected into the drift tube.

Such an arrangement allows the practical embodiment shown in Figure 2 in which the spherical emitter is a spherically coiled filament 9 which can be mounted by leads 10 onto supports, now shown, inset in the cathode block.

Filament 9 is formed in two halves, 9a and 9b, each formed by winding tungsten wire around a hemispherical mandrel. The arrangement illustrated in Figure 2 is used rather than a single hemispherical coiled filament, placed axially in the orientation of emitter 1 of Figure 1, since it provides relatively closely spaced turns facing in the beam direction rather than the open end of an axially aligned coil.

It will be appreciated that the cathode arrangement described may be used with electron guns in arrangements other than X-ray tubes.

WHAT WE CLAIM IS: 1. A cathode arrangement for an electron gun, the arrangement including a focussing cup electrode and an electron emitter comprising two adjoining coil filaments each conforming substantially to part of a spherical surface so that the two coil filaments together form at least a hemispherical electron emitting surface disposed to face an open end of cup electrode to project electrons out of said cup.

2. A cathode arrangement according to Claim 1 wherein the electron emitting surface is substantially a complete sphere.

3. A cathode arrangement according to Claims 1 or 2 wherein the axis of symmetry of the combined spherical coil arrangement is substantially perpendicular to the mean direction of projection of the electrons.

4. A cathode arrangement according to any preceding claim wherein the cup electrode is substantially cylindrical and the mean direction of projection of the electrons is parallel to the axis of the cylinder.

5. A cathode arrangement substantially as herein described with reference to the accompanying drawings.

6. An electron gun including a cathode arrangement according to any of the preceding

Claims (1)

1. claims.