ITPA - Meeting, Toronto; Session 3 - High Z studies 3 - High-Z studies (Chair - A. Herrmann) 16:25 (0:10) A. Herrmann - Introduction 16:35 (0:15) A. Kallenbach - Carbon migration on tungsten surfaces 16:50 (0:15) T. Tanabe - Materials Aspect of W as PFM 17:05 (0:15) S. Takamura - High Flux Helium and Hydrogen Exposure to Bulky Tungsten as well as Tungsten-coated Graphite 17:20 (1:00) A. Herrmann - Discussion –Talks –Preparing the issue card discussion (collecting arguments for C, W)

ITPA - Meeting, Toronto; Session 3 - High Z studies 3 - High-Z studies Presentations have 2 different focus: Plasma wall interaction and consequences for the plasma performance (target as an ideal material) –A. Kallenbach Carbon migration in a tungsten machine (C divertor, W – first wall) Plasma wall interaction and material (W) modifications –T. Tanabe Materials Aspect of W as PFM –S. Takamura – High heat flux on W-coated C with He & H plasmas (Bubbel formation)

ITPA - Meeting, Toronto; Session 3 - High Z studies 3 - High-Z studies - Discussion (I) Carbon and tungsten machines (A. Kallenbach) –75 % W coverage –Carbon content decreases slower. –Mainly due to main chamber recycling (dynamic retention, ML). –Cold divertor, but reduced C-D co- deposition. –Carbon redeposition from outer to inner divertor (1g / 1000 s, static retention).

ITPA - Meeting, Toronto; Session 3 - High Z studies 3 - High-Z studies – Discussion (II) Materials Aspect of W as PFM (T. Tanabe) Disillusion phase with respect to tungsten material properties A lot of material problems: –How relevant are they for real ITER (JET) like conditions? –Synergetic effects – Damaging of material annealing of defects Dynamic hydrogen retention …. Is there an optimum operation temperature (compare to C)? Is the ‘tungsten problem’ an ELM problem (has to be solved anyhow)?

ITPA - Meeting, Toronto; Session 3 - High Z studies 3 - High-Z studies – Discussion (III) High Flux Helium and Hydrogen Exposure to Bulky Tungsten as well as Tungsten-coated Graphite (S. TAKAMURA) –Bubble formation under He impact. –Effect of local He density in the bulk Mitigated at higher energies Stronger with shallow angle of incidence (20-40° at the target) –Can be avoided by: Proper material selection Target operation (annealing)

ITPA - Meeting, Toronto; Session 3 - High Z studies Aspects for W and C - PWI Tungsten No Hydrogen co-deposition High sputter threshold Prompt redeposition Higher energy reflection Low tolerable core concentration Needs good divertor retention (cold divertor) Reduced operational space Carbon Low-Z material ‘High’ tolerable core concentration Self adjusting radiation levels (2-3% carbon concentration in all (divertor) machines) Hydrogen co-deposition (H/C ) Chemical erosion at low temperatures low energies

ITPA - Meeting, Toronto; Session 3 - High Z studies Aspects for W and C - Material Tungsten High melting Temperature, low thermal expansion 、 high thermal conductivity, low sputtering yield, High heat shock resistance Melting. Cracking/Micro porous. Brazing/welding technology (mechanical stress). Heavy weight. Hard to be machined. Low electric resistance Carbon High heat shock resistance. No melting. High thermal conductivity. Easy to machine. Chemical erosion at low temperatures/energies. Degeneration of thermal properties due to neutrons. Brazing/welding technology. Actively cooled targets are intrinsic in-safe