A magnetron has an anode and a cathode. The cathode includes two parts joined by sleeves of ferrous alloy spaced by a sleeve of insulating material. The ferrous alloy sleeves are adapted to be connected to opposite poles of a power supply for heating the cathode. A high frequency power supply is used to heat the cathode. The ferrous alloy sleeves have a surface coating of conductive material. The currents induced by the magnetic field generated by the high frequency currents of the power supply are largely confined to the conductive coating due to the skin effect, avoiding the heating of and losses in the ferrous alloy itself which would otherwise ensue.
1. A magnetron comprising: a hollow cathode including a filament as an electron emissive part, and an outer sleeve containing a core, the hollow cathode being arranged to apply a high frequency voltage between the outer sleeve and the core to heat the filament; and sleeves of ferrous alloy spaced by a sleeve of insulating material joining the core and the outer sleeve, the ferrous alloy sleeves having magnetic flux induced in them, in use, from the high frequency voltage that heats the filament, and the ferrous alloy sleeves having a conductive surface coating with a resistivity of substantially that of copper or silver.
2. The magnetron as claimed in claim 1 , in which the frequency of the high frequency voltage is within the range of from 1 kHz to 1 MHz.
3. The magnetron as claimed in claim 2 , in which the frequency of the high frequency voltage is within the range of from 5 kHz to 500 kHz.
4. The magnetron as claimed in claim 1 , in which the conductive material is continuous on both inner and outer curved surfaces of the ferrous alloy sleeves.
5. The magnetron as claimed in claim 4 , in which a thickness of the conductive coating is within the range of from 1 micron to 50 microns.
6. The magnetron as claimed in claim 5 , in which the thickness of the conductive coating is within the range of from 5 to 30 microns.
7. The magnetron as claimed in claim 1 , in which the conductive material is copper.
8. The magnetron as claimed in claim 1 , in which the ferrous alloy of the sleeves is a nickel cobalt ferrous alloy.
9. The magnetron as claimed in claim 8 , in which the ferrous alloy is Kovar.
10. The magnetron as claimed in claim 1 , in which the insulating material is a ceramic material.
11. The magnetron as claimed in claim 1 , in which a connection of the ferrous alloy sleeves to the sleeve of insulating material is a vacuum tight connection.
12. The magnetron as claimed in claim 1 , in which the ferrous alloy sleeves include a first ferrous alloy sleeve and a second ferrous alloy sleeve, and the first ferrous alloy sleeve, the sleeve of insulating material, and the second ferrous alloy sleeve are juxtaposed along a common central axis.