Enhancements to the JET Poloidally Scanning VUV/Visible Spectrometers

Enhancements to the JET poloidally scanning spectrometers are presented, which will aid the exploitation of the recently installed ITER-like wall in JET. They include the installation of visible filter/photomultiplier tube assemblies and spectrometers and the replacement of large rotating mirrors in the JET torus vacuum with small oscillating mirrors outside. The upgrade has resulted in a more robust and reliable diagnostic than before, which is described. The visible system has been absolutely calibrated using an in-vessel light source and drifts in the mirror angle reconstructed from quadrature encoder signals are attributed to a limitation of this technique, which requires a reference signal. The use of the small scanning mirrors necessitated the inclusion of focusing mirrors to maintain throughput into the VUV spectrometers. The mirror design has taken account of the extreme sensitivity of the focusing to the grazing angle of incidence, an aspect of importance in the design of grazing incidence focusing components on future machines, such as ITER
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