Priority is claimed with respect to Great Britain Application No. GB 1005450.0 filed on Mar. 31, 2010, the disclosure of which is incorporated herein by reference in its entirety.
This invention relates to magnetrons.
Magnetrons are used in linear accelerator systems (linacs) to generate X-rays, and one use of such linacs is to generate X-rays for the treatment of tumours in radiotherapy. In an attempt to deliver the optimum dose of radiation to a tumour, linacs are being mounted on gantries which rotate around the patient, sometimes at high speed, while the X-ray dose is being delivered. This causes a problem, in that to achieve optimum performance the cathode must be held in a precise position in a hollow cylindrical anode with a high voltage between the anode and cathode. The cathode may be supported on a pair of electrically conducting arms which are anchored into the vacuum envelope at their ends.
Thus, referring to
At the free ends, the cathode support arms are connected to opposite ends of the cathode 1 by means of leads 29, 30. The cathode support arms 3, 4 terminate short of the cylindrical anode space 2, to allow room for the cathode to be inserted in an axial direction during manufacture (see
It is believed that the support arms 3, 4 are prone to pick up mechanical vibrations, which can impair the correct functioning of the magnetron.
In one embodiment of the invention there is provided a magnetron, comprising: a cathode having an axis; a vacuum envelope in which the cathode extends, the vacuum envelope including a portion extending radially relative to the axis of the cathode; cathode supply terminals; a pair of electrically conducting support arms for supporting the cathode and in electrical connection with the cathode supply terminals, the support arms having free ends connected to the cathode by leads; and a wall extending across an area of the radially-extending portion, wherein the wall is positioned along the radially-extending portion nearer to an end of the radially-extending portion that is adjacent to the cathode than to an end that is remote from the cathode, and the support arms are mounted in the wall.
Reducing the free length of the support arms by mounting them in a wall positioned intermediate the ends of the radially-extending portion in this way permits undesirable frequencies of vibration to be reduced or eliminated.
Each support arm may be in two parts secured together, one part, which could be made of tungsten, or molybdenum, or copper, or nickel, or alloys thereof, being mounted in the wall and having a greater diameter than the other part, which could be made of tungsten, or molybdenum, or alloys thereof, which is connected to the cathode. If desired, the support arms may be in more than two parts.
The wall may be formed integrally with the part of the radially-extending portion that is remote from the cathode, and may be connected to the part adjacent to the cathode by sealing material, in order to allow access to the wall for application of metallisation. The parts may be made of ceramic material.
The wall may be positioned along the radially-extending portion less than one third, or less than one quarter, of the length from the end adjacent the cathode to the end remote from the cathode.
The support arms preferably terminate outside the projection of the cylindrical anode profile, and leads, which may be of nickel wire, are welded or brazed to make the connection between the cathode and the cathode support arms during assembly of the magnetron.
One way of carrying out the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which:
a is a fragmentary end view of the magnetron of
Like reference numerals have been given to like parts throughout all the Figures.
The magnetron of the invention differs from the known magnetron of
Referring to
In contrast to the known magnetron of
The parts 19, 20 of the support arms have integral extensions 19a, 20a, and the cathode supply terminals 6, 7 are secured to the ends of the extensions. The terminals and the integral extensions are protected by the tubular member 17, which also holds off the high voltage between the anode body and the terminals 6, 7. If desired, the empty space within the tubular member 17 could be filled with rubber material in order to prevent corona discharge taking place within this space.
RF chokes are provided to prevent the leakage of RF through the radially-extending portion. Thus, the metal ring 11 has a quarter-wavelength choke 21, to prevent leakage of RF around the periphery of the opening into the radially-extending portion, and hollow sleeves 22, 23, also quarter-wavelength in length, surround the portions 19, 20 of the cathode support arms, to prevent leakage of RF along the cathode support arms 3, 4. The RF chokes 21, 22, 23 overlap each other. The RF chokes 22, 23 are provided with flared regions 22a, 23a.
In order to maintain the integrity of the vacuum seal at the wall 18, the portions 19, 20 of the cathode support arms are brazed to narrower diameter regions of the hollow sleeves 22, 23, which are in turn brazed at their upper ends to the underside of the wall 18, the mating surfaces being metallised. In addition, the flared regions 22a, 23a are designed to overlap the metallised rings (24, 25—
Such a metallising operation could be awkward to perform in the restricted space beneath the wall 18. For this reason, the tubular ceramic member 17 may be joined to the wall either during or after the metallising operations have been carried out. A layer of powdered glass 26 may be used to seal the parts together in a vacuum tight manner.
The wall 18 is positioned at least half-way along the length of the sidearm from the upper end to the lower end as seen in
The free length of the arms 3 and 4 are much shorter than in the known magnetron of
Various factors affect the resonance frequency of the cathode support arms. Thus, the resonance frequency depends on the stiffness of the arms, and it will be noted that the diameter of the regions 19, 20 of the support arms is greater than that of the regions 27, 28 that are connected to the cathode. In addition, the choice of larger cross section materials for the regions 19, 20 also has the benefit of increasing the heat loss through conduction from the cathode and its adjoining components. This may be advantageous if the magnetron is operated close to its upper limit for mean output power.
Suitable materials for the parts 27, 28 of the support arms are tungsten, molybdenum or other high temperature melting point metals or alloys. Suitable materials for the parts 19, 20 include tungsten, molybdenum and their alloys, copper, nickel and other alloys of nickel.
Of course, variations may be made to the embodiment described without departing from the scope of the invention. Thus, for example, the wall 18 could form the base of a very deep cup secured to the upper end of sleeve 17, that is, similar to the wall 13 of the prior art construction shown in
The invention is especially suitable for magnetrons with peak output powers from 2 MW. A typical range of operating frequencies is from 2850 MHz to 3010 MHz, the design being especially suitable for 2993 MHz to 3002 MHz.
It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.
Number | Date | Country | Kind |
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1005450.0 | Mar 2010 | GB | national |
Number | Name | Date | Kind |
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2437880 | Kusch | Mar 1948 | A |
6756735 | Lee et al. | Jun 2004 | B2 |
20090236991 | Wilson | Sep 2009 | A1 |
Number | Date | Country |
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201374306 | Dec 2009 | CN |
Entry |
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Great Britain Search Report of Application No. GB1005450.0 dated Aug. 3, 2010. |
Great Britain Search Report of Application No. GB1104879.0 filed Jul. 21, 2011. |
Number | Date | Country | |
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20110241542 A1 | Oct 2011 | US |