GB1215020A - High power beam tube employing a fly-trap beam collector - Google Patents

Abstract

1,215,020. Electron beam apparatus. VARIAN ASSOCIATES. 20 Dec., 1967 [22 Dec., 1966], No. 57829/67. Heading H1D. An electron beam tube includes a cathode 6, and accelerating electrode 4, wherein electrons emitted from the cathode and focused by electrode 8 to form a beam 3 are collected by collector structure 5 of the fly-trap type comprising a cavity 17 with an apertured wall 18 having a mouth forming a restricted beam entrance 19 which inhibits the escape of secondary electrons from the cavity, and an electric beamfocus structure 21 disposed at the mouth portion 19 of the collector which shapes the decelerating electric field in the presence of the beam to form equipotentials which are inwardly dished towards the collector, Fig. 2 (not shown), and include an equipotential 22 at collector potential extending across the mouth substantially normal to the electron trajectories. In the linear beam tube arrangement shown the spacing d 2 from a cathode 6 to the midplane of the accelerating electrode is preferably equal to the spacing d 1 from this midplane to the collector equipotential surface 22. A pair of shaped, aligned mesh grids 9, 11 may be arranged close to cathode emissive surface 7. The grids 9, 11 and the cathode may have central apertures 12 to permit the passage of positive ions which are collected by a target electrode 13, preferably made of getter material, e.g. titanium. Central beam passageway 14 in the accelerator 4 is of smaller diameter than the cathode emissive surface 7 and is longer than its diameter to prevent coupling between the input pulses applied to the grid 11 and the output from the collector 5. In a modification, Fig. 3 (not shown), the beam focus electrode comprises a number of separate, insulated apertured discs secured to cavity wall 18. In another embodiment, Fig. 5 (not shown), the tube is of the radial beam type with the beam directed radially outwardly from a central cathode emitter to a coaxial annular fly-trap type collector; this arrangement increases the power handling capacity of the tube. In any of the embodiments, a radio frequency resonant circuit 31 may be connected between the accelerating electrode and the collector, and a signal to be amplified may be applied to grid 11 to modulate the beam. Preferably, the pulsed potential of grid 11 is 1/30 or 1/50 of the potential of the accelerating electrode and the collector potential is depressed to nearly equal to the cathode potential, so that the forward conduction potential drop is 1-10% of the accelerating electrode potential. The input to grid 11 may be from a further resonant circuit. The collector 5 and accelerator electrode 4 may be water cooled. The tube may also be used as a switch tube.