06. 11. 2006 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)

06. 11. 2006 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)

06. 11. 2006 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).

06. 11. 2006 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)?

06. 11. 2006 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)

06. 11. 2006 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 0.4 -1)

• Chemical erosion at low temperatures low energies

06. 11. 2006 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