GB1148426A - Electron beam apparatus including filament current control means - Google Patents

Abstract

1,148,426. Automatic current control of electron beam apparatus. INTERNATIONAL BUSINESS MACHINES CORP. Dec. 15, 1967 [Jan.13, 1967], No.57022/67. Heading G3R. Electron beam apparatus is operated at a desired point on the beam current/filament current characteristic by superimposing on the normal filament working current a cyclic variation of constant amplitude which is small relative to the normal working current, detecting the consequent change in beam current, comparing this change with a reference representative of the change corresponding to operation at the desired point and using the error signal to vary the filament working current to maintain the change at the reference value. As shown the filament 14 of the apparatus 11 is fed from a D. C. supply 31 through parallel-connected transistors 33, 34 which respectively contribute the cyclical and steady components of the filament current, the former component being produced by an oscillator 44 controlling the base of transistor 33. The cyclical beam current change produced by the varying filament current, Fig. 2 (not shown) is detected by a resistor 48 connected between the filament 14 and the high voltage supply 53, the A. C. signal so obtained being amplified, rectified and smoothed in members 59, 60 to produce a D. C. voltage which is applied to a summing amplifier 61 for comparison with a reference voltage obtained from members 66, 67. The error signal is fed through an operating amplifier 69 to the base of transistor 34 to control the steady component of the filament current in accordance with the beam current change. The above-described control system ensures that the most efficient operation of the filament is obtained irrespective of changes in the characteristic due to filament ageing or replacement. To prevent the control system from locking on to the lower (undesired) one (77) of its two stable operating points, Fig. 3 (not shown), the amplifier 61 is provided with a third signal from a terminal 81 through a switch 79 whose state is dependent on the current carried by the transistor 34 (via terminals 82, 84). For values of this current below a predetermined magnitude the switch 79 is closed to apply the third signal to the amplifier 61, but upon reaching this magnitude the switch 79 opens to remove the signal and so allow the system to lock on to the desired operating point (76).