A Comparison of Multi-Alkali and GaAs Photo-Cathode Detectors for JET Edge LIDAR TS profiles

The JET Tokamak has two LIDAR Thomson Scattering systems, one for measuring the core Te and ne profiles and one dedicated to the edge parameters. The LIDAR scheme is unique to JET and is also envisaged for use on ITER. The system's spatial resolution is defined by the convolution product of its components: laser pulse duration, detector response time and digitizer speed. The original multi-alkali photocathode micro channel plate photomultipliers dictated the response time of the system, resulting in a twelve centimeter spatial resolution along the line of sight. In the edge LIDAR system, this is improved by aligning the line of sight with the flux surfaces, thus improving the effective, across flux surfaces, spatial resolution to two centimeters, depending on the plasma configuration. To meet demands for better resolution of the edge gradient, an upgrade to higher quantum efficiency detectors was proposed. Four GaAs photocathode detectors have been procured, two of which surpass expectations. These detectors are shown to have a more than two times higher effective quantum efficiency and their response time is at least twice as fast as the multi-alkali detectors. In combination with a fast digitizer this improves the spatial resolution of the system by a factor of two, down to one centimeter effective, depending on plasma configuration.
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