Priority is claim with respect to Great Britain application No. GB 1005412.0 filed Mar. 31, 2010, the disclosure of which is incorporated herein by reference in its entirety.
This invention relates to magnetrons.
Magnetrons typically use permanent magnets to set up a magnetic field through the interaction region. AlNiCo is often used as the magnetic material and is relatively easy to magnetise. As a result, it is found convenient to buy the material in a demagnetised state and to magnetise it in the finished magnetron. It is even possible to make fine adjustments to the magnetic field strength by controlled degaussing of the magnet using an alternating magnetic field generated by coils carrying an a.c. current.
Use of high energy magnetic materials such as samarium-cobalt or neodymium-iron-boron enables much smaller and lighter magnetrons to be realised but such magnetic material is much more difficult to magnetise and it is generally necessary to magnetise the material during manufacture, meaning that the magnets are bought in a fully magnetised state.
However, it may sometimes be necessary to trim the magnetic field in order that the magnetron will operate at the desired operating point of current and voltage.
Some existing methods of adjusting the magnetic field strength existing in a magnetron are described with reference to
A magnetron is an evacuated device comprising a plurality of resonant cavities surrounding an interaction region where electrons emitted from a hot cathode are subjected to the combined effects of crossed electric and magnetic fields. The magnetic field is often focussed across the interaction region by means of high permeability pole-pieces, which sometimes form part of the vacuum envelope. Detail of the magnetron is omitted from
The magnetic field can be generated by a horseshoe magnet or by a pair of magnets with a magnetically permeable return path. The field can be applied directly without pole pieces but more commonly the field is concentrated by means of high permeability pole-pieces. The pole pieces may be in intimate contact with the magnet(s) or they may connect via an intermediate pole-shoe for convenience in construction.
One known method of adjusting the strength of the magnetic field through the magnetron is by the use of corner shunts, such as that illustrated by the reference numerals 11, 12. These corner shunts are of mild steel, and some of the magnetic flux is diverted through them. This reduces the magnetic field available to extend through the magnetron itself. They can be used where it is desired to reduce the magnetic field strength in the working gap between the pole pieces 1, 2.
Alternatively, flat shunts, consisting of one arm only of the illustrated corner shunts, may be employed to reduce the magnetic field in the working gap.
Another known method of achieving this objective is to provide additional sheets of thin mild steel for the magnetic return path.
It has been proposed (U.S. Pat. No. 4,338,545) to adjust the magnetic field in the interaction space to compensate for changes in field strength resulting from temperature variation by automatic displacement of auxiliary pole pieces in response to deformation of a bimetallic member.
It has also been proposed (UK Patent No. 826 822) to displace a magnetic shunt between the pole pieces and pole shoes of a magnetron in a radial direction towards the axis of the anode in order to considerably reduce the magnetic forces to assist in the magnetron being assembled/disassembled.
In one embodiment of the invention there is provided 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.
It is possible with the arrangement to make fine adjustments to the field strength through the interaction region.
The mass may be a slider movable in the direction of a tangent to the anode casing, and may be securable with a bolt, with optional serrations to assist the clamping of the slider. The slider may be slidable on a guide mounted on an output waveguide from the magnetron, and may include a channel-shaped region in engagement with the guide.
The anode casing over which the mass is slidable may be the exterior of the anode body, or the exterior of an additional casing at least partly surrounding the anode body.
Alternatively, the magnetically permeable member may be a rotary member.
Ways of carrying out the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which:
Like reference numerals have been used for like parts throughout all the drawings.
Referring to
The magnetic field strength through the interaction region of the magnetron is adjustable by means of a slider 20 of mild steel. The slider is constrained to travel in an axial direction only, because the rear portion 21 is channel-shaped and slides over a rib 22 of the casting 18. The upper surface of the rib has side-to-side serrations, as has the mating face of the channel-shaped region, and the rib is locked in a desired position by tightening a bolt (not shown) which extends through an aperture in the slot in the upper surface of the channel into a threaded hole in the face of the rib.
The track can be marked with gradations as a setting aid for the operator.
Referring to
When a desired field strength through the magnetron has been achieved, the bolt is tightened.
Variations may of course be made without departing from the scope of the invention. Thus, in order to provide greater adjustment the slider may be wider, such as the alternative version 20a shown in dotted lines in
For finer adjustment, the tip of the slider may be triangular, or profiled in some other way.
As a further alternative, shown in fragmentary form in
As a further alternative, the rotary adjuster could be of uniform magnetic permeability, but non-circular, that is, eccentric-shaped.
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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GB1005412.0 | Mar 2010 | GB | national |