Damping RF waves with low reflection in simulations of slab or curved magnetized plasma: Parametrization, verification and implementation of Bermudez Perfectly Matched Layers

Most Radio-Frequency antenna simulations in magnetic fusion require emulating radiation at infinity at some boundaries of the simulation domain. To this end, the Perfectly Matched Layer (PML) technique amounts to stretching artificially the (real) spatial coordinates into the complex plane. Following reference [1], this contribution parametrizes, tests numerically and implements unbounded stretching functions, whose only numerical limitations arise from the PML discretization: refining the mesh can reduce the spurious reflections from the PML arbitrarily low, at the expense of a larger numerical cost. We tune the PML to ensure low wave reflection in a prescribed spectral range [kn,min, kn,max] in a direction n, with a minimal discretization (i.e. at a minimal computational cost). We extensively quantify the reflection coefficients using 1D Finite Element simulations. The minimal discretization for a regular mesh scales as kn,max/kn,min. We extend the PML formulation to a cylindrical geometry, where the wave eigenmodes involve Bessel functions. We implement slab radial and parallel Bermudez PMLs in realistic multi-2D RF simulations [2], to attenuate the propagative Slow Waves parasitically emitted by the ITER ICRF antenna into a tenuous scrape-off layer.