A VISIBLE AND UV CHARGE-EXCHANGE SPECTROSCOPY SYSTEM FOR THE TRITIUM PHASE OF JET

Results achieved by a direct optical link between torus and remote ins truments designed for the active phase of JET are reported. The system is based on a labyrinth light path and a set of remotely controllable relay mirrors avoiding a direct view of the plasma core, thus reducin g the number of neutrons scattered into the detection path. Similar la byrinth systems are currently discussed for next-step fusion experimen ts with appreciable radiation levels which inhibit the use of fibres i n the immediate neighbourhood of the torus. The JET mirror link covers a spectral range from 2000 to 8000 Angstrom which considerably extend s the range accessible by standard fibre links. The suitability of dom inant UV charge exchange spectra for routine ion temperature and ion d ensity diagnostics is addressed by comparing UV and visible CX spectra of C VI, Be IV, He II and Ne x which represent the main light impurit ies in a JET plasma. As a general rule it is found that the gain in th e increase of atomic emission cross sections for UV CX lines compared to their counterparts in the visible range is compensated for by the p resence and intensity of additional edge emission lines in the same sp ectral neighbourhood. The exception of this rule is the UV CX spectrum of Be IV at 2530 Angstrom which is characterized by only a few backgr ound spectral lines and which offers a straightforward spectral analys is. In the case of the intense, but complex, C VI UV spectrum at 3434 Angstrom atomic modelling of line ratios has led to a successful reduc tion of free parameters and to the extraction of ion temperature value s consistent with independent measurements.