W. Tierens

Making ICRF power compatible with a high-Z wall in ASDEX Upgrade

A comparison of the ASDEX Upgrade 3-strap ICRF antenna data with the linear electro-magnetic TOPICA calculations is presented. The comparison substantiates a reduction of the local electric field at the radially protruding plasma-facing elements of the antenna as a relevant approach for minimizing tungsten (W) sputtering in conditions when the slow wave is strongly evanescent. The measured reaction of the time-averaged RF current at the antenna limiters to the antenna feeding variations is less sensitive than predicted by the calculations. This is likely to have been caused by temporal and spatial fluctuations in the 3D plasma density distribution affected by local non-linear interactions. The 3-strap antenna with the W-coated limiters produces drastically less W sputtering compared to the W-coated 2-strap antennas. This is consistent with the non-linear asymptotic SSWICH-SW calculations for RF sheaths.

Higher-order hybrid implicit/explicit FDTD time-stepping

Both partially implicit FDTD methods, and symplectic FDTD methods of high temporal accuracy (3rd or 4th order), are well documented in the literature. In this paper we combine them: we construct a conservative FDTD method which is fourth order accurate in time and is partially implicit. We show that the stability condition for this method depends exclusively on the explicit part, which makes it suitable for use in e.g. modelling wave propagation in plasmas.

Unification of Leapfrog and Crank--Nicolson Finite Difference Time Domain Methods

In this paper we present a unified theory of explicit (leapfrog) and implicit (Crank--Nicolson) finite difference time domain (FDTD) time-stepping schemes. This enables us to interface leapfrog FDTD with Crank--Nicolson FDTD and we will prove that the result is stable at the Courant limit of the leapfrog part. It also enables us to construct explicit FDTD-like algorithms whose stability condition is less restrictive than that of leapfrog FDTD, and a remarkable explicit 1:2 FDTD refinement scheme which is stable up to the Courant limit of the coarse part.

Explicit and provably stable spatiotemporal FDTD refinement

Abstract In this paper we introduce an explicit and provably conditionally stable Finite Difference Time Domain (FDTD) algorithm for Maxwells equations, with local refinement in both the spatial discretization length and in the time step (spatiotemporal refinement). This enables local spatial refinement with a locally reduced time step.