GB1343497A - Electron beam tube modulation system - Google Patents

Abstract

1343497 Supply systems for electron beam tubes; protective arrangements MACHLETT LABORATORIES Inc 14 May 1971 [18 May 1970] 15049/71 Headings H2H and H2K [Also in Divisions H1 and H3-H5] A circuit responsive to an input pulse 28 to switch on an electron beam tube 34 comprises terminals C, J between which normally appears a first voltage but between which appears a second voltage during the application of the pulse, an interlock circuit 32 connected to the terminals and responsive to the pulse to detect a condition in which the second voltage is present and the input pulse is absent, and a switch 202 controlled by the interlock circuit to switch off the tube when the condition is detected. The tube 34 may be an X-ray tube or a travelling wave tube and a signal source 26 which supplies the pulse 28 includes an exposure control unit 140 which is manually adjustable to determine the width and frequency of pulses 142. Pulses 28 are derived from the pulses 142 by a system including a light source 150, a light conduit 152 and a gallium arsenide photo-detector 154, the system enabling isolation of the high voltages in the X-ray tube housing. A pulse circuit 71 includes a transistor 72 which supplies a pulse 96 to a transformer 122 and a transistor 74 which supplies a pulse 106 to a valve 68, thereby switching off the valve 68. Current therefore flows from a terminal V3 of a vattery 50 through resistors 58, 60, 62, 64 and a diode 66 to the respective terminal of the battery 50. A forward bias is thus applied between the grid and cathode of the tube 34 thereby switching on the electron beam. On termination of the pulse 106 the valve 68 is switched on thereby switching off the X-ray tube. A valve 70 is supplied by a pulse 124 derived by differentiation of the pulse 96, the pulse 124 switching on the valve 70 to sharpen the switch-on instant of the electron beam. The interlock circuit 32 comprises amplifiers 186, 188 which monitor the potentials across the resistors 60, 62, i.e. the grid-cathode bias potential for the tube 34 and the reverse current through the diode 66 since excessive reverse current through the diode 66 results in an increased potential across the resistor 62. The output voltages from the amplifiers are supplied to a logic circuit 190 which controls a light source 192. If the diode 66 is damaged by shorting due to an arc in the X-ray tube, the increased potential across the resistor 62 causes the output of the logic circuit 190 to extinguish the light source so that an amplifier 200 and a photo-detector 198 do not supply a signal to a logic circuit 201. The logic circuit 201 therefore operates the switch 202 which comprises a relay to disconnect the high voltage supply to the X- ray tube. Since the light source 192 is on when the X-ray tube is inoperative, the absence of light during the absence of a pulse 142 indicates a fault and causes the logic circuit 201 to operate the relay thereby disconnecting the high voltage supply to the X-ray tube.