R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Beryllium and carbon mixed-material studies R. P. Doerner, M. J. Baldwin, J. Hanna and D. Nishijima Center for Energy Research, University of California – San Diego R. Pungo, K. Schmid, and J. Roth Max-Plank Institute for Plasmaphysics, Garching, Germany Work performed as part of US-EU Collaboration on Mixed-Material PMI Effects for ITER

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Outline Technical results –Temporal behavior of chemical erosion suppression –Scaling with Be plasma concentration Near term US-EU goals/experiments –Next few months Longer term US-EU goals/experiments Possible new US PFC Program Direction

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Motivation for PISCES Mixed Materials Studies UC San Diego PISCES and EFDA are investigating the influence of Be plasma impurities on exposed materials interactions relevant to ITER. The ITER design: Be first-wall, W divertor, C (graphite) strike points. Diverted plasma expected to dirty: Eroded Be impurity conc. up to 10 %. PISCES-B Experiments Simulation This Region

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France PISCES-B, and Its Associated Surface Analysis Laboratory, Are Compatible with Beryllium Operations. - The PISCES-B plasma generator is contained in a separate enclosure to prevent release of beryllium particulates into the general lab. - All personnel entering the enclosure wear full coverage personal protective equipment and follow strict entry and exit procedures when performing routine maintenance, machine repair, or sample handling/exchange operations. - The entire enclosure has a specially designed ventilation system that filters all input and output air streams.

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France PISCES-B has been modified to allow exposure of samples to Be seeded plasma

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France A small beryllium impurity concentration in the plasma drastically suppresses carbon erosion -50 V bias, 200ºC, T e = 8 eV, n e = 3 e 12 cm -3 Chemical erosionPhysical sputtering

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Be rich surface layers form during exposure and shield underlying carbon from erosion ITER is expected to have 1-10% Be impurity concentration in the divertor plasma Weight loss data confirms reduction in erosion seen spectroscopically Equilibrium Be surface concentration after P-B exposure.

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France XPS data shows surface layer is largely Be 2 C Virtually all C remaining at the surface is bound as carbide Presence of carbide inhibits chemical erosion of C Carbide layer reduces sputtering yield of bound Be Subsequently deposited Be can more easily erode XPS analysis of Be on C sample surface

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Be 2 C layer formation time is measured by decrease in CD band intensity CD band intensity near C target drops w/ time as Be erosion signal from target increases CD signal approaches background device level (walls) RGA signals during plasma exposure shows similar drop in m/q = 20 & 18 (CD 4 & CD 3 ) Identical exposure of Be coated sample on the following day show changes persist and are not associated with plasma start-up or wall changes

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Increasing Be plasma concentration decreases the Be layer formation time Solid lines are exponential decay fits I CD = exp(-t/ f Be ) Decrease in CD band intensity is correlated to concentration of C in the surface layer 0.16% Be 0.04% Be PISCES-B conditions: f Be ~ 0.001, pl = 3e18 cm -2 s -1, Be = 3e15 Be/cm 2 s or 1 Be monolayer/sec Chemical erosion mitigation time depends of Be plasma concentration

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Rate of chemical erosion decay (1/ f Be ) I CD = exp(-t/ f Be ) We can obtain the scaling of the erosion decay rate by fitting data with a power law expression: 1/ f Be = f Be, obtaining = 785 s -1, = 2.07 Assuming this holds for ITER with 1%< f Be (ITER)<10%, then 16 sec < f Be (ITER) < 0.1 sec. increases with plasma beryllium concentration 10 s 100 s 1000 s s (Plasma Be concentration %)

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Equilibrium Be surface concentration after P-B exposure. 0.04% Be 0.16% Be From K. Schmid et al., Nucl. Fusion 44 (2004)815. Recall data from previous viewgraphs and I CD = exp(-t/ f Be )

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Erosion mitigation time scaling is also valid for previous PISCES data sets Surface composition AES data (solid circles) from K. Schmid et al., NF (2004) were measured after 0.55 – 2.8 hrs. Grey shaded region corresponds to those exposure times Open circles correspond to AES data collected after hr plasma exposures Open squares correspond to CD intensity measurements made at hrs. during plasma exposure K. Schmid et al., NF (2004).

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Near term collaboration goals Extend CD decay scaling toward ITER –Incident Ion Flux –Ion Energy Quantify results of Be injection with simultaneous CD 4 puffing into plasma –Target behavior –Witness Plate Deposition Be/W alloy formation conditions

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Longer term collaboration goals Heat pulse deposition to mixed-material targets –Pulse positive bias to targets (collect electrons) Undergoing circuit debugging on PISCES-A –Measure surface temperature evolution Fast (~1 sec) response with filtered photodiodes NIR (0.9 m – 1.6 m) spectra acquisition in 0.5 msec timescale for blackbody temperature fit (EU camera) –Measure surface layer response during & after pulses Be seeding and CD 4 puffing on W targets Role of oxygen in Be-rich codeposits

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France Possible New Direction for the US-PFC Community Carbon may lead to insurmountable tritium accumulation in ITER Removal of hydrogenated carbon codeposits at sufficient rates to benefit ITER has not been demonstrated ITER technology phase uses a tungsten divertor target Metal PFCs also still have a variety of problems –Alloy formation, melting, tritium accumulation and removal, fatigue, etc.

R. Doerner, Oct. 18, 2005 EU PWI TF meeting, France US PFC Community is proposing to focus its efforts towards an all-metal ITER scenario De-emphasize C work both experimentally and in modeling, some critical C investigations would continue Modeling will stress identifying ITER edge, SOL and divertor plasma scenarios compatible with metal PFCs Experiments will study mixed-materials, tritium retention and high heat load response of metals Direction is seen as complementary to the JET ITER-like wall project