A new multicamera soft x-ray detector system has been constructed for the pumped divertor phase of JET to provide improved spatial and temporal resolution. It is based on 12 compact pinhole cameras each of which uses a 35 element PIN photodiode array. The cameras are housed in a secondary vacuum and view the plasma through beryllium windows 250-μm-thick which absorb x rays with energies below ∼2 keV. The front-end electronics, located up to 10 m away from the detectors, sample data at 250 kHz, with a 100 kHz passband. The diodes are operated at zero bias voltage. The system ran successfully during the 1995 phase of JET operation with poloidal mode numbers up to m=5 being successfully reconstructed. Detector noise levels (<1 nA) were small compared to signal statistical noise. Some deterioration in diode performance was observed after exposure to an integrated neutron fluence of ∼1013 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 investigate fast XUV/x-ray events which develop at lower energies near the plasma edge or in the divertor. Finally, a radiation hardened system is being developed for D-T operation.
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Alper et al. (1997) studied this question.