This application claims the priority of Great Britain Application No. 0805277.1, filed Mar. 20, 2008, the contents of which is incorporated herein by reference.
This invention relates to magnetrons.
A known magnetron will now be described with reference to
The cathode 4 is tubular, and has a heater extending along its axis, and a D.C. supply to the heater, as well as a high negative voltage for the cathode, is supplied to the cathode by means of conducting supports 5,6, which extend into an upper region 7 of the magnetron, the interior of which is within the vacuum envelope. The conducting supports connect to terminals on a part of the exterior of the upper region that is not shown. Couplers (not shown) extend into a resonant cavity and withdraw power into an output section 8, which can be coupled to a waveguide.
Referring to
The cathode is supported on radial arms 5, 6 that enable the magnetic field to be applied directly by a separate electromagnet (not shown). The gap over which the magnetic field is applied is desirably minimized so the electromagnet is as small as possible and uses least power. The vacuum gap between the ends of the tubular cathode structure and the end walls 1,2 of the magnetron has to be sufficient to hold off the negative voltage, typically 50 KV, that is applied to the cathode relative to the anode and the magnetron body, including end walls, under normal working conditions. Experience has shown that the cathode to end wall gap is not adequate to prevent arcing under all conditions (particularly when driven with line-type modulators) and very occasionally this can have serious consequences when the arcing causes the end wall to puncture. It is believed that in addition to the applied
pulse voltage across the cathode to side wall gap there are RF voltages picked up from where the cathode supports pass near the anode, particularly if there is a projection such as if the anode is provided with a strap 11. These picked-up voltages may be increased by resonances on the cathode supports or in the space between end plate and anode.
The heater connection 12 on the cathode is a sharp point, which further enhances the voltage stress in this area. The result is that the heater connection can form a seat for arcing, which can confine any arcing that occurs to the region of the wall 1 that is immediately adjacent, and thus increase the risk of perforation.
The Applicants contemplated counteracting this risk by the expedient of increasing the gap between side wall and cathode but this would mean the outline of the magnetron would need to change. However, there are thousands of equipments currently in use, which require the current profile for the magnetron, so such a modification would be disadvantageous. It is also undesirable to make anything more than the minimum change to the interior of the magnetron, as any change risks upsetting its operation.
The invention provides a magnetron comprising a cathode, an anode surrounding the cathode, the region between the anode and the cathode being within a vacuum chamber, a heater for the cathode having a D.C. supply connection at an end of the cathode, and a cover of conducting material interposed between the D.C. supply connection and the adjacent end of the vacuum chamber.
The cover to the heater terminal on the cathode hides any sharp point, thereby reducing voltage stress.
The cover plate could be made of any conductor that is vacuum tube compatible. Nickel or a nickel alloy would be suitable because of its availability, ease of machining and ease of joining.
The invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which:
Like parts have been given like reference numerals throughout all the drawings.
The invention can best be appreciated by considering
There will of course be the same voltage between the cover 13 and the adjacent wall 1 of the vacuum chamber (since the face of the plate follows the line of the original support 5), but the sharp point 12 is now electrostatically shielded, so that there is no tendency for such arcing as may occur to take place preferentially in the region of the wall 1 immediately adjacent the connection 12. Such arcing as occurs will be spread over the surface of the cover and over a corresponding area of the wall. Thus, the risk of perforation of the wall have been reduced or eliminated.
The invention is especially applicable to high power magnetrons, that is, magnetrons with peak output powers exceeding 1 MW. A typical range of operating frequencies is from 1 GHz to 20 GHz, the design being especially suitable for S-band operation, that is, from 2 GHz to 4 GHz. Such magnetrons are suitable for use in linear accelerators.
Number | Date | Country | Kind |
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0805277.1 | Mar 2008 | GB | national |
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2815467 | Gardner | Dec 1957 | A |
3859558 | Harada et al. | Jan 1975 | A |
4686413 | Sato et al. | Aug 1987 | A |
5216327 | Myers et al. | Jun 1993 | A |
5861716 | Ogura | Jan 1999 | A |
6132565 | Lin | Oct 2000 | A |
6420821 | Kishiki et al. | Jul 2002 | B1 |
Number | Date | Country |
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1992135 | Jul 2007 | CN |
0627757 | Dec 1994 | EP |
1804554 | Jul 2007 | EP |
825566 | Dec 1959 | GB |
Number | Date | Country | |
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20090236991 A1 | Sep 2009 | US |