GB2259181A

GB2259181A - Magnetron

Info

Publication number: GB2259181A
Application number: GB9123924A
Authority: GB
Inventors: Peter Frederick Lewis
Original assignee: EEV Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1991-08-30
Filing date: 1991-11-11
Publication date: 1993-03-03
Anticipated expiration: 2011-11-11
Also published as: GB9123924D0; GB9118593D0; GB2259181B; DE4228687A1; FR2680912A1

Abstract

A magnetron has a vacuum envelope comprising an anode having a plurality of internal resonant cavities and a cathode located within the anode. In Fig. 3 the anode block 1 is located within a magnetic field in between a pair of pole pieces 45, 46 which are each attached to opposing walls of a metal housing 40 which functions both as a yoke for the pole pieces and a Faraday cage to prevent the emission of unwanted electromagnetic radiation from the magnetron. In Fig. 2 the magnetron has an elongate cathode 8 which is supported at each end by an elongate conducting pin 9 which serves for supplying electrical current to the cathode, and a choke or shielding arrangement 15 surrounds each pin 9 for preventing the emission of electromagnetic radiation. The shielding 15 is formed by two similarly dimensioned, coaxially spaced tubes 16, 17 each of length 1/4 of the output wavelength of the magnetron and connected respectively to the anode and the cathode, and a larger diameter third tube 20, preferably of the same length as tubes 16, 17, surrounds the axial separation 18 of the tubes 16, 17. A method of attaching the conducting pin 9 to the cathode 8 is also disclosed in which pin 9 is inserted in a first hole 11a in cathode end cap 11 and a welding, brazing or like tool is inserted in a second hole 14 which connects with the first hole 11a to effect the join between the pin 9 and end cap 11. <IMAGE>

Description

-.?1 '1 - 1 MAGNETRON The invention relates to a magnetron and especially

to one which, in at least one aspect, includes a high degree of shielding to prevent the emission of electromagnetic radiation.

0 The invention provides in one aspect a magnetron having a vacuum envelope comprising an anode having a plurality of resonant cavities, a cathode located within the anode, an output probe of the vacuum envelope extending outwardly from the anode, the vacuum envelope being located within a magnetic field in between a pair of pole pieces, the pole pieces being attached to opposing walls of a metal housing, the output probe extending out through an opening in the housing which is operative to provide both a yoke for the pole pieces and a Faraday cage to prevent the emission of unwanted electromagnetic radiation from the magnetron.

In a preferred aspect a coil surrounds each pole piece to provide an electromagnet within the housing, each coil being surrounded by an enclosure, conductors extending to and from each coil through the housing wall to within its respective enclosure for supplying electrical power to the 0 - 2 coils, thereby to prevent electromagnetic radiation being coupled into the coils and radiated from the conductors.

The housing may comprise two planar sheets of metal bent into a U shape and arranged together with the upright limbs overlapping one another to define four walls of the housing, a further two walls each comprising an apertured grille.

It is also preferred that the vacuum envelope is detachable from the metal housing to allow the envelope to be replaced without the expense of replacing the metal housing and pole pieces. In a high power magnetron, i. e. one having a power output in excess of about SKW or so, this is a great advantage since the pole pieces and metal housing can be relatively large and expensive to manufacture, especially when they incorporate a high degree of R.F. shielding.

In another aspect the invention provides a method of assembling a magnetron which comprises an anode having a plurality of internal resonant cavities, the method including the steps of inserting an elongate cathode centrally within the anode, an end cap being secured to an end of the cathode, the end cap including a first hole 4 1 extending within the end cap transversely to the longitudinal axis of the cathode, and a second hole extending between a surface facing away from the cathode and at the interior of the first hole, the method including the further steps of inserting, within the first hole, an elongate conductor operative in the assembled magnetron to provide a means of support and a means of passing electrical current to the cathode, and, having inserted the conductor within the first hole, locating a welding, brazing or like tool within the second hole, and operating the tool to electrically and mechanically connect the elongate connector to the end cap.

