Edge density measurements with a fast Li beam probe in tokamak and stellarator experiments

High energy neutral Li beam probes have advanced to the point where they are a standard diagnostic tool on W7-AS and ASDEX-Upgrade, both in terms of the Li beam injector and the reconstruction algorithm to arrive at densities ne(z) along the beam from the Li[2p-2s] resonance line profile. With beam energies in the range 30–70 keV and neutral equivalent currents greater than 1 mA, it is possible to derive ne(z) profiles for line densities nez < 4 × 1014 cm−2, with a radial resolution of about 0.5 cm and time response of less than 0.2 ms. By way of example, the diagnostic layout on the W7-AS stellarator is sketched and salient results from experiments discussed. Densities over the range 1012–1014 cm−3 are accessible, permitting full coverage of the core density gradient region on W7-AS. Examples from the ASDEX tokamak that involve the H-mode and pellet injection are presented to exemplify the time response.