A simple current mode preamplifier for electron multipliers with high voltage biased anode

Abstract

We developed a simple current mode preamplifier for the microchannel plate (MCP) with the anode biased to high voltage. It uses a capacitor to block dc high voltage and measures the MCP current through the capacitor. Then it compensates exactly the distortion caused by the capacitor so the amplifier can measure any length of signals without distortion. It is much less expensive and simpler than using the phosphor screen system to isolate the dc high voltage. We derived a formula for that compensation and implemented it by using an op-amp's circuit. It gives an approximate time resolution of 10 mu s which is restricted by the op-amp's characteristics. So, it may be improved by using fast op-amp's. This can also be used for channeltron and other electron multipliers. For the test of the preamplifier, we measured the photoelectron currents induced by the soft x rays radiated from the KT-1 tokamak of which the pulse length was 20 ms. The data revealed well, without distortion, the time evolution of the line radiation. (C) 1996 American Institute of Physics.