Plasma parameters and tungsten sputter rates in a high-frequency CCP

Abstract

Monitoring and investigating the fusion plasma in ITER will be crucial to pave the way to a fusion power plant. However, the harsh conditions in the vacuum vessel are detrimental for the optical diagnostics systems. Replacing the element with a direct line of sight to the fusion plasma by a metallic mirror shifts the problem to this component. The flux of impurities onto these mirrors accumulate to deposits, which degrade their optical properties over time. It has been proposed to address this issue by igniting discharges in front of the mirrors during the maintenance phases allowing the deposited material to be sputtered away and recover the mirror properties. To further the knowledge for such an option, in this work, plasma parameters and sputter rates in a high-frequency (60 MHz) capacitive discharge in argon at pressures below 10 Pa are studied. The powered electrode consists of tungsten as a cheap rhodium proxy—the material of the metallic mirrors in ITER—and to simulate tungsten deposition. Its size is equivalent to a mirror for charge-exchange recombination spectroscopy at ITER (8.5 cm × 18 cm). The discharge is studied using and interpreting voltage measurements, microwave interferometry, electrical probe measurements, and optical emission spectroscopy. These investigations provide the opportunity to identify the optimal conditions for the process based on various requirements, such as damage threshold of the mirror crystal and severity and type of contamination.