Patent GB-1494077-A (GB)
Title: Electron beam generators
IPC: H01J37/065
Filed: 1973-11-15
Abstract: 1494077 Electron beam apparatus WELDING INSTITUTE 12 Nov 1974 [15 Nov 1973] 53083/73 Heading H1D An electron beam generator system comprising an electron gun 16 for generating a pencil beam, high voltage generator 36 connected to gun cathode 18, and oscillator 38 of #2 Hz output, with primary and secondary coils 44, 46, an electrode of the gun being connected to receive the high voltage generator output and being connected to or coupled to the output of the secondary coil so that the output of the latter is superimposed on the high voltage generator output. The bias supply and/or cathode heating supply for example may be locally superimposed on the high tension voltage at the beam generator, not only reducing the high voltage cable required but also improving response to operational changes. Flashover damage is also reduced. The high voltage generator may itself be supplied from an inductively coupled oscillator so that the high voltage cable is dispensed with. The primary coil may be outside the vacuum enclosure which has a non- conducting slit. In Fig. 1, cathode 18 heating is from 40 MHz oscillator 38 connected through insulator 42 to two-turn driving coil 44 coupled to single-turn coil 46 (also Figs. 2-4, not shown) paralleled with tuning capacitor (50) and provided with corona ring 60 Coil 46 is connected to a further two-turn coil 48. Annular cathode 18 providing a hollow beam is mounted by legs (52), one of which, with bolt (58) and connection (56) (Fig. 2) conveys the high voltage. The cathode may be a disc or button rather than an annulus and of Ta or W. The above arrangement also reduces the effect on the electrons of magnetic fields due to circulating current. Alternatively connection of coil 46 in series with a ribbon filament provides direct heating. In Fig. 6 (not shown), filament heating is via the H.T. cable and the bias voltage superimposed locally on the H.T. Pulse modulated (e.g. 10kHz) 80 kHz oscillator (38) is applied to multi-turn coil (70) secondary (74) in slitted corona ring (60) connected to the centre tap of coil (74) and control electrode (20). Coil (74) output may be connected to a solid state rectifier bridge within the column, but preferably two sides of the truncated cathode (62) with electrodes (76) are used as diodes connected to coil (74) and constitute a full-wave rectifier. This arrangement facilitates "peaking" of the beam current with filament current changes as discussed with respect to Fig. 8 (not shown), clamping of the filament legs providing cooler diode portions and modified characteristic (B). Coil specifications are given. A low voltage (e.g. 30 kv.) generator (Fig. 7, not shown) uses earthed primary coil (70) coupled through ferrite rod (80) to (coil 74), the housing 10 being either of insulating or slitted metallic material. Coil (70) may alternatively be in housing (10). A tuning capacitor across primary and secondary coils increases terminal voltages. The method may also be applied to cathode heating by electron bombardment by connection in series with a diode between cathode and filament. Similar alternatives are discussed applied to other electrodes.