GB2271021A - Magnetron - Google Patents

Abstract

A magnetron includes 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. A shielding arrangement 15 surrounds each pin 9 for preventing the emission of electromagnetic radiation. The shielding 15 is formed by two coaxially spaced tubes 16, 17 of the output wavelength of the magnetron and connected respectively to an anode block 1 and the cathode 8, and a longer diameter third tube 20, preferably of the same length as tubes 16, 17 surrounds the axial separation 18 of the tubes 16, 17.

Description

MAGNETRON The invention relates to a magnetron and especially to one which includes a high degree of shielding to prevent the emission of electromagnetic radiation.

This invention provides 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 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 and it is also preferred that the first tube is secured within the walls of an opening extending through the anode, the third tube being of substantially the same length as the first tube and being 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 betwen 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 in Figure 3.

A vacuum envelope for a magnetron shown in Figures 1 and 2 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 located.

An elongate conducting pin 9 is attached to each end of the coil 8 for supplying electrical current to the 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 11 a extending from an edge into the rim 12 of the end cap 11. A second hole 14 extends between a surface 1 2a facing away from the cathode across the first hole and through the spigot portion 13.During assembly, the length portion 9a of the conductor is inserted into the first hole 11 a, 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.

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. 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 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 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 removeably 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 provideed 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 failsure of the envelope are reduced as compared to an arrangement integral with the pole pieces. This is of especial advantage when the magnetron is of high power, i.e. one having an output in excess of about 5KW 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.

Claims (5)

1. 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 surrounding and radially spaced from a length portion of the conductor and with a length substantially equal to 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.

2. A magnetron, according to claim 1, in which the third tube is connected to the anode.

3. A magnetron, according to claim 1 or 2, 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.

4. A magnetron, according to any of claims 1, 2 or 3 in which a terminal is connected to the free end of the elongate conductor, and a sleeve formed from an insulating material extends between the terminal and the anode block.

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