ITER VSRS first mirror plasma cleaning using high frequency gas discharge – understanding RF circuit design and plasma effects

The line averaged measurement of the visible continuum emission of the core plasma (55.E6 VSRS) is going to be one of the first optical diagnostics in the ITER tokamak chamber It will contain so called “First Mirror”(FM) as a front end optical component. The top candidate FM material for general applications is single crystal Molybdenum. It has a good sputter resistance and a good compatibility with the radiofrequency (RF) mirror cleaning method. For the VSRS diagnostic system, the 100 mm diameter FM located at the front end of the optical system looking into the tokamak chamber is expected to experience contamination with beryllium and tungsten. Small levels of contamination (on the order of 10 nm) can significantly degrade optical performance of the mirror. In the VSRS design, a cleaning system will be implemented to maintain or restore the FM optical performance. The preferred cleaning technique is capacitive coupled, radio frequency discharge (CCP) which utilizes ion fluxes in gas discharge plasmas to sputter contaminations on the mirror surface. A discharge operating at 13.56 MHz, or in a 30.80 MHz frequency range in an inert gas such as He, Ar or Ne can be considered. The exact choice of RF, power and gas is a subject of subsequent R&D work. In the present report, we analyze the composition of the RF electrical circuit with a plasma load at the end Characteristics of the critical plasma cleaning system components are assumed and discussed based on the numbers of ion fluxes and energies obtained in the previous research for the UWAVS ITER diagnostics. The proposed analysis can be useful for other diagnostic systems containing FM where a gas discharge based FM plasma cleaning approach is applicable. Figs. 1-3 illustrate composition of the VSRS system with FM and SM positions. Although the available information is not complete, a generic approach for a plasma cleaning system circuit design can be proposed based on the previous experience.