ITER visible spectroscopy reference system first mirror plasma cleaning in radio-frequency gas discharge – circuit design and plasma effects

Abstract

Abstract The Visible Spectroscopy Reference System (VSRS.55.E6) is going to be one of the first optical diagnostics in the ITER tokamak chamber. For the VSRS diagnostic system, the 100 mm diameter first mirror (FM) located at the front end of the optical system is expected to experience contamination primarily with beryllium and tungsten. Small levels of contamination (on the order of 10 nm) can significantly degrade the optical performance of the mirror. In the VSRS design, a plasma FM cleaning system will be implemented to maintain or restore optical performance. An RF discharge operating in a 30–60 MHz frequency range in an inert gas such as Helium, Argon, or Neon is a candidate technology for the VSRS FM cleaning. In the present report, we analyze the composition of the high-frequency electrical circuit with a plasma load. Characteristics of critical components including the generator, matching unit, cables outside and inside the vacuum vessel, air-to-vacuum interfaces and the electrode are considered. The effect of the plasma cleaning is discussed based on the numbers of ion fluxes and energies obtained in the similar research program. The proposed analysis may be useful for other diagnostic systems using RF plasma for cleaning and connected to long RF-feed and water cooling lines.