Time-resolved erosion measurements at a reference limiter in ISX-B using laser-induced fluorescence☆

Abstract Laser-induced fluorescence has been used to measure the density and ionization length of neutral metal atoms released from a reference limiter during plasma discharges in the Impurity Study Experiment (ISX-B) tokamak. A tunable, pulsed laser beam is introduced into the tokamak at the midplane along a major radius and passes in front of a stainless steel reference limiter positioned between the wall and the main limiter radius. Neutral iron atoms released from the limiter are detected by saturation pumping of the ground-state transition at 302 nm, followed by off resonance detection of the resulting fluorescence radiation. The sampling rate is 15 Hz; additional time resolution is provided by overlaying similar discharges with different time synchronization. In ohmic discharges in ISX-B, a nearly constant impurity release rate of 8 × 10 14 atom cm −2 s −1 occurs during the steady-state part of the discharge, with substantial peaks during the breakdown, rampup, and termination of the shot. Measured ionization lengths are suggestive of particle velocities typical of sputtering. These results are presented with similar data from neutral-beam-heated shots.