RF circuit analysis for ITER visible spectroscopy reference system first mirror plasma cleaning

Abstract The Visible Spectroscopy Reference System (55.E6.VSRS) is going to be one of the first optical diagnostic systems in the ITER tokamak chamber, for which plasma cleaning system of front-end optics has to be implemented. In order to extract light out of the fusion plasma towards diagnostic sensor systems, a set of optics forming a labyrinth will be employed. The VSRS front-end contains a 100 mm diameter first mirror (FM) that is expected to experience contamination with beryllium and tungsten degrading its performance. To maintain or restore the FM optical performance, a high-frequency-discharge-based plasma cleaning system will be implemented. The high-frequency power transmission circuit will contain four parallel mineral insulated radio-frequency (RF) cables, pre-matching elements, signal splitters and a number of interfaces. The circuit ends with the FM, where plasma is ignited in front of it. The FM is combined with the RF power electrode. The whole RF signal transmission circuit is modelled based on realistic characteristics of RF components such as cables and feedthroughs, their dimensions and ITER operational  requirements. The goal is to understand the RF power budget, power distributions and losses in the components critical for the design. First simulation results confirm that the presence of a passive L-C pre-matching element next to the FM is critical for acceptable power consumption in the circuit. The results are also useful for other ITER optical diagnostics where RF plasma cleaning of the FM is foreseen and where similar power transmission circuits may be used.