GB901017A - Improvements in asynchronous beam scanning device - Google Patents

Abstract

901,017. Electron and particle beam apparatus. GENERAL ELECTRIC CO. March 21, 1960 [March 23, 1959], No. 9883/60. Class 39 (1). In a particle beam generator such as an electron beam apparatus for irradiating articles 25, the beam is formed into a series of pulses occurring periodically in accordance with a time varying accelerating potential and the beam is projected through a deflecting field which is asynchronous with the pulse frequency and has a different magnitude at the beginning of each pulse in a predetermined number of successive beam pulses so that the beam pulses will initiate, reach maximum energy, and terminate at successively different points on the window 21 when the beam is deflected on the long dimension of the window during each pulse, as for example is shown in Fig. 6. The beam 24 is pulsed by a control grid 15 to which rectangular pulses are applied by a bias control 40 which is operated from the high voltage end of the secondary 30 via a capacitance 37 formed between the tube cap and the earthed enclosing tank so that the beam is pulsed through 72 degrees of the accelerating voltage cycle of 360 degrees, about its positive peak. The accelerating voltage may have an amplitude of one million volts and a frequency of 180 cycles per sec. with a corresponding beam pulse duration of <SP>1</SP>/ 900 sec. The accelerating potentials are derived from a generator 48 driven by a motor 49 which also drives through gearing 52 a generator 51 supplying current to the scanning coils 50 which deflect the beam 24 lengthwise of the window 21. The lengthwise scanning flux voltage curve 53, Fig. 5, is of such a frequency that it passes through more than a whole cycle during a tube current pulse 45. Two successive occurrences of the peak current and accelerating voltages are shown by the dots 60, 601. If with a pulse duration of 1/900th second a scanning frequency of 1005 cycles per second is utilized, then twelve complete scans occur before the beam pattern is repeated on the window, with a resultant uniformity of loading over the window area.