Abstract
This paper presents an experimental comparison between the plasma cleaning and the laser cleaning techniques of diagnostic first mirrors (FMs). The re-deposition of contaminants sputtered from a tokamak first wall onto FMs could dramatically decrease their reflectance in an unacceptable way for the proper functioning of plasma diagnostic systems. Therefore, suitable in situ cleaning solutions will be required to recover the FMs reflectance in ITER. Currently, plasma cleaning and laser cleaning are considered the most promising solutions. In this work, a set of ITER-like rhodium mirrors contaminated with materials tailored to reproduce tokamak redeposits is employed to experimentally compare plasma and laser cleaning against different criteria (reflectance recovery, mirror integrity, time requirement). We show that the two techniques present different complementary features that can be exploited for the cleaning of ITER FMs. In particular, plasma cleaning ensures an excellent reflectance recovery in the case of compact contaminants, while laser cleaning is faster, gentler, and more effective in the case of porous contaminant. In addition, we demonstrate the potential benefits of a synergistic solution which combines plasma and laser cleaning to exploit the best features of each technique.
Highlights
Metallic first mirrors (FMs) will be crucial components of optical plasma diagnostics in ITER and future fusion reac tors [1]
This paper presents an experimental comparison between the plasma cleaning and the laser cleaning techniques of diagnostic first mirrors (FMs)
In this work we presented an experimental comparison between plasma cleaning and laser cleaning, which are the two candi date cleaning techniques for the ITER first mirrors (FMs), in conditions potentially relevant for ITER scenarios
Summary
Metallic first mirrors (FMs) will be crucial components of optical plasma diagnostics in ITER and future fusion reac tors [1]. Laser cleaning is employed in highly-specialized applications (e.g. conservation of cultural heritage and micro electronics) which require a remotely-operated, solventfree cleaning process [20] These features, along with its versatility, make this technique a very appealing possibility for FMs cleaning. We provide a direct experimental comparison between plasma and laser cleaning techniques, in conditions that are potentially relevant to simulate the cleaning of ITER FMs. In addition, we explored the possibility of a synergistic approach in which laser and plasma cleaning techniques are combined to recover at best the original mirror properties. The insights gathered from the direct comparison of the techniques allowed us to develop a combined cleaning strategy that foresee a laser cleaning treatment followed by a plasma cleaning session This combined strategy ensured a complete recovery of specular reflectance without any detect able damage of the mirrors’ surface
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