Changes of optical properties of retroreflector installed in LHD

Abstract

This article describes changes of optical properties of retroreflectors installed on the inner wall of the vacuum vessel of the Large Helical Device. They were made of stainless steel and were used for a CO 2 laser polarimeter. The reflectivity for a CO 2 laser beam dropped up to 40% after the start of glow discharge cleanings and main plasma experiments. Then it decreased gradually. The reflectivity in a wavelength range shorter than 10 m decreased significantly through one experimental campaign ͑about 4 months͒. On the other hand the reflectivity in a wavelength range longer than 50 m was larger than 70%. Decrease in the reflectivity in the central region of mirrors was more significant ͑the reflectivity for visible beams became almost zero͒ than that in the edge. The distribution of the reflectivity along the radial direction is expected to be related to the shape of the retroreflector. The parallelism of the reflected beam to an incident one also deteriorated. The changes in the polarization angle and in the ellipticity of reflected light polarization were not observed from the visible to the far infrared range.

Modification of optical characteristics of metallic amorphous mirrors under ion bombardment

“…if mirrors are screened by a buffer structure (HL-2A [23]) or a diaphragm (cylindrical, conical or L-type, LHD [25], HL-2A [26]), the appearance of contaminating deposit cannot be avoided. Qualitatively similar situation is realized for retroreflectors (Tore Supra [22], LHD [25,27,28]) as their central part is recessed relatively to the edges, and therefore the direct plasma impact to this part is weakened.…”

Development of in situ cleaning techniques for diagnostic mirrors in ITER

“…Coupling the modeling data with the experimental results of [2,[5][6][7][8][9] leads us to the discouraging result: for many UV and VIS diagnostics, the mirror lifetime will be limited to a few ITER discharges. On the other hand, the first results of modeling taking into account the realistic geometry of diagnostic ducts [10] have demonstrated the efficiency of duct geometry to mitigate impurity fluxes towards the mirrors.…”

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

“…Mirrors are expected to be installed in a vacuum vessel, and their reflectivity might be degraded by two orders of magnitude in the near-IR and visible range because of impurity deposition or sputtering on the mirror surface [12]. Although the transmission losses in several optical components (window, beam splitter, etc.)…”

“…The latter is particularly critical for a hollow retroreflector, where impurity deposition tends to occur mostly in the central region. For example, in LHD, the reflectivity at the 1 µm range decreased by a factor of 10-100 because of impurity deposition during one experimental campaign (about four months) [12]. Passive shielding measures or active in-situ cleaning of the reflecting surfaces are options for maintaining a sufficiently high reflectivity.…”

“…A Si photodetector with a variable conversion gain of 10 3 -10 11 V/W (OE-200-SI, FEMTO Messtechnik GmbH) will be used to detect the second harmonics. Assuming that the reflectivity of the retroreflector deteriorates to 1% [12], the power of the fundamental and the second harmonic (P 1 ) for a laser source with a power of 1 W and a PPGM:SLT crystal are 1 × 10 −2 W and 1.4 × 10 −4 W, respectively, at the position of the second nonlinear crystal. Here, the transmission loss other than that at the retroreflector is neglected.…”