Using the Stix Finite Element RF Code to Investigate the Power-Phasing Scan on Alcator C-Mod

Ion cyclotron range of frequency (ICRF) power plays an important role in heating and current drive in fusion devices. However, experiments show that in the ICRF regime there is a formation of a radio frequency (RF) sheath at the material and antenna boundaries that influences sputtering and power dissipation. Methods to mitigate the formation of the RF sheath have been studied through the means of optimizing the ICRF antenna. A power-phasing scan was done on Alcator C-Mod in which the amount of power was changed on the inner versus outer two straps of the 4 strap antenna showed a minimization of enhanced potentials and impurities around fractions between ~0.5 to 0.8. New capabilities in the realm of representing the RF sheath numerically now allow for these kind of experiments to be simulated. Given the size of the sheath relative to the scale of the device, it can be approximated as a boundary condition (BC). A new parallelized cold-plasma wave equation solver called Stix implements a non-linear sheath impedance model BC formulated by J. Myra 2015 ~\footnote{J. Myra, et al., Phys. Plasmas 22, 062507 (2015)} through the method of finite elements in pseudo-2D using the MFEM library [http://mfem.org]. This research will discuss the results of Stix's rectified potential values simulating this power-phasing scan done on Alcator C-Mod.