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 instruments 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 reducing the number of neutrons scattered into the detection path. Similar labyrinth systems are currently discussed for next-step fusion experiments with appreciable radiation levels which inhibit the use of fibres in the immediate neighbourhood of the torus. The JET mirror link covers a spectral range from 2000 to 8000 AA which considerably extends the range accessible by standard fibre links. The suitability of dominant UV charge exchange spectra for routine ion temperature and ion density 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 impurities in a JET plasma. As a general rule it is found that the gain in the increase of atomic emission cross sections for UV CX lines compared to their counterparts in the visible range is compensated for by the presence and intensity of additional edge emission lines in the same spectral neighbourhood. The exception of this rule is the UV CX spectrum of Be IV at 2530 AA which is characterized by only a few background spectral lines and which offers a straightforward spectral analysis. In the case of the intense, but complex, C VI UV spectrum at 3434 AA atomic modelling of line ratios has led to a successful reduction of free parameters and to the extraction of ion temperature values consistent with independent measurements.