Faraday screen sheaths and impurity production during ion cyclotron heating

TLDR

When a plasma interacts with a material surface under an applied voltage, a sheath forms. The study develops a one‑dimensional model of the magnetized ion sheath to investigate the RF sheath near the Faraday screen of an ion cyclotron heating antenna. The model is applied to the RF sheath to analyze its behavior. The RF sheath rectification of the applied voltage creates a large DC potential drop that accelerates ions, causes sputtering, and produces high‑Z impurity influx that matches fast‑wave experiments, supporting the DC acceleration mechanism as a plausible explanation for the observed impurity release.

Abstract

When a plasma interacts with a material surface subject to an applied voltage, a sheath results. A one-dimensional model, specific to the magnetized ion sheath, is developed and applied to the radiofrequency (RF) sheath which forms near the Faraday screen of an ion cyclotron heating antenna. The RF sheath rectification of the applied voltage is shown to provide a large DC potential drop which can accelerate tons and cause sputtering. Numerical estimates of the high-Z impurity influx are compared with fast wave experiments, and it is concluded that the DC acceleration mechanism is a plausible explanation for the observed high-Z impurity release. Means of controlling the sputtering are examined, including operating with low densities at the Faraday screen.