Effects of active mode control on edge profiles and plasma-surface interactions in T2R

H. Bergsåker with contributions from S. Menmuir, M. Henriksson et al.

Recent work on edge, impurities and discharge termination:

• Impurity flux from spectroscopy• Impurity flux from passive probes• Edge profiles • Conclusions

T2R inner wall configuration

• Major radius: R=1.24 m• Minor radius a=0.183 m• Feedback control of resistive

wall modes• Wall: Stainless steel with Mo

limiters ( 8% of surface)• Diagnostics: magnetics, 1

chord interferometer, 3-points Thomson scattering, visible and VUV spectroscopy, soft X-ray tomography, Langmuir probes, surface probes, TOF particle spectrometer, surface probes etc.

Time resolved flux of Mo and Cr with and without feedback in H plasma

Collector probe, surface analysis by RBS

Graphite collectors were exposed to 2 – 32 complete discharges at the wall position. The areal densities of collected metals were measured by Rutherford Backscattering Spectrometry. Here a collector exposed to 8 discharges with feedback in H.

Discharge averaged metal flux from probes

sample Cr Fe Ni Mo Exposure ion feedbackE15 / cm² /s E15 / cm² /s E15 / cm² /s E15 / cm² /s ms

10 0.87 2.5 0.33 0.54 543 H on11 1.1 3.8 0.49 0.76 148 H off17 1.8 4.6 1.2 0.80 489 D on18 5.1 9.2 3.4 1.6 153 D off20 2.1 4.9 0.93 0.91 909 D on

The average deposition rates of metals were higher without feedback than withfeedback (40-10%).The average deposition rates of metals were higher in D-plasmas than in H-plasmas (60-130%).The fluxes derived from deposition rates agree with spectroscopy within a factor 2, the main discrepancy being that the H/D difference is not visible in thespectroscopic data.The composition of deposited metals does not change much, approximatelyCr:Fe:Ni = 0.35:1:0.15, while the composition of SS316L is 0.25:1:0.18.Mo / Fe = 0.2, larger than the overall area ratio.

Discharge averaged Cr flux

Filled: discharge in DOpen: discharge in H

Discharge averaged Mo flux

Filled: discharge in DOpen: discharge in H

Estimating particle energies from trapping

D is trapped within the implantation range when implanted in graphite. This can be used to estimate the flux and energy of the impinging partivles [2]. The present results suggest saturation close to 1016 D/cm2 and consequently energies below 100 eV, as opposed to the similar measurement in a smaller RFP device [2].

Sputtering

• Sputtering yield for Stainless steel and Molybdenum by H+ and D+. The yields are everywhere at least a factor 2 higher for D than for H [3].

• The metal deposits on the collectors indicate 60-130% higher fluxes in D plasmas compared to H plasmas, but this difference is not visible in the spectroscopic data

• The flux and energy of hydrogen impinging on wall and limiters appears insufficient for physical sputtering to play a major role.

Thermal evaporation

• Evaporation rates as a function of temperature for Cr, Fe, Ni, Mo. The rates are calculated for elemental metals with vapour pressures from the Clausius-Clapeyron equation. At any given temperature the evaporation rate is around a factor 2 higher for Cr than for Fe and many orders of magnitude lower for Mo.

• The deposition rate of metals on the collector may be slightly enhanced (50%) for Cr compared to the SS316L wall composition. The Mo deposition rate is 20% of that of Fe.

Arcing

• Unipolar arcing is a well known erosion mechanism

• The photo shows arcs tracks at a limiter surface when the vessel was opened in February 2003.

Conclusions

Discharges in T2R can be greatly prolonged by active feedback suppression of resistive wall modes [1]. Discharges without feedback terminate early and the termination is accompanied by significant metal release into the plasma. The calibration of spectroscopic measurements of metal flux involves assumptions e.g. of plasma edge conditions. A quantitative comparison has been made of absolute metal flux measurements by spectroscopy and collector probes / surface analysis.

The only significant edge profile modification close to termination without feedback was about 5 eV higher Te just inside limiter radius.

The spectroscopic data and the collector measurements of metal fluxes are in reasonable agreement, except for the difference between D and H discharges. The collectors show higher metal deposition in D than in H discharges, but the flux and energies of hydrogens are insufficient to explain the large metal flux as due to physical sputtering. Thermal evaporation is possible, but there is probably insufficient difference betweeen Mo and SS components for this to be a determining erosion mechanism. Other candidates for metal release at discharge termination: arcing or impurity sputtering.