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

Abstract

A comparison of the ASDEX Upgrade 3-strap ICRF antenna data with the linear electromagnetic 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.

Nonlinear plasma sheath potential in the ASDEX Upgrade 3-strap antenna: a parameter scan

“…In further figures in this paper, we will omit data beyond the vertical range of the antenna, and use a common non-slanted coordinate system for the vertial coordinate (also shown in figure 2). Also in figure 2, the green dashed lines indicate where SSWICH-SW imports an E field map from another code (here RAPLICASOL, see the next section), to use as boundary condition in (1). The orange dashed lines indicate where SSWICH-SW uses E = 0 as boundary condition.…”

A post-processing method to simulate the generalized RF sheath boundary condition

“…where  (1) and  (2) are the amplitudes of the electrostatic potential for the incoming and outgoing waves respectively. By convention k sw,x is taken as the root with k sw,x > 0.…”

“…The sheaths impact the wave fields and the local wave fields impact the sheaths. The rectification of rf voltages across the sheath is believed to contribute to power losses [1] and enhanced sputtering, the latter motivating the investigation of new antenna designs [2,3].…”

3-Dimensional density profiles in edge plasma simulations for ICRF heating

“…It is important to understand the behaviour of Ion Cyclotron Range of Frequencies (ICRF) waves in edge plasmas and near the antenna, in order to understand the physical mechanisms underlying experimental observations such as ion sputtering [1,2], RF induced convection [3], and variation in power coupling to the plasma.…”