Interpretative modelling of JET's thermal helium diagnostic

Electron density, n(e), and temperature, T-e, in the MkIIa divertor have be en measured using neutral helium line ratios. Experimental evidence indicat es that the spectroscopic signal for helium injected from the vertical targ et is dominated by recycling. When helium is injected from the horizontal t arget it is possible to differentiate between injected and recycled spectra l sources. To determine the spatial location of the neutral helium emission , and hence of the n(e), T-e measurements, an interpretative model has been developed which uses the impurity/neutral codes, DIVIMP/NIMBUS, the atomic database ADAS and the diagnostic lines of sight. Measurements and simulati ons have been performed using the various injection points over a range of target conditions. Comparisons with other divertor diagnostics, such as the divertor interferometer and target Langmuir probes are presented. (C) 1999 JET Joint Undertaking, published by Elsevier Science B.V. All rights reser ved.