EP0326273A2

EP0326273A2 - Directly heated cathodes

Info

Publication number: EP0326273A2
Application number: EP89300425A
Authority: EP
Inventors: Alan Hugh Pickering
Original assignee: EEV Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1988-01-26
Filing date: 1989-01-18
Publication date: 1989-08-02
Also published as: GB8801633D0; GB2214705A; JPH01281635A; EP0326273A3

Abstract

A directly heated cathode for generating an electron beam along an axis (A) comprises two nested, dish-shaped conductors (4,5) on the axis joined electrically and mechanically on the axis by a spot weld (7), and supported by respective co-axial cylinders (1,2). A heater current supplied by the cylinders (1,2) flows between the conductor rims by way of the spot weld (7), and does not interfere with the electron beam generated by the outer conductor (5).

Description

This invention relates to a directly heated cathode which may, for example, be used to generate the electron beam in a travelling wave tube, magnetron or cathode ray tube. Directly heated cathodes are those which are heated by a current flowing in the electron-emissive component itself, rather than in a separate heater circuit.
Many conventional directly heated cathodes are complex and cumbersome due to the need for cooling mechanisms, to avoid mechanical deformation which would otherwise arise from thermal stress in the cathode and its support. This deformation would adversely affect the life of the cathode and would deflect the electron beam from its intended axis. The influence on the electron beam of magnetic fields generated by the heating current passing through the cathode has also been difficult to minimise.
The present invention seeks to overcome both these problems by providing a directly heated cathode comprising two thin, thermally deformable conductors which are both rotationally symmetric about a common axis and are connected electrically and mechanically in the region of their common axis, and means for supplying a heater current to flow between their respective peripheral regions by way of their said connection.
The rotational symmetry of the conductors is maintained in spite of thermal expansion and contraction, thus avoiding deflection of an electron beam emitting surface on one of the conductors. The conductors are thin and thermally deformable, so that cooling, although possible, is not necessary. Further, the symmetry of the conductors and the axial position of their connection ensure that the heater current is also distributed symmetrically about the axis and does not give rise to any magnetic field which would deflect the electron beam from the common axis.
Preferably, the conductors are nested, dish-shaped conductors whose peripheral regions are their rims. Thermal strain in these conductors is accommodated by relative axial movement of the rim and the base, and by a symmetrical deformation of the base. Preferably, each dish-shaped conductor has a substantially square section, the rim extending substantially parallel to the axis of the conductor.
The current supplying means preferably comprises two cylinders, each of whose end edge is connected to the peripheral region of a respective one of the conductors for mechanical support.
One way in which the invention may be performed will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a longitudinal section taken on the axis of the electron-emissive end region of a directly heated cathode embodying the invention, and
Figure 2 is a transverse section taken on the line X-X of Figure 1.

A directly heated cathode comprises inner 4 and outer 5 nested, dish-shaped conductors which are rotationally symmetric about a common axis A. The conductors 4, 5 each have a substantially square section, as shown in Figure 1, so that the rim extends parallel to the axis A. The base of each conductor 4, 5, is concave to produce the desired focussing of the emitted electron beam. Allowance is made for deformation to a different curvature when hot. The dish-shaped configuration also allows for relative axial movement of the rim relative to the base as a result of thermal expansion.
The conductors 4, 5 are thin sheet pressings of a tungsten-nickel alloy with a high tungsten content, preferably about 25 microns thick.
The outer surface of the base of the outer conductor 5 supports an electron-emissive surface, which may be a thin layer of barium, strontium or calcium oxide formed in a well-known manner. The surface may, for example, be a porous mush of sintered nickel powder impregnated with barium oxide.
A mesh 8 close to, but spaced (by about 1mm) from, the outer conductor 5 acts as a grid to control the electron current emitted from the cathode. The mesh 8 forms part of a dish-shaped conductor 6 similar to, but larger than, the inner and outer conductors 4,5, supported by a further coaxial support cylinder 3. This mesh 8 may be of nickel, molybdenum or suitable alloys, and is treated to reduce thermionic emission. In common with well-known practice, a further grid (not shown), known as a shadow grid, is interposed between the mesh 8 and the outer conductor 5, and is connected electrically to the outer conductor 5, its wires being aligned with those of the mesh 8 to protect them from molecules or ions which are inevitably emitted from the volatile electron-emissive surface.
The inner surfaces of the two conductors 4, 5 are connected mechanically and electrically on the axis A, over a region which is small in comparison with the size of the conductors, by a weld 7 which may be made by spot welding or arc or laser welding.
The rims of the conductors 4, 5 are brazed or welded to the outer end surfaces of respective support cylinders 1, 2 which are co-axial with the conductors 4, 5. The support cylinders provide mechanical support and also provide a means for supplying a heater current to the respective conductors 4, 5. To this end, the support cylinders 1, 2 have electrical terminals (not shown) for connection to a variable heater current supply unit (not shown).
The heater current flows from the rim of one of the conductors radially through that conductor, through the weld 7, and radially through the other conductor to the rim of that other conductor. Due to the rotational symmetry of the conductors and support cylinders and to the axial position of the weld 7, the distribution of heater current is also rotationally symmetric, and will not give rise to any magnetic field which would tend to deflect an electron beam on the axis A from that axis. Thus the electron beam emitted along the axis A by the outer conductor 5 remains undeflected by the heater current.
The heating of the cathode by the passage of the heater current is ohmic, and the distribution of the heat generated is therefore a function of the current density. The distribution of cathode temperature may be adjusted by appropriate choice of the lengths of the rims of the conductors 4, 5, and of the diameter of the central weld 7. Further, if necessary, holes could be laser cut in the rim of the outer conductor 5 so as locally to increase the current density and hence the ohmic heating.
As described above, thermal deformation of the cathode is accommodated so that the electron-emissive surface is maintained normal to the axis A, so that the electron beam is not deflected off the axis. The composition and thickness of the conductors 4,5,6 is such as to give sufficient flexibility for such thermal expansion, leading to a satisfactory working life: no provisions for cooling are required. The whole assembly has the advantage of being simply-constructed, rigid and robust, and no special provisions need to be taken to allow for expansion during warm up. It is anticipated that warm-up times as low as 2 seconds could be attained with such a cathode.
Further, the inductance of the heater is very low, enabling it to be used with a switched-mode power supply.

Claims

1. A directly heated cathode comprising two thin, thermally deformable conductors which are both rotationally symmetric about a common axis and are connected electrically and mechanically in the region of their common axis, and means for supplying a heater current to flow between their respective peripheral regions by way of their said connection.

2. A cathode according to claim 1, in which the conductors are nested, dish-shaped conductors, whose peripheral regions are their rims.

3. A cathode according to claim 2, in which the dish-shaped conductors each have a substantially square section, the rim extending substantially parallel to the axis of the conductor.

4. A cathode according to claim 1, 2 or 3, in which the supplying means comprise two cylinders, each of whose end edge is connected to the peripheral region of a respective one of the conductors for mechanical support.

5. A cathode according to any preceding claim, in which the outer surface of the outer conductor is coated with an oxide to enhance its electron emissivity.

6. A cathode according to any preceding claim, whose peripheral region has a plurality of apertures for determining the heater current density over the conductors.