Investigation of plasma cleaning on molybdenum crystals for diagnostic mirrors in fusion devices

Sammanfattning: In the long-term energy scenario, nuclear fusion is a promising low-carbon and sustainable energy source. The unavoidable exposure of the wall materials to the plasma is a big challenge in realizing a fusion reactor. The plasma-facing components (PFCs) in the reactor wall will experience erosion deposition in the surface materials.  First Mirrors (FMs) are one of the PFCs and the main component of optical plasma diagnostics. One of the main issues that FMs have to address, is the re-deposition of impurities from the plasma. The presence of contaminants on the surface of the mirror affects their reflectivity. In-situ cleaning methods are proposed to restore their performance, such as plasma cleaning.  In this project, the investigation of different methods of plasma cleaning on Molybdenum crystals is performed. The metal mirrors are deposited with Tungsten to simulate a “contamination” on their surface. Characterization of the  Molybdenum mirrors was performed before and after the deposition and consequent plasma cleaning, to compare their performance. The results of this investigation have shown that the Magnetron Scattering technique is the more effective among the different methods experimented on. The performance of the FMs was generally regained and the remaining deposition was limited to a few angstroms. The reflectance measurements on the metal mirror after the plasma cleaning were comparable to the results on the sample before the deposition. The method was also relatively fast and the cleaning rate of the deposition was estimated to be ~ 0.6 nm/sec.