Mitglied der Helmholtz-Gemeinschaft Application of rarefied gas dynamics for design of the ITER optical diagnostics 64th IUVSTA Workshop, May 16-19th 2011 V. Kotov, D. Reiter, IEK-4 - Plasma Physics
Mirrors in ITER Optical diagnostics bring most of information about plasma the picture is a courtesy of ITER www.iter.org Mirrors redirect light to the protected instruments First mirror faces the plasma directly Degradation of reflectivity due to incident particles 64th IUVSTA Workshop, May 16-19th 2011 V. Kotov, D. Reiter, IEK-4 - Plasma Physics Folie 2
Degradation of the mirrors First Wall Duct Mirror Be,C D,T Be,C Protected in the diagnostic ducts: no direct ion fluxes Fast atoms (D, T, He): erosion Impurities (Be, C, Fe, W etc): formation of deposits Flux to the mirror<aperture flux How effective is the protection? Free molecular flow approximation (MFP>3 m, size 1 m) straightforward numerical modelling 64th IUVSTA Workshop, May 16-19th 2011 V. Kotov, D. Reiter, IEK-4 - Plasma Physics Folie 3
Special feature 1: non-thermal incoming particles 3D distribution function f (E, θ, φ). Neutral particles come: From the plasma facing components From plasma: charge-exchange and elastic collisions f(e),1/ev 1 0.1 0.01 0.001 0.0001 1e-05 #1679 UP, D UP, Be EP, D EP, Be f(sin 2 (θ)) 20 15 10 5 UP, D UP, Be EP, D EP, Be #1679 1e-06 1e-07 0 1 2 3 4 5 6 7 8 9 10 E, kev 0 10 20 30 40 50 60 70 80 θ, DEG. Reflection from kev plasma: high-energetic tail of D, T Distribution of sources: grazing incidence prevails 64th IUVSTA Workshop, May 16-19th 2011 V. Kotov, D. Reiter, IEK-4 - Plasma Physics Folie 4
Special feature 2: particle-surface interaction Reflection coefficients R(E,θ; E,θ ) for fast, >10 ev, particles (D, T, He): Binary collisions with lattice atoms TRIM code, W. Eckstein, IPP Garching Reflection coefficients of impurities (Be, C etc.): Empiric, molecular-dynamics First Wall Duct Mirror Be,C D,T Be,C Important process: sputtering of deposited films from the duct wall Caveat: the data on impurity reflection and the sputtering of deposits are incomplete and uncertain Parametric studies and worst cases 64th IUVSTA Workshop, May 16-19th 2011 V. Kotov, D. Reiter, IEK-4 - Plasma Physics Folie 5
The numerical tool: EIRENE f(r, v, t) + v rf(r, v, t) = Q(r, v, i, t)+ t + σ(v, V ; v, V) v V f(v )f b (V )dv dv dv f b (V) σ(v, V; v, V ) v V f(v)dv dv dv Linear Monte-Carlo particle transport code Fixed background for test particles Primary application: interaction of magnetized plasma with rarefied neutral gas Models for elementary processes, particle-surface interaction Arbitrary 3D geometry, MPI-parallelized Widely used in magnetic fusion community Developed at FZJ by D. Reiter et al., see www.eirene.de 64th IUVSTA Workshop, May 16-19th 2011 V. Kotov, D. Reiter, IEK-4 - Plasma Physics Folie 6
The results of parametric studies M1 A (716.52, 285.753, 28.9404) Gross Deposition Rate (Be), nm/s 2e 05 3.2e 05 5e 05 8e 05 0.00013 0.0002 An example: core-cxrs diagnostic ANSYSgenerated grid for CAD geometry Experimentally found limits for erosion can be reached after >10000 ITER discharges. Good :-)... for deposition: after 1-1000 discharges. Too bad :-( Impurity flux attenuation by a factor 100: factor >10000 is desirable 64th IUVSTA Workshop, May 16-19th 2011 V. Kotov, D. Reiter, IEK-4 - Plasma Physics Folie 7
Protection by baffles ØD l 1 l L L M l 2 M ØD ØD f Example: Impurity reflection coefficient R N = 0.1 Enhanced re-erosion Attenuation factors (L/D=20): W/o baffles: <1500 With baffles: <20000 (!) (D f = 2D, l 1 = 0, l 2 = 2D, l = D) But this is not a universal solution: Relies on R N <1, formation of molecules: R N 1 Geometry is not suitable for all diagnostics Protection by gas puff as an option??? Possibly small flux: to minimize disturbance of plasma! 64th IUVSTA Workshop, May 16-19th 2011 V. Kotov, D. Reiter, IEK-4 - Plasma Physics Folie 8
Summary Life-time and performance of the ITER optical diagnostics is limited by the degradation of mirrors Protecting effect of the diagnostic ducts has to be estimated Free-molecular flow problem with two special features: Non-thermal sources of test particles Complex particle-surface interaction The 3D test particle Monte-Carlo code EIRENE is applied Analysis shows that erosion can be effectively reduced But reduction of deposition could be insufficient Protection by gas puff (transition flow!) may be an option This work has been done in frame of the ITER Service Contract C4T/09/71/OLT CHD/DIAGNOSTIC 64th IUVSTA Workshop, May 16-19th 2011 V. Kotov, D. Reiter, IEK-4 - Plasma Physics Folie 9