PWI questions of ITER review working groups WG1 and WG8 : Materials Introduction EU PWI TF V. Philipps, EU PWI TF meeting, Oct 2007, Madrid V. Philipps, B.N. Bazylev, I.S. Landman, L. Colas R. Neu, K. Schmid, R. Dux, R. Doerner, J. Roth, Ch. Linsmeier, V. Bobkov, A. Loarte, A. Kallenbach, A. Kirschner

1.Document the considerations excluding the use of Be also for high heat-flux components in the divertor power handling : Be armour must be thin to allow margins for transients erosion: Ero with no Be from the main chamber (worst case) maximum gross Be erosion 50nm/s with redeposition : 9nm/sec (maximum). with Be/D 0.1% 1nm/s. Calculations indicate Be/D < 0.1% in the outer divertor net Be erosion EU PWI TF V. Philipps, EU PWI TF meeting, Oct 2007, Madrid

2. Clarify the need for additional protection limiters in the vessel to protect the latter during start-up, plasma control excursions and transient events. Support these statements by consistent start-up scenarios The definition of expected wall loads in steady state and in transients done in another group steady state wall loads edges must be protected against parallel power flux (between panels, portplugs, upper dump plates needs a double rough like shaping ( uncertainty of X point position) transients: effective control of ELMS, disruptions, VDes must be on board EU PWI TF V. Philipps, EU PWI TF meeting, Oct 2007, Madrid

3. Melting of the Be wall during transients (including the radiation flash from mitigated disruptions Data on Be melt layer behaviour are from Memos code calculations, not much validated, validation better for W ( RF cooperation) Melting by ion impact protection by vapour shielding Radiation impact no protection, larger evaporation depths. Melt hills and craters of several microns per ELM by Lorentz force,adds up during multiple ELM melting Effect of melt surface roughness on power loading and melting not included and not well known. ELM size Rayleigh Taylor (RT) instability, can be somewhat mitigated by macrobrush design with brush < 4cm ELM size Kevin Helmholtz instability, does not depend on the brush size, large droplets and melt layer splashing. Increasing ELM size EU PWI TF V. Philipps, EU PWI TF meeting, Oct 2007, Madrid

4. Assess design measures to suppress impurity production associated with ICRF on ITER 5. W erosion influx in start-up, steady state and during ELMs W influxSteady stateELM induced Fast ionsICRH induced Main chamber W/s W/s1.2 x10 16 /s W/s Divertor But still uncertainties on The real impurity composition of the ITER plasma (both intrinsic and seeded) The plasma transport in the SOL in particular the outer region, The estimation of the ELM induced impurity erosion The effect of ICRH induced impurity production, evaluated in a separate EFDA task (estimation of small contribution of W erosion by fast ions is more solid). EU PWI TF V. Philipps, EU PWI TF meeting, Oct 2007, Madrid

Conclusions: W plasma impurity contamination can be kept below the critical values under scenario I steady state plasma conditions, for which no accumulation is expected. But : W sources estimated under reference scenario of a partially detached inner and outer divertor small W influx from the lower divertor high flux areas. Loss of detachment increases significantly the gross W influx and the plasma contamination depends on the divertor impurity retention No statement was done on advanced scenarios which may lead to impurity accumulation Start up in a full W ITER difficult or impossible according to calculations. But largely by avalanche effects due to W self sputtering. AUG demonstrate start up with a full W wall with the inner wall as start up area. More information is needed in the area to make firm conclusions but a full W ITER would imply a severe risk to obtain successful routine plasma start up. TEXTOR had severe start up problems with a full set of W coated limiters EU PWI TF V. Philipps, EU PWI TF meting, Oct 2007, Madrid

6. Evaluate the influence of mixed-material formation on plasma compatibility and tritium retention 1. Be-W alloy formation: Present view: inner ITER divertor and the dome is in Be deposition mode Effective Be 12 W formation between K (at lower temperatures the Be diffusion in W is to low and a pure Be layer forms on top of the W, at higher temperatures Be evaporates leading to a thin Be allow layer) the temperature of the Be layer deposition areas on the upper inner W target is not high enough for Be-W alloy formation Temperature excursions by ELMS& disruptions or X point Marfe formation can lead to formation of some Be-W alloying Outer divertor: no (thick) Be layer is expected to form. 2. C- chemical erosion: Complete suppression of C erosion on inner target No full suppression of C chemical erosion on outer target EU PWI TF V. Philipps, EU PWI TF meeting, Oct 2007, Madrid

3. T-retention Retention in Be by implantation saturates,estimated amount of retention from T implantation in the whole Be first wall about 7 gT. Retention by codeposition with Be/C Larger scatter in database Temperature the most important variable. Mixed Be/C and W/C codeposited layers seems not to retain significantly more D compared with the pure materials Layer structure seems to affects the retention: layers deposited at higher ion energies tendency to retain more D than those with lower ion energies. At present a value of D/Be = 8% on the plasma facing side of the inner divertor and a lower value of 1% on the dome area of ITER is recommended. EU PWI TF V. Philipps, EU PWI TF meeting, Oct 2007, Madrid ITER Be walls: gT /shot Inner divertor alone

ITER C- outer divertor: erosion deposition modelling (ERO-code), carbon target, no background impurity flux Carbon target Gross erosion: 7.7·10 22 C/s, 98% redeposition C- deposition on dome 2% C/sec Beryllium Target Gross erosion: 3.2·10 22 Be/s 90% redeposition 10% deposition on dome Be/s x 0.4 x 400= D,T/shot 0.5g T- retention in C layers on dome alone Net erosion Deposition