Magnetron

1. A magnetron, comprising:

an anode;

an anode casing at least partly surrounding the anode;

a pair of permanent magnets on each side of the anode defining an interaction region and creating a magnetic circuit defining a magnetic field through the interaction region;

a mass of magnetically permeable material positioned in a vicinity of the magnetic circuit, the mass being arranged to be slidable over the anode casing; and

a locking device to secure the position of the mass to set the strength of the magnetic field through the interaction region.

2. A magnetron as claimed in claim 1 , wherein the mass comprises a slider movable in a direction tangentially to the anode casing.

3. A magnetron as claimed in claim 2 , wherein the locking device comprises a bolt to enable the slider to be locked into position.

4. A magnetron as claimed in claim 2 , further including a surface with serrations operatively arranged with the slider to assist in clamping of the slider.

5. A magnetron as claimed in claim 2 , further including an output waveguide and a guide mounted on the output waveguide, wherein the slider is slidable on the guide.

6. A magnetron as claimed in claim 5 , wherein the slider includes a channel-shaped region in engagement with the guide.

7. A magnetron as claimed in claim 1 , wherein the mass comprises a rotary member.

8. A magnetron as claimed in claim 7 , wherein the rotary member has a magnetic permeability that is non-uniform in a circumferential direction.

9. A magnetron as claimed in claim 1 , wherein the permanent magnets comprise alloys including at least one of samarium, neodymium or other rare earth metals.

10. A magnetron as claimed in claim 9 , wherein the permanent magnets comprise allows of samarium-cobalt or neodymium-iron-boron.