Effects of Modified Radial Transport Model on Runaway Electron Current

The classical formula for the radial transport of electrons in perturbed magnetic field [1] has been modified to have a lower radial diffusion for high-energy electrons. Different energy dependence factors are tested for a DIII-D shot with Neon shattered pellet injection. The modeling is done with the CQL3D bounce-averaged Fokker-Planck (FP) code [2,3] coupled to the 3D extended MHD code NIMROD [4]. Time-dependent data on plasma profiles (Ohmic current density, temperatures, toroidal electric field, densities of all ionization states of Neon, magnetic field fluctuation, etc.) is read from NIMROD and mapped to CQL3D grids. The CQL3D advances the FP solution for electrons, with adjustment of electric field when the REs appear, in such a way as to maintain a nearly constant current density during thermal quench. Based on the survey results, a range in the magnitude and shape of the radial diffusion coefficient is determined that is consistent with the RE current observed in the experiment.

[1] A.B. Rechester, M.N. Rosenbluth, PRL 40, p.40 (1978)

[2] R.W. Harvey, V.S. Chan, S.C. Chiu et al., Phys. Plasmas 7, 4590 (2000).

[3] R.W. Harvey and M.G. McCoy, www.compxco.com/cql3d.html.

[4] C. R. Sovinec, A. H. Glasser, T. A. Gianakon, et al., J. Comp. Phys. 195, 355 (2004).