Results from integrated modeling of RF-induced ponderomotive effects on edge/SOL transport

Fast time-scale RF fields in the scrape-off layer result in slow time-scale ponderomotive forces which can perturb the local density profiles near the RF antenna structures. We have coupled the Vorpal (FDTD EM+plasma solver) and UEDGE (2D edge plasma transport) codes in a manner enabling numerical study of these ponderomotive terms and their effects on edge/SOL transport and profiles, and present results from studies with the coupled codes.

We look at a 2D poloidal plane model, with an antenna strap and box for the RF, and tokamak H-mode diverter scrape-off layers for the transport timescale. We review how different terms can dominate for ions and electrons, in particular, with electrons seeing a gradient-density term that can dominate for the typical steep H-mode pedestal density profiles. We also highlight how the ponderomotive forces are dominated by the strong fields at the corners of the antenna box.

In NSTX-like scenarios with experimentally realistic plasma profiles and antenna parameters, we observe that the ponderomotive contribution to parallel electron momentum is comparable to or larger than other edge transport processes, for representative RF input power fluxes. This parallel momentum source transports density away from the region immediately in front of the RF antenna, necessitating an iteration between the RF and Transport codes, in order to converge to an RF-perturbed equilibrium. We discuss details and difficulties of this iteration process.