22nd IAEA Fusion Energy Conference, Geneva, Switzerland, 13-18 October 2008 Reflectivity Reduction of RetroReflector Installed in LHD due to Plasma Surface Interaction 3 1 3
Background and Objectives (1/2) In ITER, many first reflectors will be used for plasma diagnostics using laser beam. One of the concerns about them is the reduction of the optical reflectivity under operation due to PSI. Because they directly face to the plasma, the mirror surface suffer bombardment of energetic plasma particles and also deposition of impurity atoms. These phenomena will change the optical properties of the mirrors and thus reduce the accuracy of the diagnostics. Example of lines of sight of the poloidal polarimeter on ITER
Background and Objectives (2/2) First reflectors are also widely used in many plasma confinement devices working at present. Their experience of the first mirrors are valuable for R&D of reliable reflectors for ITER. In LHD (Large Helical Device at NIFS), retro-reflectors were
Examined Retro-Reflectors of LHD 2cm Retro-reflector made of three gold plated SS flat reflectors This was used in the cycle 4 for the CO2 laser polarimeter. Surface morphology, internal structure and optical reflectivity were examined by SEM, TEM, optical spectrometers (λ=190nm 2500nm, λ=2.5µm 20µm for point a~e.
Wave Length Dependence of Reflectivity Almost no reflection in visible region for points d and e, which areas were used for the measurements. Large reduction of reflectivity even for infrared region.
Morphology and Chemical Components SEM images and EDS spectrum for points a~e 800 1000g 720 Point e Counts 640 0 0.00 1.00 C b Au, Fe 2 µm 2 µm c FeKa 4.00 5.00 6.00 kev 7.00 AuLa AuLl FeKesc 3.00 FeKb CrKa 2.00 AuMr Au 8.00 9.00 10.00 Energy (kev) Meso-scale projections Surface roughness = 200~400nm e Fe, Cr, Ni, O Fe, Cr, Ni deposition AuMa 80 sputtering Cr AuMz CKa Fe, Cr, Ni 160 FeLa 240 CrKb 400 320 a Fe O 480 CrL1 OKa CrLa Counts 560 2 µm Roughness increase 2 µm
Modification of Reflector s Surface Im p pl uri as tie m s a Deposition of Fe, Cr, Ni, O, etc. Meso-scale projections inclining to the plasma direction Aperture edge Balance of erosion and sputtering About 2µmthichk Cube corner Eroded by plasma bombardment.
Process of Surface Modification 1. Impurity atoms and ions come from the wall as well as from the peripheries of the retro-reflector. 1. Impurities deposited on the mirror surface are transported further by the successive sputtering with energetic particles (>a few 100eV) toward cubic corner at the center and concentrate around it. 2. Hollow cubic corner structure is the reason why the thick deposition is formed at the center. Retro-reflector is an amplifier of the impurity deposition. Cubic corner
Evaluation of Ref. with Rough Surface Reflectivity Reduction by Surface Roughness (H.E. Bennett, 1963) δ: average roughness θ: reflection angle λ: wave length R: Reflectivity estimated from the average roughness δ R0: Reflectivity of a perfectly smooth surface Relactive Reflectivity 4πδ cos θ R = exp R0 λ 2 1.0 a 200nm 0.8 0.6 d c 0.4 50nm 400nm 1µm 0.2 0.0 50nm 100nm 1200nm 00nm 2300nm 00nm 3400nm 00nm 4500nm 00nm 600nm 5800nm 00nm 61000nm 00nm 8a 00nm c0 0 n m 10 e 0 5000 10000 15000 20000 Wave Length (nm) Large reduction of the reflectivity above infrared at point d can not be explained by the surface roughness (200~400nm). Other mechanisms of reflectivity reduction
Obs. of Sub-surface Structure by TEM Cross-sectional specimen preparation technique was used to obtain thin-foil specimens for TEM by using Focused Ion Beam (FIB) device. + 3mmφ TEM Disk 30keV Ga TEM e- beam sample 20mm Thickness =100~200nm 200nm Cross-sectional specimen ~200nm
Cross-sectional TEM Micrograph Top surface C depo B A MgF2 coating layer structure ( bright, dark ) Sudden change from flat to wavy Inclination Fine cavities Au coating 1µm SS base
Details of the Microstructure (A) Flat layer structure, cavities + fine FeO crystals. Fine dense holes dark layer were formed under He-GDC. (B) dark layer: flat half-cycle layer (cross section of blister) (A) large FeO crystals, less cavities in bright region
2-D Distribution of Impurities bright image region=high oxygen content larger O/Fe ratio ratio STEM / BF C O Fe Au B A 20nm
Comparison of Layer Structure of Deposition with Discharge History After New Year Holidays Start of main discharge (48H He-GDC) He-GDC start Leak from NBI (6days stop) He-GDC 26 24 22 20 18 16 1 7 5 3 H-MPD 15-14 13-9 25 27 23 21 19 17 8 6 4 2 Dark regions are formed under He-GDC. Bright regions are formed under main discharges. Domes (blisters of deposited layer) were formed after the leakage.
