Patent GB-814650-A (GB)
Title: Improvements in and relating to positive-ion draining structures in electron discharge devices
IPC: H01J25/12
Filed: 1955-09-26
Abstract: 814,650. Velocity modulated tubes. VARIAN ASSOCIATES. Sept. 26, 1955 [Nov. 4, 1954], No. 27398/55. Class 39(1). In a klystron or travelling wave tube positive ions are drained from a region in the path of the electron beam by one or more floating members adjacent the region and electrically isolated from the exterior of the envelope, the floating member or members being biased solely by caught electrons of or derived from (e.g. secondary emission) the beam and having a secondary emission ratio of less than unity so that it goes or they go negative and so tends or tend to collect positive ions from the region. The electrode may be provided between the gun and resonator of a reflex klystron or in the or each drift tube of a two cavity or multi-cavity klystron or in a travelling wave tube. In one form, Fig. 1A the ion collector 48 is an annulus of U cross section and is mounted on an insulator 47 within a multiapertured shield 45 which prevents interaction of the beam and the ion collector. Fringe electrons of the primary beam enter through an annular aperture in 45 and bias the ion collector negatively, the design of the ion collector structure being such as to prevent the escape of secondary electrons while the positive ions reach the ion collector through holes 46. If little or no shielding is required between the beam and ion collector, holes 46 can be made larger or 45 can be omitted entirely The ion collector shown is disposed between accelerator grid 32 and the buncher, and a further similar ion collector is disposed in the drift tube between buncher and catcher, Fig. 1, (not shown). In other forms, Figs. 2, 2A (not shown) secondary electrons emitted under primary electron bombardment by the first grid of the buncher go to an annular ion collector and bias it negatively, or the secondaries may be derived from a ledge, which intercepts the fringe electrons of the beam and is disposed adjacent to the ion collector. In a reflex klystron, Fig. 10 (not shown) an ion collector as in Fig. 1A may be used but its annular opening faces towards the reflector, so that the ion collector is biased negatively by reflected electrons ; the ion collector can also collect secondary electrons emitted from adjacent electrodes. In another form a plurality of ion collectors of the form shown in Fig. 5 are disposed in a drift tube at different circumferential and axial positions. Fringe electrons of the primary beam enter through aperture 64 and strike cylindrical ion collector 67 to bias it negatively so that it can collect positive ions through aperture 65. Electrode 67 is designed to prevent escape of secondaries and is mounted on insulator 66 within shield 63. The ion collectors may be of glass, ceramic or copper and the latter are insulated from the conductive tubes in which they are embedded by glass or ceramic insulation. The positive ions considered are those generated by collision of electrons and residual gas molecules, and the disadvantages of their presence are set out as undesired modulation of output power or frequency or beam current, due to high frequency ion oscillations or to low frequency oscillations associated therewith, or occurring as the frequency of applied modulating voltages is varied over a range near the ion oscillation frequency.