GB2310539A

GB2310539A - Electromagnets for magnetrons

Info

Publication number: GB2310539A
Application number: GB9603650A
Authority: GB
Inventors: Michael Barry Clive Brady; Paul Simon Burleigh
Original assignee: EEV Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1996-02-21
Filing date: 1996-02-21
Publication date: 1997-08-27
Anticipated expiration: 2016-02-21
Also published as: GB2310539B; GB9603650D0

Abstract

The electromagnet 2 comprises a permanent magnet 8 and an electrically conductive coil 9 disposed about a ferrous core 10. A layer 11 of thermal insulation is interposed between the coil and the permanent magnet in order to reduce degradation of the permanent magnetic field caused by heat produced by the current-carrying coil, which was a problem hitherto. Coupling between the ferrous core and the surrounding coil increases the magnitude of the variable magnetic field produced by the coil. The insulating layer may be PTFE or an air gap. A similar arrangement is positioned at the other end of the cylindrical cathode 5 and anode 6 of the magnetron.

Description

AN ELECTROMAGNET This invention relates to electromagnets, and particularly although not exclusively to electromagnets for magnetrons.

Magnetrons are devices which produce high-frequency microwave radiation, and are typically used in industrial heating systems. A typical magnetron comprises a cylindrical cathode surrounded by a coaxial cylindrical anode having vanes on its inner surface so as to form cavity resonators. Electrons emitted from the cathode are attracted to, and hence travel towards, the anode. To set up a travelling electron microwave in the magnetron a coaxial magnetic field is introduced which deflects electrons as they travel towards the anode, causing the electrons to travel in a cycloidal path.

The magnetic field is typically provided by an electromagnet comprising a permanent magnet with a surrounding current-carrying coil. The permanent magnet produces a constant magnetic field whilst the coil provides a further magnetic field, the magnitude of which may be adjusted as required by varying the amount of current in the coil. It has been found that the performance of this device degrades with time.

The invention provides an electromagnet comprising a permanent magnet, an electrically conductive coil disposed about a ferrous core, the core being in contact with the permanent magnet, and means for thermally insulating the coil from the magnet.

It has been realised that the provision of thermal insulation between the coil and the permanent magnet reduces degradation of the permanent magnetic field caused by heat produced by the current-carrying coil. In addition, the invention permits a greater variable magnetic field to be achieved than hitherto, due to coupling between the coil and the ferrous core.

Advantageously, the ferrous core may be of mild steel and the layer of thermal insulation may be manufactured from PTFE (poly-tetra-fluoroethylene).

The invention will now be described by way of example, with reference to the accompanying drawing, Figure 1, which is a sectional view of a magnetron assembly incorporating an electromagnet constructed in accordance with the invention.

Referring to Figure 1, the magnetron assembly comprises a magnetron unit indicated generally by the reference numeral 1, electromagnets 2, 3 and screen box 4 which screens the magnetron unit and electromagnet from unwanted external radiation and external magnetic fields. The magnetron unit 1 comprises cylindrical cathode 5, from which electrons are emitted, and coaxial cylindrical anode 6 having a plurality of vanes 7 on its inner surface. For clarity, the power supply to the magnetron unit and the output for extracting microwave energy from the magnetron unit are not shown in this diagram.

The electromagnet 2 supplies a magnetic field in order to deflect electrons as they travel from cathode to anode, establishing cycloidal electron motion. The fields associated with the electrons induce r.f. fields in the resonant cavities formed by the anode 6 and the vanes 7 and, due to the closed nature of the circuit which provides positive feedback, high-frequency microwave oscillations are set up.

The magnetic field provided by the electromagnet 2 must be of sufficient strength to deflect electrons away from the anode 6, and to cause the electrons to rotate about the cathode 5. Hence, the electromagnet 2 provides a magnetic field which can be varied in order to sustain the required magnetic field strength.

Electromagnet 2 comprises permanent magnet 8, electrically conductive coil 9, ferrous core 10 and thermally insulating layer 11. The permanent magnet 8 produces a constant background magnetic field. A plate 12 is affixed to the magnet 8, and is manufactured from a ferrous material e.g. mild steel. The plate 12 unifies the magnetic field potential across the face of the magnet, reducing fringe effects at the edges of the face of the magnet. Thermally insulating layer 11, which may be, for example, manufactured from PTFE, is interposed between magnet 8 and coil 9. This layer 11 prevents heat produced by the current-carrying coil 9 from coming into contact with, and thereby degrading, the permanent magnet 8. The passage of electrical current through the coil 9 gives rise to a magnetic field, the strength of which increases with increasing current. The coil is wound around ferromagnetic core 10, of e.g. mild steel, which enhances the magnetic effect of the current. The coil 9 is mounted onto an inner surface of the screen box 4 with an interposed ferrous plate 13 which may be, for instance, a plate of mild steel and which prevents magnetic saturation occurring.

A second electromagnet 3, identical to the electromagnet 2 described above, is mounted on the opposite inner surface of screen box 4 to that on which electromagnet 2 is mounted. Electromagnet 3 is arranged so that the magnetic field it produces is of the same orientation as that produced by electromagnet 2, thus reinforcing the magnetic field in the magnetron.

Variations may be made without departing from the scope of the invention. For instance, core 10 and plates 12 and 13 need not be of mild steel as any ferrous material would suffice. Insulation layer 10 may be made from thermally insulating material other than PTFE, or could even be an air gap. Furthermore, the invention is not restricted to use in magnetron arrangements as it could be used in any device employing an electromagnet.

Claims

1. An electromagnet comprising a permanent magnet, an electrically conductive coil disposed about a ferrous core, the core being in contact with the permanent magnet, and means for thermally insulating the coil from the magnet.

2. An electromagnet, as claimed in claim 1, in which the ferrous core is mild steel.

3. An electromagnet, as claimed in claim 1 or 2, also having a ferrous plate mounted on the permanent magnet.

4. An electromagnet, as claimed in any one of claims 1 to 3, in which the insulation means comprises PTFE.

5. A magnetron including an electromagnet as claimed in any one of claims 1 to 4.

6. A magnetron including an electromagnet as claimed in claim 5, also having a second electromagnet arranged such that the magnetic fields produced by the electromagnets reinforce each other.

7. An electromagnet, substantially as hereinbefore described with reference to, or as illustrated in, the accompanying drawing.