Studies of reflectivity degradation of retroreflectors in LHD and mitigation of impurity deposition using shaped diagnostic ducts and protective windows

Abstract

Maintaining the reflectivity of first mirrors is indispensable in future fusion devices. While a retroreflector (corner cube mirror) is useful for laser diagnostics, impurities tend to accumulate and form a thick deposition layer in the central region, which causes degradation of reflectivity, due to the hollow shape of the retroreflector. Two mirror structures are tested to retain the reflectivity in the Large Helical Device (LHD). One is a bending mirror structure with a protective cylinder with fins and it could maintain the reflectivity over a three-month experimental campaign. The other is a cover window just in front of the reflector. Candidates of the window materials were exposed to the LHD plasmas and the degradation of the transmissivity of ZnSe and silicon, which are used for infrared and far infrared laser light, respectively, were small.

Experimental Simulation of the Behaviour of Diagnostic First Mirrors Fabricated of Different Metals for ITER Conditions

“…Qualitatively similar results were obtained later when exposing mirrors of the same metals in the LHD facility [54]: 1.1 for ratio of SS and Mo erosion depths and 15.5 for the ratio of Cu to SS erosion depths, instead of ~7 and ~2.5, according to data in [5], correspondingly.…”

Impurity Deposition on a First Mirror Surface during Hydrogen Discharges in LHD

“…This is because the deposition particles tend to accumulate in the central region, as shown in Ref. [5]. It is possible that a uniform deposition layer (almost uniform color) on the mirror surface during the glow discharges might be sputtered to the central region from the peripheral region during the main discharges.…”

“…It would be very rare that a corner cube mirror for laser diagnostics is installed near a divertor, and the mirror surface faces it. In this specific case, however, the carbon deposition during main discharges might increase, and additional measures, such as a cylinder with fins or a protective window [5], heating of the mirror, or in situ mirror cleanings, would be necessary. We speculated that the non-uniformity of the deposition on the three mirror surfaces might be explained as follows.…”

“…Mirror materials with a low sputtering yield, such as monocrystal molybdenum and rhodium, are being developed to reduce the surface roughness by sputtering [4]. Protective measures to mitigate impurity deposition are also proposed [5]. In author's e-mail: takiyama@ms.nifs.ac.jp situ cleaning methods have been examined in fusion devices [6] or in laboratories [7].…”

Conceptual Design of Electron Density Measurement System for DEMO-Relevant Helical Plasmas

“…Passive shielding measures or active in-situ cleaning of the reflecting surfaces are options for maintaining a sufficiently high reflectivity. Passive shielding is achieved by covering windows or placing a bending cylinder with fins in front of the retroreflector [13]. In-situ cleaning can be achieved by using glow or electron cyclotron resonance discharges to remove carbon deposition by chemical sputtering and laser cleaning.…”