Design and analysis of first mirror plasma cleaning electrical circuit for Edge Thomson scattering ITER diagnostics

Abstract

The ITER Edge Thomson scattering system provides measurements of the electron temperature and density profiles at the periphery plasma of the ITER tokamak for physics studies and advanced plasma control. The front-end of the system contains a relatively large 23 cm × 25.8 cm metal first mirror (FM). Radio-frequency plasma is considered to remove contaminants from the FM. The new design approach for the power transmission circuit for plasma cleaning includes a pre-matching element next to the FM and a notch-filter. The FM is water-cooled and the water cooling pipe length is tuned as a notch filter for the driving frequency. The power dissipation in the circuit elements was analyzed in a series of simulations at frequencies 30, 40 and 50 MHz. New plasma impedance values used for the model were measured in argon and helium plasma at 1–10 Pa in a special mock-up of the front-end optics compartment. The results define inductance, capacitance and geometry of pre-matching elements and help to predict the performance of the plasma cleaning circuit.