Member of the Helmholtz Association A. Litnovsky et al., Third rehearsals on PSI-18, Jülich, Monday, May 19, 2008 Investigations of castellated structures for ITER: the effect of castellation shaping and alignment on fuel retention and impurity deposition in gaps A. Litnovsky P. Wienhold, V. Philipps, K. Krieger, A. Kirschner, D. Matveev, D. Borodin, G. Sergienko, O. Schmitz, A. Kreter, A. Pospieszczyk, U. Samm, S. Richter, U. Breuer, J. P. Gunn, M. Komm, Z. Pekarek and TEXTOR Team

Slide 2 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Motivation Investigations at TEXTOR Castellated structures had rectangular and shaped cells Castellated vertical target of ITER divertor Shaping made to minimize deposition in gaps Radioactive tritium (the fusion fuel) can be accumulated in the gaps in between the cells The divertor and the first wall of ITER will be castellated by splitting it into small-size cells to insure thermo-mechanical durability of ITER Limiter with castellated structures exposed in the SOL of TEXTOR

Slide 3 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Exposure in TEXTOR Metallically shiny plasma- facing surfaces: exposure under erosion-dominated conditions 16 discharges, 112 s. T e ~20 eV N e ~6*10 12 cm -3 Averaged fluence 2.2×10 20 D/cm 2 Bulk temperature 200 o C-250 o C Local heating of plasma closest edge up to 1500 o C Details of exposure: 20 o Toroidal gap Poloidal gap Poloidal direction Toroidal direction B t, I p SOL Plasma 20 o BtBt Shaped cells Rectangular cells We discriminate: Poloidal and toroidal gaps Shaped and non-shaped cells Cell dimensions: 10x10x12/15 mm, gap width: 0.5 mm W castellated limiter

Slide 4 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Analyses after exposure EPMA scan NRA scan SIMS (calibrated with Dektak) Accuracy of C, D quantification is ~ 20%. Secondary-Ion Mass spectrometry SIMS (FZJ) Quantification of C and D in the deposits Stylus profiling with DEKTAK (FZJ), Metal mixing in the deposit and its quantification Nuclear Reaction Analysis, NRA (IPP Garching) Electron Probe MicroAnalysis, EPMA (RWTH Aachen) Trapping ratio of carbon and deuterium in gaps Depth distributions of elements Tasks Typical scheme of investigations

Slide 5 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, ,7% Observations: Strong W intermixing on the plasma-closest edges of gaps (up to 70 at. % of W); W-fraction decreases rapidly with the depth of the gap: λ W <λ c W intermixing in the deposits: typical examples Metal mixing in the deposits will make cleaning of the gaps difficult in ITER at. %

Slide 6 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Typical profiles of C and D deposition in poloidal gaps 0 C, D (x10), at./cm 2 Distance along the gap, mm C rect. C shaped. D rect. D shaped. Less peaked profiles in shaped gaps Bottom of the gap (later in this presentation) NRA data Erosion zone Maximum deposition on the shadowed side Plasma Plasma open side Plasma shadowed side 0 30

Slide 7 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 D=3.03 C=20.4 shadowed Summary: Comparable C amount Less D in the gaps of shaped cells Low D/C ratio: D/C<5% Poloidal gaps: results D= 0.54 C=11.2 open D=0.96 C=13.9 open D=9.69 C=21 shadowed D, [10 15 at] C, [10 16 at] Rectangular cellsShaped cells Further shape optimization required Up to 70 at. % W in the deposit e.g. by making the roofs less steep

Slide 8 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Summary: Toroidal gaps: results BtBt Left of field line direction Right of field line direction C deposition: 13-25×10 16 at. D accumulation: 1-10×10 15 at. W fraction: 0-4 at. % C deposition: 28-45×10 16 at. D accumulation: 1-15×10 15 at. W fraction: 5-10 at. % Less deposition on the left side W intermixing is relatively low Significant C deposition and D accumulation detected Shape optimization is necessary

Slide 9 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Results: comparison of poloidal and toroidal gaps Total integrated amount ShapedRectangularType of gaps D, (10 15 ) at Poloidal gaps Toroidal gaps, row Toroidal gaps, row 2 C, (10 16 ) at Poloidal gaps Toroidal gaps, row Toroidal gaps, row 2 For parallel plasma fluence: 6.6*10 20 D/cm 2 with 3% C in the plasma Material trapped in the gaps Comparable amounts of C and D in toroidal and poloidal gaps Toroidal gaps cannot be excluded from analyses of fuel retention and impurity deposition D trap <1.2*10 -4 (<0.01% of impinging D flux) C trap <0.1 (<10% of impinging C flux)

