GaAs photomultiplier for LIDAR Thomson scattering

Abstract

Edge light detection and ranging (LIDAR) Thomson scattering at Joint European Torus (JET) is used for measuring temperature and density gradients of the edge plasma. The accuracy of the present measurements is limited not only by the spatial resolution of the system but also by photon statistics. The accuracy of the measurements can be increased by at least a factor of 2 by improving the sensitivity of the detection system. The present detectors are fast, 18 mm diameter microchannel plate photomultipliers (MCP PMTs) with multialkali photocathodes. The quantum efficiency is 5% at 650 nm decreasing toward the red. We present the characteristics of a newly developed MCP PMT with a GaAs photocathode. Four 18 mm diameter photomultipliers with a V-stack MCP and a preliminary prototype unit have been produced and tested to fully equip the edge LIDAR diagnostic. The GaAs photocathode of the new detectors has a quantum efficiency in excess of 20% in the spectral range of 520–850 nm. The effective quantum efficiency has been measured and is at least four times that of the present detectors. Investigations of the other characteristics required for use in a LIDAR system show good results. The pulse response time is slightly faster than that of the current tubes, potentially offering the possibility of improving the spatial resolution to better than the present value of 12 cm. The effect of varying the gate voltage between 100–300 V has been investigated. Lower gate voltages cause less ringing on the output signal and only increase the effective noise slightly.