Development of vacuum RF feeder lines for front-end mirrors plasma cleaning in ITER optical diagnostics

The Visible Spectroscopy Reference System (VSRS.55.E6) is going to be one of the first optical diagnostics in the ITER tokamak. The VSRS front-end part shall contains the 100 mm diameter first mirror (FM). The FM is expected to experience contamination with beryllium and tungsten. Levels of contamination on the order of 10 nm degrade the FM optical performance. In the VSRS, a high-frequency RF-discharge-based plasma cleaning will be implemented to maintain or restore the optical performance. The system also contains more than 30 m long RF power feeding components, partly in vacuum, and a number of interfaces. In the present report, we analyze the design requirements for the RF electrical circuit with a plasma load to prevent power loss, parasitic discharges and vacuum arcing. The proposed analysis is useful for long RF circuits for plasma cleaning of front-end optics.