Slide 10 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Preliminary results: No metal intermixing in the deposits; No clear poloidal / toroidal difference in the deposition: deposition via neutrals? Deposition at the bottom of a castellation: first investigations nm Up to 200 nm thick deposits; Same behavior observed in the long-term experiment with ALT tile [1]. [1] A. Kreter et al., Meeting of the ITPA TG on DSOL, Avila, Spain, Bottom surfaces contain about 14% of C amount deposited in poloidal gaps Deposition at the bottom of gaps must be taken into account and understood

Slide 11 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Modeling of deposition in the gaps Model Reflection or neutral collisions alone cannot explain observed deposition profiles Partial qualitative agreement with experiments (with chemical erosion) Current status Modeling algorithms further to be improved Transport along straight lines, reflection at the walls Neutral collisions included Chemical erosion Homogeneous mixing model (HMM)

Slide 12 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Summary At least two times less D in the poloidal gaps of shaped cells. Less than 30% difference in C deposition. Geometry has to be further optimized; Significant amount of W found intermixed into deposit. This will provide difficulties in cleaning deposits in the gaps; Modeling of C deposition reproduced only partly the experimental deposition patterns. Further improvement of algorithms is needed. Toroidal gaps contain comparable amount C and D as poloidal ones and cannot be excluded from analyses of carbon transport and fuel accumulation; The limiter was exposed in the erosion-dominated conditions. Nevertheless, there are deposition-dominated conditions in the gaps. About 10% of impinging C and less than 0.01% of impinging D fluxes was trapped in the gaps; Deposition at gap’s bottom cannot be described by the simple particle reflection and calls for the further clarification of deposition mechanisms;

Slide 13 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Thank you

Slide 14 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Further steps Better characterization of the deposits at the bottom of the gaps: NRA in IPP Garching. New exposure of castellated limiter: cells with conventional and optimized shaping at three different angles to magnetic field. Plasma background in the gaps: N e, N i, T e, T i, v par (J. Gunn, CEA) to be introduced in the modeling codes Gap Additional slide 1

Slide 15 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 C and D Trapping ratio in the gaps Additional slide 2 To be compared with: Maximum D content in the poloidal gaps: 1.1*10 16 D Maximum D content in the toroidal gaps: 2.7*10 16 D Maximum C content in the poloidal gaps: 3.5*10 17 C Maximum C content in the toroidal gaps: 6.5*10 17 C Parallel plasma fluence: 6.6*10 20 D/cm 2 to e-side: 2.2*10 20 D/cm 2 to i-side: 4.2*10 20 D/cm 2 assuming 3% of C in plasma: to e-side: 6.6*10 18 C/cm 2 to i-side: 12.6*10 18 C/cm 2 Which yields to: D trap <1.2*10 -4 (<0.01% of impinging D flux) C trap <0.1 (<10% of impinging C flux)

Slide 16 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Toroidal gaps: quantification of deposits Additional slide 3 2D deposition patterns were selected, based on the color of a deposit For each pattern the thickness was taken based on NRA/EPMA/SIMS results This thickness was cross-calibrated with colorimetrical tables Thickness was then re-calculated to the total amount of atoms and multiplied to the area of the deposition patterns plasma flow A2A2 A3A3 A1A1 A4A4 A2A2 A3A3 A1A1 A4A4 A6A6 A7A7 A5A5 A8A8 A6A6 A5A5 P. Wienhold

Slide 17 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Toroidal gaps: the nature of deposition Additional slide 4 BtBt Left of field line direction Right of field line direction plasma viewing side plasma shadowed side Plasma BtBt Similar deposition patterns on toroidal gaps independently on plasma-shadowing Cannot be described based on misalignment of magnetic field May be explained by gyro-motion of particles Gap width might be decisive BtBt Gap width 0.5 mm R L (D + ) ~ 1.6 mm R L (C 4+ ) ~ 1 mm D +,C x+

Slide 18 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Distance along the gap, mm C, at/cm 2 Plasma flow A B A: Plasma facing side of the gap SIMS (calibrated with Dektak) EPMA scan ~ 1 mm EPMA value (without W mixing) ~ 25 nm NRA value ~ 42 nm SIMS/DEKTAK value ~ 50 nm EPMA modeled value (41 at. % W in the deposit) EPMA, NRA, SIMS/DEKTAK NRA scan W intermixing in the deposit Additional slide 5

Slide 19 of 13 A. Litnovsky et al., PSI-18, Toledo, Spain Monday, May 26, 2008 Plasma-open sides Plasma-shadowed sides Additional slide 6