Such a method of assembly has been found to be reliable and efficient. In a preferred aspect the above steps are repeated to secure another elongate conductor to the other end of the cathode. The end cap may include a spigot portion and the cathode a helical coil, the method including the step of welding, brazing or otherwise electrically and mechanically connecting the spigot portion within an open end of the coil.

In yet another aspect the invention provides a magnetron comprising an anode having a plurality of internal resonant cavities, and a cathode supported within the anode A 4 by a support means extending outwardly from the anode, the support means comprising an elongate conductor, insulated from the anode, for supplying electrical current to the cathode, the conductor being surrounded by shielding to prevent the emission of radiation from the conductor the shielding comprising a first tube surrounding and radially spaced from a length portion of the conductor and with a length substantially equal to a 1/4 of the wavelength of the output signal of the magnetron, the first tube being electrically connected to the anode; a second tube of substantially the same length and diameter as the first tube surrounding and radially spaced from another length portion of the conductor, the first and second tubes being axially separated from one another, the second tube being electrically connected to the cathode; and a third tube, of greater diameter than the first and second tubes, being radially spaced from and extending across the axial separation between the first and second tubes.

Such a shielding arrangement has been found, unexpectedly, to provide a high degree of electromagnetic shielding. The third tube is preferably connected to the anode.

In a preferred aspect the first tube is secured within 11 the walls of an opening extending through the anode, the third tube being of substantially the same length as the first tube and secured within the walls of a stepped recess surrounding the opening in the anode. A terminal may be connected to the free end of the elongate conductor, and a sleeve formed from an insulating material may extend between the terminal and the anode.

In order that the invention may be well understood, an embodiment thereof will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a view from one side of a vacuum envelope for a magnetron with the end plates removed to show the interior of the envelope; Figure 2 is a cross-sectional view along lines A-A of Figure 1; Figure 3 is a cross-sectional view of the housing and magnet assembly within which the envelope of Figures 1 and 2 is located, the envelope not being shown in section in this view; and Figure 4 is a view looking in the direction of arrow B - 6 in Figure 3.

A vacuum envelope for a magnetron shown in Figures 1 and 2 comprises a comprises a generally cylindrical anode block 1 having an open-ended internal cavity 2. A plurality of circumferentially spaced anode vanes 3 extend into the central region of the cavity 2 to define a plurality of resonant cavities 4. Alternate vanes 3 are strapped together at each end by one of a pair of rings 5, 6 in known manner.

The vanes 3 each stop short of the centre of the cavity 2 to define a cylindrical space 7 within which a cathode comprising a coil 8 of thoriated tungsten is An elongate conducting pin 9 is attached to each the coil 8 for supplying electrical current to the located end of cathode. Each conducting pin 9 extends out through an opening 10 in the anode block 1. Each conducting pin 9 is connected to the coil 8 by means of a top-hat shaped end cap 11 including an annular rim 12 and a spigot portion 13 which is brazed or welded within the adjacent open end of the coil 8. The end cap 11 includes a first hole 11a extending from an edge into the rim 12 of the end cap 11. A second hole 14 extends between a surface 12a facing away from the cathode across the first hole and through the spigot portion 13 1 7 - During assembly, the length portion 9a of the conductor is inserted into the first hole 11a, and then a welding, brazing or like tool, introduced to a position within the second hole and operated to both electrically and mechanically connect the conductor 9 to the end cap 11 and thus to the coil 8. Such a method of assembly has been found to be reliable and convenient.

The elongate conducting pins 9 are both arranged to supply current to the cathode and to provide a means of supporting the cathode in spaced relation from the anode. The portion of the pin 9 extending outwardly from the anode block is surrounded by a choke or shielding arrangement 15 arranged to limit the transmission of electromagnetic radiation coupled into the conductor from the resonant cavities 4 of the magnetron. The shielding arrangement 15 comprises a first tube 16 extending outwardly from the walls of the opening 10 in the anode block. The tube 16 surrounds and is radially spaced from a length portion of the pin 9 and is of a length substantially equal to a 1/4 of the wavelength of the output signal of the magnetron. A second tube 17 of substantially the same length and diameter as the first surrounds a further length portion of the conducting pin 9. A small axial spacing 18 separates the two tubes 16, 17 so as to electrically insulate them from one another.

