GB2422050A

GB2422050A - Inductive output tube

Info

Publication number: GB2422050A
Application number: GB0510135A
Authority: GB
Inventors: Edward Stanley Sobieradzki; Steven William Hurrell; Stuart Andrews; Ian Milsom
Original assignee: e2v Technologies Ltd
Current assignee: e2v Technologies Ltd
Priority date: 2005-05-18
Filing date: 2005-05-18
Publication date: 2006-07-12
Also published as: GB0510135D0

Abstract

An IOT intended for use at high frequency, for example, over 1GHz, has a grid/cathode gap of the order of fractions of a millimetre, and this causes a problem on switch-on of the IOT due to the thermal expansion undergone as the cathode is heating up. According to the invention, a structure 10 is provided for mounting the cathode 1 on the grid-supporting structure 7, the structure 10 comprising, for example, two metal cylinders 11, 12 of a molybdenum/rhenium alloy separated by an annulus of ceramic material 13 to provide DC isolation between the cylinders 11, 12. The expansion coefficients of the metal cylinders 11, 12 are matched to that of the cathode 1 and the position along the length of the cathode at which it is supported such that expansion of the cathode on the grid side of the support position is compensated by an equal expansion of the two metal cylinders connecting to the grid support, in order to maintain the grid/cathode gap constant over the temperature excursion on start-up.

Description

ELECTRON BEAM TUBE

This invention relates to electron beam tubes, especially inductive output tubes.

Inductive output tubes (lOTs) include a cathode and anode, separated by a control grid to which an r.C input is applied SO as to density modulate a linear beam of electrons travelling between the cathode at a highly negative potential and the anode at ground.

The output is extracted by electromagnetic induction in a gap between the anode and a drift tube in an output cavity. Thereafter the electron beam strikes a collector.

The grid cathode spacing is critical to the performance of the JOT, but can vary during warm-up of the tube. Naturally, the cathode must be heated on start-up to provide the supply of electrons fhr the electron beam. Typically, the grid and the cathode are individually secured to the inner wall ofthe vacuum tube. J'hus. on start-up, the cathode expands and, as it is supported at the end remote from the grid, expands towards the grid. Eventually, the grid itself becomes heated and also moves, away from the cathode.

Thus the cathode grid spacing reduces as the cathode warms up, before stabilising to the desired thermal equilibrium spacing.

For operating frequencies in the hundreds of Ml lz, (sub I (iHz) region, the equilibrium gap is of the order of 0.25 millimetre, and the variation of the gap on start-up is not critical.

However, for operation at microwave frequencies, where the cathode grid gap is of the order of a tenth of a millimetre or less, some form of temperature compensation of the gap is necessary. To this end, it has been proposed in US Patent No. 6 664 720 to support the grid on the adjacent end of the cathode, so that the only variation of the gap stems from expansion of the small length of cathode between its tip and the position where the grid is supported.

A niore exact form of temperature compensation for the gap has been proposed for l'ravelling Wave Tubes, but its use in lOTs, which typically have much greater diameters, has not previously been proposed.

The invention provides an inductive output tube, including a grid, a cathode, a structure for mounting the grid within the interior of the vacuum envelope, and a structure fOr supporting the cathode on the grid mounting structure, the expansion co-efficients of the cathode and cathode supporting structure being such that the grid cathode spacing remains substantially constant as the cathode warms up in use.

Such an arrangement provides superior consistency of the cathode grid gap than the 1O1' temperature compensation arrangement previously proposed and, while a similar arrangement has been proposed for TWTs, such an arrangement has not hitherto been contemplated fOr lOTs.

One way of carrying out the invention will flow he described in detail, by way of example, with reference to the accompanying drawing, which is a sectional view of a part of an inductive output tube, partly in schematic form, in accordance with the invention.

Referring to the drawing, the lOT comprises a cathode indicated generally by the reference numeral I with a dished tip 2 and a grid 3 which is spaced from the tip 2 by a fraction of a millimetre, the gap being exaggerated in the drawings. A linear beam of electrons flows from the cathode I at a highly negative potential, to an anode 4 at ground potential. Amplification takes place between the input which is applied as an r.f signal to the grid, and the output which is extracted inductively from the region of the vacuum envelope 5 between the anode 4 and a drift tube 6. The electron beam continues through the hollow anode 4 and drift tube 6 to a collector (not shown).

In the usual way, the grid is supported by a structure indicated generally by the reference numeral 7 on the interior of the vacuum envelope. The structure in this instance comprises an annular ring of ceramic material 8, which connects to the interior of the envelope, and a clamping ring 9, also of ceramic material, between which the grid is held.

In accordance with the invention, a structure indicated generally by the reference numeral 1 0 is provided, for mounting the cathode I on the gridmounting structure 7.

The structure 10 comprises a two-past thin-walled metal cylinder, the parts I 1, 12 being separated by an annular insulator 1 3 of ceramics material, to provide D.C. isolation between the cathode I and grid 3. The upper end, as seen in the drawing, of the upper part 11 of the cylinder, is connected by a flange 14, for example, by welding, to the periphery of the cathode I. Ihe materials of the cathode 1, and the Iwo parts 11, 12 of the cylinder, are chosen in relation to each other and in relation to the position along the length of the cathode at which the flange 14 meets the cathode, so that expansion of the part of' the cathode beneath the flange, as seen in the drawing, is matched very closely by that of the two- part cylinder, as the cathode and the grid warm up. The cathode is of course provided with the usual heater (not shown). Suitable materials for the parts 11, 12 are a molybdenum/rhenium alloy.

Typically, the temperature excursion undergone is of the order of 10000 C. At an operating frequency in the Gigahertz range, for example, 1.3 GHz, the cathode grid gap is of the order of 0.1 millimetre.

The uppermost part of the cathode I, as seen in the drawings, has connections (not shown) to the atmospheric side of the vacuum envelope, which are sufficiently flexible to allow expansive movement of that end of the cathode without resulting in movement of the cathode itself with respect to the grid.

The advantage of the invention is that it provides a mechanism that maintains precise cathode to grid spacing throughout the temperature transients associated with switch-on of the tube. The invention is advantageous for lOl's operating at frequencies in excess of I Gllz.

Claims

I. An inductive output tube, including a grid, a cathode, a structure for mounting the grid within the interior of the vacuum envelope, and a structure for supporting the cathode on the grid mounting structure, the expansion co-efficients of the cathode and cathode supporting structure being such that the grid cathode spacing remains substantially constant as the cathode warms up in use.
2. An inductive output tube as claimed in claim I, in which the cathodesupporting structure includes a pair of coaxial cylinders separated by an annulus of insulating material.
3. An inductive output tube as claimed in claim 2, wherein the coaxial cylinders are made of metal.
4. An inductive output tube as claimed in claim 3, wherein the metal is a molybdenum! rhenium alloy.