Numerical simulations of wall impurity emission from the RF limiters of the Alcator C-Mod ICRH antennas using the hPIC2 and RustBCA codes*

In this work, we performed numerical simulations of the wall impurity flux produced during ICRH operations in Alcator C-Mod using the hPIC2 particle in cell code and the RustBCA ion surface interaction code. We considered a deuterium plasma impacting on the molybdenum-covered RF limiters surrounding the ICRH launcher. The electromagnetic fields were calculated using the STIX electromagnetic solver in the frequency domain, which included nonlinear RF sheath boundary conditions. The output from STIX was then utilized to find the ion impact energy-angle distributions (IEAD) using the hPIC2 code at different levels of plasma contamination (Mo atomic concentration 0-5%). The resulting IEADs show the characteristic two-peak energy structure that displayed an increasing energy displacement with larger concentrations of Mo. The RustBCA code was then used to analyze the sputtering behavior of the wall and obtain maps of the Mo impurity fluxes. A parametric study as a function of the antenna strap phasing reveals a minimum on impurity emission also observed in the experiments.