A magnetron has an anode body ( 1 ) and including a ceramic sleeve ( 7 ). In higher power generators, stray radiation is emitted from this sleeve in addition to the main power launched from the antenna into the waveguide ( 2 ), and RF absorbing material is provided. Such absorbers, however, tend to be frequency-selective, and can overheat. According to the invention, a non-metallic jacket ( 13 ) containing a dielectric liquid such as water surrounds the sleeve. This provides absorption over a broad band of frequencies, and it is easy to make the jacket have a sufficiently high thermal capacity, for example, by arranging a flow of liquid through it.
1. A magnetron having an axial direction, comprising: a vacuum envelope; an electrically insulating sleeve forming part of the vacuum envelope; supply leads for connecting to a magnetron cathode and extending through the insulating sleeve; and having an absorber to absorb stray microwave radiation radiated through the electrically insulating sleeve, the absorber comprising a non-metallic jacket containing a dielectric liquid, wherein the jacket surrounds the electrically insulating sleeve.
2. The magnetron as claimed in claim 1 , in which the electrically insulating sleeve is extensive in the axial direction of the magnetron.
3. The magnetron as claimed in claim 1 , in which the electrically insulating sleeve comprises a sidearm.
4. The magnetron as claimed in claim 1 , in which the jacket includes a cooling circuit to permit liquid to flow through the jacket.
5. The magnetron as claimed in claim 4 , in which the cooling circuit comprises a coiled pipe within the jacket to contain the flowing liquid.
6. The magnetron as claimed in claim 4 , in which the cooling circuit comprises a pipe which has undulations including sections parallel to the axis of the magnetron and extends around a periphery of the jacket.
7. The magnetron as claimed in claim 1 , in which the dielectric liquid comprises water.
8. The magnetron as claimed in claim 1 , in which the jacket comprises a plastics material.
9. The magnetron as claimed in claim 1 , in which the magnetron includes a water-cooled electromagnet having a cooling circuit for providing a main field, and a single supply is connected to the cooling circuit for the electromagnet and to a cooling circuit for the jacket.
10. The magnetron as claimed in claim 1 , in which the magnetron includes a water-cooled anode-block and a single supply is connected to the cooling circuit for the anode and to a cooling circuit for the jacket.
11. The magnetron as claimed in claim 10 , in which incoming water flow has a low impedance path to the cooling circuit for the anode and a high impedance path to the cooling circuit for the jacket.
12. The magnetron as claimed in claim 1 , wherein the non-metallic jacket is a hollow jacket containing the dielectric liquid, surrounding and sufficiently spaced from the insulating sleeve to prevent build-up of moisture by condensation.