JET SOFT-X-RAY DIAGNOSTIC SYSTEMS

A new multicamera soft x-ray detector system has been constructed for the pumped divertor phase of JET to provide improved spatial and tempo ral resolution. It is based on 12 compact pinhole cameras each of whic h uses a 35 element PIN photodiode array. The cameras are housed in a secondary vacuum and view the plasma through beryllium windows 250-mu m-thick which absorb x rays with energies below similar to 2 keV. The front-end electronics, located up to 10 m away from the detectors, sam ple data at 250 kHz, with a 100 kHz passband. The diodes are operated at zero bias voltage. The system ran successfully during the 1995 phas e of JET operation with poloidal mode numbers up to m=5 being successf ully reconstructed. Detector noise levels (<1 nA) were small compared to signal statistical noise. Some deterioration in diode performance w as observed after exposure to an integrated neutron fluence of similar to 10(13) n. cm(-2). Tests have shown that operating with 3 V reverse bias can offset much of the fall in performance from radiation damage . Using a similar 16 element. PIN photodiode, a windowless camera has been developed for installation in the JET primary vacuum, to investig ate fast XUV/x-ray events which develop at lower energies near the pla sma edge or in the divertor. Finally, a radiation hardened system is b eing developed for D-T operation. (C) 1997 American Institute of Physi cs.