Patent GB-836737-A (GB)
Title: Vacuum pump
IPC: H01J41/16
Filed: 1957-03-22
Abstract: 836,737. Discharge apparatus. WISCONSIN ALUMNI RESEARCH FOUNDATION. March 22, 1957 [March 23, 1956], No. 9426/57. Class 39(1). In an ion pump gas molecules are ionized by electron bombardment in an ionization space defined by an electron-permeable electrode having two parts, one of which attracts electrons into the ionization space and the other drives the ionized gas to a collector surface on which gettering material is continuously deposited so as to trap the gas molecules. The apparatus shown comprises an earthed housing formed by a cylindrical wall 10, which may be of non- magnetic stainless steel, open at one end 12 and closed at the other end by a plate 14. A valved port 16 provides access to a mechanical pump. Two electrode parts 18 and 22 define a cylindrical ionization space. Part 18 consistsof a plurality of wires 20 stretched across a ring 19 which is supported by a plurality of wires 35 from insulators 36. The wires 20 are prevented from vibrating by two interwoven wires crossing them at right-angles. Part 22 consists of a plurality of wires 25 extending between rings 23 and 24 which are supported by wires 30 from a plurality of shielded insulators 26 mounted on a stainless steel framework 28. The upper ring 23 is substantially closed by the electrode part 18 and the lower ring 24 is provided with wires similar to those of electrode 18. The wires and rings may all be of tungsten. A multi-strand thermionic filament 37 covering a large area is supported by springs 38 from insulators 64 and is supplied at terminals 40 with current from a secondary winding 41 of a transformer 42. A wire 44 of titanium or other gettering material is fed into the housing from a container 46 and is evaporated from a post 48 of graphite or other suitable material heated by a filament 50 which is supplied from a secondary winding 52 of the transformer 42. A source of potential 53 maintains the post 48 positive with respect to the filament 50. A window and mirrors (not shown) may be provided to enable the temperature of the post 48 to be estimated before the gettering material is fed to it. A molybdenum member 54 may be provided to direct the particles of getter material to the inner surface of the wall 10 adjacent the electrode part 22. A copper or stainless steel plate 56 and a tungsten grid 58 serving to shield the evaporating apparatus from the ionization space are supported by conductive members 60 from the plate 14 and are thus at earth potential, but may alternatively be insulated from the housing and biased negative with respect to the filament 37. Cooling fluid is circulated through a conduit 70 surrounding the wall 10. An axial magnetic field is provided by a coil 72 supplied from a current source 73, the arrangement being preferably such that the field is weaker at the centre of the coil than at the ends. The electrode parts 18 and 22 are maintained positive with respect to the filament 37 and the wall 10 by sources of potential 76 and 78. The parts 18 and 22 may be at the same potential in which case they may be connected together inside the housing and only one source of potential need be employed. The filament 37 is maintained positive with respect to the plate 56 and grid 58 by a source 80. A primer is formed by a plurality of tungsten rods 84 disposed around a conductive hub 86 supported by a conductive rod 88. Short lengths of getter material 90 such as titanium are around the rods 84 and are evaporated by heating the appropriate rod. To initiate operation the pressure is first reduced by the mechanical roughing pump. The filaments 37 and 50 are then brought up to temperature and the potentials applied to the post 48 and electrode parts 18 and 22. Current is then applied to one of the rods 84 of the primer. When the temperature of the post 48 is sufficiently high the wire 44 is fed to the post at a constant rate. Electrons emitted by the filament 37 are drawn by the positive potential of the electrode part 18 into the ionization space. Positive ions which move into the space between the electrode part 22 and the wall 10 are driven into the surface of the wall by the positive potential of part 22 and are buried by the continuous deposition of gettering material. The electron-permeable electrode may alternatively comprise nine parallel plates of different widths or an arrangement of radially disposed plates occupying a volume similar to the volume defined by the electrode parts 18 and 22.