8 - The other end of the tube 17 remote from the axial spacing 18 is secured to an electrical terminal 19 attached to the conducting pin 9. A further tube 20 of substantially the same length as the first and second tubes but of relatively enlarged diameter is secured within a stepped recess 21 in the side walls of the hole 10 in the anode block 1. The third tube 20 encloses the majority of the length of the first tube 16 and extends across the axial spacing 18 to surround a small portion of the length of the second tube 17. The third tube 21 includes a outwardly extending flange 22. A sleeve 23 formed of an insulating material such as a ceramic extends between the terminal 19 and the anode block together with three other rings 24 formed from an insulating material. The shielding arrangement 15 described above has been found, unexpectedly, to provide an especially high degree of electromagnetic shielding.

The envelope further comprises an output probe 25 extending from one of the vanes. A pair of ring shaped channels 26, 27 extend in side-by-side relation about the anode block 1 coaxially with the major axis thereof. The two channels 26, 27 are spaced apart by a dividing wall 28. A cylindrical sleeve 30 surrounds the anode block 1 and includes appropriate openings to receive the shielding arrangements 15 and output probe 25. The wall 28 dividing - 9 the two channels 26, 27 stops short of the sleeve to provide a connecting passageway 30 of much reduced cross-sectional area with respect to the channels 26, 27. A coolant inlet 31 and a coolant outlet 32 each extend into a respective one of the two channels 26, 27. In use, coolant flows round one of the channels through the connecting passageway and round the other channel before passing to the outlet. Such an arrangement is relatively easy to provide on the anode block and provides efficient cooling. A pair of end plates 34 are provided at each end of the anode block to seal off each end of the cavity 2 so that the interior can be evacuated. The output probe 25 includes a peripheral flange 35 for removably attaching the envelope within its housing, as is described below.

Referring now to Figures 3 and 4, the envelope just described with reference to Figures 1 and 2 is shown located within a box shaped housing 40 which serves as a Faraday cage to prevent the emission of electromagnetic radiation from the magnetron. The housing 40 comprises two planar sheets of metal 41, 42 which are folded so that they are U- shaped when seen in cross-section. The two sheets are then arranged so that each of the upwardly extending limbs 43 of the two housing members 41, 42 overlap one another. The remaining two sides of the housing are provided by an - apertured grille 44. Electrical cables 58 are connected through cylindrical insulating sleeves 59 to the terminals 16 of the magnetron for supplying current to the cathode coil 8.

The magnetic field is provided for the magnetron by a pair of magnetic pole pieces 45, 46 formed from a ferromagnetic material secured to opposing walls of the housing. In this way, the housing 40 can provide a yoke for the magnetic circuit. A coil of wire 48, 49 surrounds each of the pole pieces 45, 46. The coils 48, 49 are operative as in a Helmholtz pair to provide an additional magnetic field so as to control the power output of the magnetron in generally known fashion. A cylindrical enclosure 50, 51 surrounds each of the coils 48, 49 respectively. Each of the enclosures 50, 51 is arranged to prevent electromagnetic energy from the magnetron being coupled into each of the respective coils 48, 49. An opening 52, 53 is present in each enclosure to receive one of the respective magnetic pole pieces 45, 46. A terminal block 54, 55 is provided on the outside of the housing adjacent a respective one of the coils 48, 49. The terminal blocks 54, 55 each include conductors 56 which extend through the housing wall where they are connected to their respective coil. Bolts 56 extend through the housing wall 42 and through the flange 35 of the vacuum envelope to removably attach the envelope within the housing.