Mechanisms of Reflectivity Reduction 1 Surface roughness 2 Formation of oxide (Fe(Cr)O) Sponge-like structure by nano-size He bubbles Reflectivity (%) 3 a e wustite FeO) Wave Length (nm) Contributing Elemental Factors hollow corner cube structure, flux of impurities and plasma particles, inclined impurity flux, sputtering erosion, blistering of deposited layers ratio of Fe flux and O flux, easily oxidizable metal or not bombardment of energetic He plasma and He neutrals Reflectivity (%) Mechanism Wave Length (nm)
Summary (1/2) In order to know the mechanisms of reflectivity reduction of retro-reflector used in LHD, its material properties have been examined extensively by using SEM and TEM. Mechanisms of reflectivity reduction of the retroreflector used in LHD 1. Surface roughening (projections) by deposition of metallic impurities (Fe, Cr, etc.) and oxygen. 2. Formation of FeO fine crystals with low reflectivity. 3. Formation of porous material due to nanascale helium bubbles formation.
Summary (2/2) One should pay a special attention on the roles of oxygen and helium. Retro-reflector is very sensitive to impunity deposition due to its structure acting as an amplifier for deposition. Standing on the present results, a new retroreflector with long life time is now being tested in LHD. Nano-scale PSI study with cross-sectional TEM observation technique is very useful for understanding complicate PSI in the actual plasma machines.
Details of Surface Morphology at e The mirror surface is covered by deposition of about 2µm thick at the point e. Deposition composed of Fe, Cr, Ni, O etc.. Meso-scale bulges cover the deposition surface Surface roughness = 200~400nm Bulges Incline to the aperture direction. 2.6µm high 1µm Original Au surface 2µm
Micro-Structure at point A C Rather homogenous structure Mixture of FeO about a few 10nm and subnano-size crystalline He bubbles with nano-size & a few 10nm B A 30nm
Micro-Structure at point B C Very large holes between flat layer and wavy layer Complicated dark area B A 30nm
Micro-Structure at point C Bright area FeO C Crystalline of FeO, a few 10nm Less bubbles (cavities) Formed under He discharge Dark area B Nano-size crystalline He bubbles Formed under long He GDC A FeO 30nm
Process of Surface Modification (2/2) 1. Due to the directional flow of impurities, projections inclining against flow are developed, once their nucleus are formed by some reason (in usually deposited surface is flat even under directional flow.) 2. Surface roughing due to the projections may reduce the optical reflectivity. How the projections start? deposition Flow of impurities (Fe, Cr, O, etc.) reflector
What Happen at Point A? Dark image layer (with He bubbles) Flat (parallel) suddenly very wavy (perpendicular at some parts) Mo Deposit in TRIAM-1M (Al substrate) 500nm D ion irradiation vacuum deposited nano crystalline Mo (SS substrate) 200nm 500nm Formation of hydrogen blisters under H discharges after leakage of water
Possible Mechanism of Blister Formation H Plasma Dis. He-GDC Oxidation by leakage H piles-up at the interface H H Plasma Dis. blistering H+ H gas
Measures to Suppress Reflec. Reduction Hood Reduction of influx of metallic impurities, oxygen, H and He., Fin Hood Retro-reflector Fin reduction of impurity at the inner wall of hood by sputtering. Flat mirror Bending of optical path by putting a flat mirror in the hood complete suppression of impurity concentration at the hollow cube corner. Under testing in LHD (T. Akiyama et al.)
Effects of Fin on Impurity Deposition Exposure He-GDC in LHD 48 hours at a wall equivalent position He ion energy~200v 72 Oxygen free Cu 946.351 specimen Mo SUS W Retractable material transfer system
Change of Specimen Put at the Bottom Specimen; Mo (~100nm) TEM/BF TEM/DF Without Fin Mo counts Fe 10nm Cr Ｏ Ｎ Cu ｉ kev With Fin Not only Cu from the fin wall but also Fe and Cr from vessel wall deposit. Thickness of deposit ~10nm Damage by energetic He ions TEM/BF TEM/DF Mo counts 10nm Fe kev A little deposition 1nm 僅か Damage by energetic He ions