1. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Plasma interactions with Be surfaces R. P. Doerner, D. Nishijima, T. Schwarz-Selinger and members of the PISCES Team Center for Energy Research, University of California – San Diego, USA Work performed as part of: Plasma-Surface Interaction Science Center (MIT and UTenn) US-EU Collaboration on Mixed-Material PMI Effects for ITER

2. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 The PISCES-B divertor plasma simulator is used to investigate ITER mixed materials PSI. PISCES-B is contained within an isolated safety enclosure to prevent the release of Be dust.

3. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 PISCES-B has been modified to allow exposure of samples to Be seeded plasma P-B experiments simulate Be erosion from ITER wall, subsequent sol transport and interaction with W baffles or C dump plates, as well as investigation of codeposited materials using witness plates PISCES

4. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Outline of Presentation Erosion in the plasma environment –Comparison to TRIM and ion beam data –Surface characterization –Role of morphology Redeposition/sticking efficiency Summary PISCES

5. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Be erosion yield measurements in PISCES Two techniques are used to measure physical sputtering yield Weight loss measurements, use low density plasma to reduce redeposition (i.e. long ionization mean free path). Line emission spectroscopy, uses high density plasma to minimize geometrical loss terms (i.e. short ionization mean free path). These presentation will focus on the weight loss technique and changes to spectroscopic measurements that are normalized to weight loss measurements. [He plasma on Be, weight loss are 5-10 times lower than TRIM calculations, He plasma on C, weight loss agree with TRIM calculations]. PISCES

6. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Significant variations in the Be sputtering yield are measured Incident ion energy ~100 eV J. Roth et al., FED 37(1997)465. PISCES discrepancy between - JET - PISCES-B - ion beam – TRIM - sputter yields (< 45%) (< 0.7%) (< 8%) (< 3.5%)

7. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Uncertainties in sputtering yield measurements Uncertainties in weight loss measurements: Surface contamination Incident ion energy Ion flux measurement Ion species mix (molecular ions) Redeposition fraction Surface morphology changes Atomic D adsorption on the surface (secondary ion yield) Uncertainties in absolute spectroscopic measurements: Atomic physics database Electron energy distribution (non-Maxwellian) Angular distribution of sputtered particles Geometrical loss fraction PISCES

8. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Native beryllium oxide surface is removed early during the plasma exposure Distinctive oxygen lines near 777 nm can monitor erosion of O from surface Background helium plasma does not change (second order He I line) Larger ion energy will remove oxide layer quicker BeO (0,0) molecular band emission (@ 470.86 nm) is not detectable PISCES

9. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 AES reveals a relatively ‘clean’ Be surface after sputtering yield measurements PISCES

10. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Incident ion energy is corrected for plasma potential. Ion flux is uniform over sample surface. From B. LaBombard et al, JNM 162-164 (1989) 314. PISCES-A space potential measurements show V pl ~ 1-2 T e E ion = |V bias | - 1.5T e Target From D. Whyte et al, NF 41 (2001) 47. Ion flux calculated from upstream Langmuir probe agrees with total current collected on the sample manipulator when biased into I sat (with ~10%) PISCES

11. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Molecular ion fractions are calculated from zero-d rate balanced model, based on measurements Model uses the atomic and molecular processes to the right Rate equations predict molecular ion fractions based on n e, T e, B, N H2 Turbulent radial transport is assumed (1/B scaling) Model is verified with molecular ion species measurements From E. Hollmann et al., Phys. Plasmas 9 (2002) 4330. PISCES

12. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Molecular ion effects change both the shape and magnitude of sputtering yield curve PISCES Shape of measured yield agrees with molecular ion model predictions. Magnitude is a factor of ~5 too low.

13. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 ERO calculates 10-20% ionization of sputtered Be in the PISCES-B plasma Shape of Be I (457 nm) axial profile agrees well with experimental profiles Magnitude of profiles are normalized Molecular ion fractions provided by previous model No Be deposition observed on W or C targets exposed to D plasma (no wall source) PISCES From D. Borodin et al, Phys. Scr. T128 (2007) 127.

14. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Be surface before plasma exposure after Surface morphology evolution with time / fluence spectroscopy: mass loss:  morphology change can account for a factor of 2 in reduction of the yield PISCES

15. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Similar yield evolution with time/fluence is documented in the literature  morphology change can account for a factor of 2 reduction of the yield 1keV,  H 2 + 7.3E21 ions/cm 2 Mattox and Sharp, J. Nucl. Mater. 1979: PISCES

16. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 1)“maximum” – static TRIM + MD 2)“minimum” – SDTrimSP with 50% of D (reasonable limit) Plasma atoms remaining in the near surface also can reduce the sputtering yield by a factor of 2-3 From C. Björkas PISCES

17. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Gross erosion is expected to remain constant with increasing Be redeposition/influx N = G * (1-R), where N = net erosion, G = gross erosion, R = redeposited fraction In PISCES-B, Gross = net erosion with no Be seeding (λ ion is large compared to r plasma, so redeposition is small) Ion influx from Be oven is identical to sputtered atoms that are ionized in the plasma and redeposited on the target, this allow a controlled and independent variation of the Be influx to the target PISCES

18. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Erosion/deposition balance in Be seeded high flux D discharges Use Be oven seeding to balance surface erosion to test input parameters of material migration models Mass loss measures net erosion Spectroscopy measures gross erosion (Be I line) Y Be → Be ≈ Y D → Be, and low concentration of Be When incident/seeded Be ion flux = sputtered flux of Be, net erosion should = 0. Mass loss ion fluence:  10 22 /cm 2 target temperature < 320K PISCES

19. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 No change in mass loss is measured when Be seeding flux equals sputtering of Be by D ADAS database is used Be flux from Be II (313.1 nm) and background plasma flow velocity (E. Hollman JNM, PSI-19) Be ion flux is verified during no bias discharges, when weight gain is measured (net deposition) Net erosion stays constant, implying gross erosion must increase Erosion yield of 0.15% can only be compensated by seeding 2.8% Be Mass loss ion fluence:  10 22 /cm 2 D/Be plasma target temperature < 320K 2.8% PISCES

20. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Beryllium seeded He discharges target: bias: < -40V results in E ≈ 30eV He ion flux: 5·10 18 cm -2 s -1 Be seeding:n Be /n D = 0 – 4 % sputter yield He on Be @ 30eV: Y = 0.15 % (measured) targetoven PISCES : During ‘no Be seeding’ discharge, Gross ≈ Net erosion Gross erosion

21. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Different behavior is observed experimentally Net erosion stays constant until influx >> sputtering rate Gross erosion increases with increasing Be influx to target Reduced sticking of depositing Be, or increased re-erosion could explain observations Similar reduced sticking needed to model CD 4 and WF 6 injection experiments in TEXTOR (from A. Kirschner) Possibly similar to C-MOD modeling difficulties reported by J. Brooks (PSI19) PISCES

22. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 What PMI issues are still unresolved Sputtering Yield – we believe we can explain the observed, low sputtering yields in PISCES-B, due to surface morphology and fuel atoms within the target surface (no Be seeding) –How many gas atoms are in the surface during exposure? Gross/Net Erosion – drastic differences in behavior of gross erosion during Be seeding indicates the simple theory regarding the benefits of prompt redeposition may need to be revisited –10 times more influx is needed to balance erosion and force net erosion to zero –Low sticking probability or high re-erosion possible explanations PISCES

23. R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Be seeding fraction (i.e. influx) is at most in the percent range, so Be self-sputtering and Be reflection can be neglected