Quantifying impurity transport in runaway electron plateaus in DIII-D

The impurity ion diffusion coefficient in post-disruption runaway electron (RE) plateaus is found to be about 10x higher than classical ion-ion diffusion. The impurity level of RE plateaus has a key role in all aspects of RE plateau dynamics, and is crucial for predicting damage resulting from RE plateau-wall strikes. DIII-D experiments have pursued both stationary and dynamic measurement of RE plateau impurity radial transport. In the stationary experiments, profiles of electron density and radiated power are matched with a 1D radial impurity transport model. This steady-state 1D analysis arrives at impurity ion diffusion coefficients of order 1 - 10 m²/s. Dynamic experiments use small (1 Torr-L), carbon granules fired into the RE plateau, with subsequent toroidal and radial transport of  ions analyzed spectroscopically. The dynamic experiments, performed in a small number of conditions, find impurity diffusion coefficients of order 5 m²/s, consistent with the steady-state estimates. Future experiments and implications for ITER will be discussed.