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Description
The Core Plasma Thomson Scattering (CPTS) diagnostic system is designed to precisely measure the electron temperature and density in the ITER core plasma region. During the fusion plasma operation, the metallic first mirror in the CPTS first mirror unit (FMU) is expected to degrade gradually due to plasma exposure, which requires radio-frequency (RF) plasma cleaning for the periodic CPTS mirror recovery. In addition, the water cooling for the FMU inevitably leads to the DC grounding of the mirror, which brings new challenges to the RF cleaning system design. To finally realize efficient contamination removal and to avoid damage to mirror materials and RF components, it is of great necessity to test the plasma cleaning for the FMU to characterize the load impedance as an input parameter to further improve the FMU design and circuit matching.
Argon plasma cleaning tests in the FMU mock-up were first conducted using mirror coupons (coated with an Al2O3 layer) at 13.56, 40.68 and 60 MHz with selected powers and pressures. The surface composition and reflectivity of the coupons before and after cleaning were analysed by X-ray photoelectron spectroscopy and a spectrophotometer. The cleaning result showed that complete contamination removal could be performed at 13.56 MHz for both configurations with and without DC grounding, which was not the case for higher frequencies due to reduced ion energies in this mock-up. Unlike the cleaning without DC grounding, structural material was deposited on both mirrors in the mock-up after cleaning with DC grounding, due to enhanced wall sputtering and weaker mirror cleaning. Furthermore, for all the tested frequencies, the whole RF circuit experienced substantial heating due to the highly power reflective load as confirmed by the standing wave voltage ratio measurement. Finally, the dependencies of the load impedance on power, pressure and frequency were characterized using three different devices: the offline substitution method (i) by a VNA, or the real-time measurement (ii) by an Octiv V/I probe or (iii) a dual directional coupler. This measurement allowed circuit modelling for power distribution analysis and the CPTS FMU pre-matching element design in the future.
The work leading to this publication has been funded by Fusion for Energy under the contract F4E-OMF-0847-01-01. This publication reflects the views of only the authors, and Fusion for Energy cannot be held responsible for any use which may be made with the information contained therein.