Including Finite Larmor Radius Effects on RF Heating of Fusion Plasmas using Weak-Form Contributions in COMSOL

Abstract: In a fusion plasma, the ions have to be heated to reach fusion relevant temperatures. One possibility is to launch an electromagnetic wave in the radio frequency band into the plasma. This wave can resonate with the ions at their cyclotron frequency and hence, the method is called ion cyclotron resonance heating. If the Larmor radius is of similar length scales as the wavelength, finite Larmor radius (FLR) effects are important. This introduces additional possible wave modes. To accurately predict the heating, simulations of these additional modes can be important. In order to describe the FLR effects in simulations, this work applies a Taylor expansion to second order in the perpendicular wavenumber of the dielectric tensor. In real space, the Taylor expansion corresponds to a series of spatial derivatives. These derivatives are implemented as weak-form contributions for a onedimensional finite element method (FEM)simulation. The approach is realized in a fork of the FEMIC code, coupling plasma physics in MATLAB with the FEM solver in COMSOL. The discretization of the FEM solver is adapted using a Helmholtz filter to provide the required degree of smoothness. Our approach proves successful to simulate FLR effects within the limitations caused by the modeling choices. We compare results with an all-order FLR method, showing good qualitative agreement. This work serves as a proof of concept to describe challenges on the way towards incorporation of second order FLR effects in twodimensional simulations in FEMIC.