Title
Radio-frequency plasma to clean ITER front-end diagnostic mirrors in geometry of Edge Thomson Scattering system
Author
Ushakov, A.
van Veldhoven, J.
Rijnsent, C.G.J.
Yatsuka, E.
Verlaan, A.L.
Stephan, U.
Steinke, O.
Maniscalco, M.P.
Moser, L.
Bassan, M.
Yokoyama, M.
Beekum, E.R.J.
Takaki, H.
Publication year
2023
Abstract
The ITER Edge Thomson scattering (ETS) system provides electron temperature and density profile measurements in the ITER tokamak. In collection optics, the front-end metallic first and second mirrors are expected to experience contamination with beryllium, tungsten and construction materials. Plasma cleaning based on a low-pressure radiofrequency discharge is expected to sputter contaminants. In the plasma cleaning system, a water-cooled first mirror is combined with a powered electrode. Water cooling was realized as a notch filter for the driving frequency with the electrode grounded for a DC-voltage. To understand plasma cleaning effects, a new test model reproducing the ETS First and the Second mirror geometries in a vacuum chamber was developed. Ion energies and fluxes were measured for 40–50 MHzdischarges in argon and helium at 1–10 Pa with and without the notch filter for various power transmission schemes. Powers in plasma were increased to 300–400W to achieve ion fluxes suitable for cleaning. 40 MHzdischarges were used for cleaning as being more stable. In helium at 5 Pa the ion flux of 1.3·1019 ions·m−2 s−1 and the ion energies of 120–140 eV were considered for cleaning. Sputtering rates of metal layers were measured at 4–5 nmh−1 forW/WO3 films. Sputtering rates were non-uniform over the electrode and were lower than 0.5nm h−1 at the edges. At 40–50 MHz, two independent discharges could be ignited at the First and the Second mirrors in argon and helium, and were maintained with minimum interference. Redeposition rate on the walls was estimated as 1–1.5nm h−1, mainly consisting of the chamber construction materials. Parasitic discharges were observed at powers above 200Win plasma and influenced plasma stability at pressures 1–2 Pa. The results are important for a number of ITER optical diagnostics where plasma cleaning of front-end water-cooled diagnostic mirrors shall be used.
Subject
Edge Thomson Scattering
Plasma cleaning
First mirror
Radio-frequency discharge
ITER optical diagnostics
Capacitively coupled plasma
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http://resolver.tudelft.nl/uuid:c082af61-fa6c-429a-a45b-48ecbe8052e0
DOI
https://doi.org/10.1088/1402-4896/aca5c5
TNO identifier
981057
Publisher
IOP Publishing, Bristol, UK
Source
Physica Scripta, 98 (98)
Document type
article