Observation of low temperature VUV tungsten emission in JET divertor plasmas

The properties of tungsten make it ideal for use as a plasma facing surface in the divertor of large plasma machines such as JET and ITER. However, the intense heat and particle fluxes that fall on the divertor surfaces lead to its release from these surfaces into the plasma and it is necessary to model its transport from the divertor and plasma edge into the plasma core. This requires measurement of spectral features over a wide temperature range. In large machines the W influx is often determined from W I line intensities, there being few measurements of discrete W lines from other low ionization stages. Their observation is highly desirable because the transport of neutral W differs markedly from that of the W ions. A change in the line-of-sight of a VUV survey spectrometer on JET to view directly into the divertor has led to the observation of numerous discrete low temperature W lines in the VUV spectral region. The spectrum of an intense influx in which W IV to W VIII features are observed has been analysed in order to provide spectral classifications so that these lines can be used for diagnostic purposes. The first observation of a VUV low temperature W magnetic dipole (M1) transition is reported for the W VIII ionization stage. The analysis shows where further line identifications are needed and that the provision of the highest quality atomic data for these ionization stages is desirable. W VI and W VII line intensity measurements are used to illustrate their use in determining the electron temperature of the emitting plasma region and the W concentration. Dependences of the W line intensities on plasma parameters shows the optimal conditions for the W release and suggests the site of its release.