Because the conductors extend into the coil within the enclosure, little electromagnetic energy will be coupled from the magnetron into the coil and radiated from the conductors 56 or any leads (not shown) connected thereto. Furthermore, because the envelope can easily be removed from the housing, maintenance costs in the event of failure of the envelope are reduced as compared to an arrangement integral with the pole pieces. This is of an especial advantage when the magnetron is of high power, i.e. one having an output in excess of about SKW or so, and where the size and the need for a high degree of shielding means that the R.F. enclosure provided by the housing, together with the pole pieces and the coils, can be an expensive item to manufacture.

A

Claims

1. A magnetron having a vacuum envelope comprising an anode having a plurality of internal resonant cavities, a cathode located within the anode, an output probe of the vacuum envelope extending outwardly from the anode, the vacuum envelope being located within a magnetic field in between a pair of pole pieces, the pole pieces being attached to opposing walls of a metal housing, the output probe extending out through an opening in the housing which is operative to provide both a yoke for the pole pieces and a Faraday cage to prevent the emission of unwanted electromagnetic radiation from the magnetron.

0

2. A magnetron, according to claim 1, in which a coil surrounds each pole piece to provide an electromagnet within the housing, each coil being surrounded by an enclosure, conductors extending to and from each coil through the housing wall to within its respective enclosure for supplying electrical power to the coils, thereby to prevent electromagnetic radiation being coupled into the coils and radiated from the conductors.

3. A magnetron, according to claim 1 or 2, in which - 13 the envelope is detachable from the metal housing and pole pieces.

4. A magnetron, according to any of claims 1 to 3 in which the housing comprises two planar sheets of metal bent into a U-shape and arranged together with the upright limbs overlapping one another to define four walls of the housing, a further two walls each comprising an apertured grille.

1

5. A method of assembling a magnetron which comprises an anode having a plurality of internal resonant cavitiesf the method including the steps of inserting an elongate cathode centrally within the anode, an end cap being secured to an end of the cathode, the end cap including a first hole extending within the end cap transversely to the longitudinal axis of the cathode, and a second hole extending between a surface of the end cap facing away from the cathode and the interior of the first hole, the method including the further steps of inserting, within the first hole, an elongate conductor operative in the assembled magnetron to provide a means of support and a means of passing electrical current, to the cathode, and, having inserted the conductor within the first hole, locating a welding, brazing or like tool within the second hole, and operating the tool to electrically and mechanically connect - 14 the elongate conductor to the end cap.

6. A method, according to claim 5, in which the steps are repeated to secure another elongate conductor to the other end of the cathode.

1

7. A method, according to claim 5 or 6, in which the end cap includes a spigot portion and the cathode includes a helical coil, the method including the step of welding, brazing or otherwise electrically and mechanically connecting the spigot portion within an open end of the coil.

8. A method of assembling a magnetron substantially as described with reference to either of Figures 1 and 2 of the drawings.

9. A magnetron comprising an anode having a plurality of internal resonant cavities, and a cathode supported within the anode by a support means extending outwardly from the anode, the support means comprising an elongate conductor, insulated from the anode, for supplying electrical current to the cathode, the conductor being surrounded by shielding to prevent the emission of radiation from the conductor, the shielding comprising a first tube z - 15 surrounding and radially spaced from a length portion of the conductor and with a length substantially equal to '/4 of the wavelength of the output signal of the magnetron, the first tube being electrically connected to the anode; a second tube of substantially the same length and diameter as the first tube surrounding and radially spaced from another length portion of the conductor, the first and second tubes being axially separated from one another, the second tube being electrically connected to the cathode; and a third tube, of greater diameter than the first and second tubes, being radially spaced from and extending across the axial separation between the first and second tubes.

10. A magnetron, according to claim 9, in which the third tube is connected to the anode.

A

11. A magnetron, according to claim 9 or 10, in which the first tube is secured within the walls of an opening extending through the anode block, the third tube being of substantially the same length as the first tube and secured within the walls of a stepped recess surrounding the opening in the anode block.

12. A magnetron, according to any of claims 9, 10 or 11, in which a terminal is connected to the free end of the 1 - 16 elongate conductor, and a sleeve formed from an insulating material extends between the terminal and the anode block.

13. A magnetron substantially as described with reference to any one of the drawings.

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