RF discharge forin situmirror surface recovery in ITER

Abstract

Almost all optical diagnostic systems in ITER will require the implementation of mirror recovery and protection systems. Plasma cleaning is considered to be the most promising technique for the removal of metal deposits from optical surfaces. The engineering and physical aspects of RF discharge application for continuous or periodic plasma treatment are discussed with a focus on implementation under ITER conditions. The ion flux parameters obtained in capacitively coupled (CC) RF discharge were measured in the mock-up of a plasma cleaning system. The uniformity of sputtering in CC RF discharge with and without a magnetic field was studied experimentally for the cylindrical discharge reactor geometry and compared with numerical simulations. The sharp increase in the sputtering rate resulting from the non-uniform radial distribution of the ion flux was observed near the electrode edges. The longitudinal magnetic field improves sputtering uniformity. It was demonstrated that Al/Al 2 O 3 deposits can be removed in the Ne and D 2 plasma of CC RF discharge but longterm exposition results in the degradation of the polycrystalline molybdenum mirror surface. The efficiency of Al sputtering in the atmosphere containing O 2 and N 2 fractions was studied in the D 2 /O 2 and D 2 /N 2 plasma of glow discharge. The addition of 2% of oxygen or nitrogen increases the sputtering yield by 3-4 times as compared with that in a nominally pure D 2 discharge. The impact of metal deposits on the performance of diagnostic mirrors is discussed. It was shown that an ultrathin metallic film with a thickness as low as a few nm may cause a significant degradation of diagnostic mirrors with a transparent coating.

ITER perspective on fusion reactor diagnostics—A spectroscopic view

“…The latter is currently considered as most promising. It uses the mirror itself as the driven electrode, allowing good coupling of the cleaning plasma to the mirror surface, can build on the industrial experience with RF plasmas and has shown in lab experiments to be able to clean both small and large surfaces [7,8,10]. Also tests in magnetic field have been performed [11].…”

Functional materials for tokamak in-vessel systems—status and developments

“…Useful data on the potential effect of ITER-type reactors on mirrors have been obtained from the diagnostic systems in existing large-scale fusion facilities [36][37][38][39]. A further issue for deposition on in-vessel mirrors is the possible use of cleaning techniques for periodic removal of deposited material, where local discharge in deuterium and laser ablation are being developed [3,[40][41][42][43][44][45][46][47].…”

In situcleaning of diagnostic first mirrors: an experimental comparison between plasma and laser cleaning in ITER-relevant conditions

“…Currently, the two most investigated solutions are plasma cleaning [4][5][6][7][8][9][10] and laser cleaning [11][12][13][14][15][16][17], both of which may have potential advantages and drawbacks.…”