UWAVS first mirror after long plasma cleaning: Surface properties and material re-deposition issues

One of the important aspects of the service life of the first and second optical mirrors (FM and SM) in ITER UWAVS diagnostics is to understand the influence of plasma cleaning on materials re-deposition and optical surface quality after multiple cleaning cycles. The capacitively coupled RF 30–60 MHz system is a candidate for the UWAVS mirror cleaning. It generates ion fluxes of tens of electron-volts, sputtering contaminants as well as construction materials. In the present report, we discuss materials re-deposition after cleaning with 30–60 MHz discharges inside the prototype of the front-end vacuum compartment. Typical ion energies were below 100 eV at the RF electrode and 20–30 eV at the grounded wall. Materials sputtered from the FM were deposited at various locations and on the SM. The deposition rates were estimated as 0.05 nm/hour or lower. The experiments showed advantages of Molybdenum as a plasma facing construction material due to lower sputtering. Traces of Fluorine were found, which may indicate cable damage at higher powers. The single-crystal Mo-mirror mock-up was etched at 0.5–1 nm/hr. rate. The mirror reflectivity showed satisfactory results after 60 h of exposure, with 50–60 nm removed. The overall effect on the reflectivity considering full service time is yet to be investigated.