Dynamic measurement of impurity ion transport in runaway electron plateaus in DIII-D

The first dynamic (time-dependent) measurements of impurity ion radial (cross field) and parallel (along-field) diffusion coefficients for post-disruption runaway electron plateaus are presented. Small (∼1 mm diameter) carbon or silicon pellets are fired into the edge of steady-state runaway electron (RE) plateaus, and the resulting radial and toroidal transport of singly charged impurity ions (C+ or Si+) is monitored with spatially distributed visible spectrometers. Consistent with previous steady-state particle balance estimates of Ar+ radial transport, radial (cross field) diffusion coefficients D⊥ ≈ 2-5 m2/s are obtained, about 10 × larger than expected from neo-classical theory. Parallel diffusion coefficients D∥ ≈ 30-80 m2/s are estimated, also much (≈ 50 ×) larger than classical. It is speculated at present that these large diffusion coefficients may be due to turbulent transport. Indications of fairly significant (almost 2 ×) toroidal variation in electron density are seen in the RE plateaus, and this appears to cause some toroidal variation in impurity radial diffusion rates. Indications of slow (≈1 Hz) toroidal rotation in the impurity ions are observed, although the uncertainty in this measurement is large.