<title>Overview Of Pulsers For Nanosecond Gating Of Image Shutter Tubes</title>

Abstract

The capability of generating a useful optical shutter of a few nanoseconds or less utilizing gated proximity-focussed microchannel-plate (MCP) wafer tubes or silicon intensified target (SIT) vidicon tubes depends strongly on the driving electrical pulse. A proximity-focussed MCP wafer tube can be optically shuttered by applying a short electrical pulse between the photocathode and MCP interface.' This interface is electrically reverse-biased by approximately 30V to prevent photocathode electrons from reaching the MCP. The gating pulse, typically 80V, is of opposite polarity to generate an effective forward bias to "shutter" the system. Light intensity ratios for gated on to off conditions (shutter ratios) of greater than 105 are obtained. The more recently developed gated SIT vidicon tubes2 are gated by applying an effective forward bias between the photocathode and a 50% transmission grid in close proximity to the photocathode. Equivalent shutter ratios have not been achieved as yet, with 103 shuttering efficiency measured for typical SITs. Assuming the optical gate width is determined by the electrical gate width one requires pulses with rise and fall times of less than a nanosecond and amplitudes in excess of 80 volts into a 50 ohm impedance. Such pulses have permitted shutter times of %1.5 ns and L800 ps for the MCP and SIT tube systems respectively while preserving their resolution capabilities. The problem of matching an electrical pulser's driving impedance to that of the optical shutter is one still under study. The intrinsit impedance of a proximity-focussed MCP optical shutter is that of a distributed capacitance and resistance.1r3 A measurement of the resistance and capacitance vs a frequency network of 10 MHz with an HP4191A impedance analyser indicated a relatively constant equivalent series capacitance of 31 pf and a photocathode equivalent series resistance in the range from <100 to 300 for individual intensifiers. The capacitance values are roughly independent of frequency up to ',1,120 MHz. For frequencies >120 MHz, and therefore gate rise times less than %3 ns, the equivalent series inductance becomes effective causing varying impedance values which complicates the problems of electrically driving these systems. This paper will provide a summary of some of the electrical gate pulsers utilized in studying both proximity-focussed MCP imaging intensifiers and gated SIT FPS vidicon tubes. All of these pulsers are designed to drive 500 impedances although work is currently being directed to lower impedances. The circuit diagram for a 1.6 ns FWHM, 80V pulser based on avalanche transistors is given in Fig. 1. An overview of the MCP image intensifier and its associated divider string is also shown. The output pulse with the system terminated in 500 without the MCP intensifier attached is given in Fig. 2a. This pulser is capable of driving a MCP intensifier from '1,1.2 ns to 5 ns total optical shutter on times. The optical resolution is reduced to below 443/mm for shuttering intervals <2 ns for total on/off times. The variable optical shutter times are obtained by varying the reverse bias applied between the photocathode and MCP. There is a limit as to how low a reverse bias one can use. This is determined by the "ringing" caused by improper impedance matching of the pulser to the optical shutter. This mismatch can be seen in Fig. 2b, obtained by observing a 1% sample signal at the input to the optical shutter. The second pulse will turn on the optical shutter thereby extending the shutter duration. Two alternative methods of driving proximity-focussed MCP image intensifiers involving the above pulser are to form a bipolar pulse (shown in Fig. 3) through feedback summation and to attempt improved impedance matching by coupling through a toroid (pulse transformer) and a constant impedance network. The toroid system is schematically shown in Fig. 4. Both methods reduce the total available amplitude by 50%, resulting in insufficient