ARIES Town Meeting “Edge Plasma Physics and Plasma Material Interactions in the Fusion Power Plant Regime” Meeting Summary and Recommendations Mark Tillack (UC San Diego) Alan Turnbull (General Atomics) Chuck Kessel (PPPL) presented at the ARIES Project Meeting July 2010 Bethesda, MD held May 2010 at UC San Diego

Meeting highlights 42 participants, 17 organizations, incl. EU & Japan Presentations are archived on the web ◦ DOE OFES VLT highlight submitted Journal article in preparation ◦ Contributions just starting to trickle in ◦ Planning to submit to Nuclear Fusion Remains to be decided how the results of this meeting might be used to advance ARIES

Key questions addressed What is the status of our current understanding and predictive capabilities in edge plasmas and PMI? What R&D is needed in order to advance this field toward the power plant regime? What contributions can new devices make toward advancing this field?

Town Meeting Sessions 1.Background and power plant requirements 2.Physics of the edge - current understanding and projections to ITER and power plants 3.Modeling of the tokamak edge 4.Experimental benchmarking of models for power plants (panel) 5.Innovative solutions 6.New device contributions to edge physics benchmarking (panel) 7.Conclusions and future plans

Recommendations for ARIES Team are presented in 7 topics 1.SOL heat flux to the divertor 2.Core and divertor radiation 3.Erosion/redep and particle fluxes 4.ELM’s 5.Disruptions 6.Innovative concepts 7.ITER vs. power plant

Topic 1. SOL heat flux to the divertor Rapid progress is being made understanding power flows in the edge. ◦ Per Boedo, “heat flux predictions are nearly in hand, particle fluxes are still unpredictable” Joint Research Task FY10 milestone expected to provide validated correlations for heat flux – soon. ITER/JET reference scaling, (1/P SOL  ) may be wrong. Should we consider JRT more reliable that other scaling laws? Also noteworthy, some analysis of ARIES-AT edge power flows already exists.

1. SOL heat flux to the divertor Recommendations for ARIES Wait for joint research milestone report, and use it to define reference SOL power flows for power plants. ◦ Do we want to use optimistic results or pessimistic results? ◦ Is this a conservative vs. aggressive range we should include? Try to exploit UEDGE more effectively. ◦ UEDGE has lots of inputs. It has been found to be postdictive, NOT predictive. So we need to understand sensitivities. This is not a “run- it-once” situation.

Topic 2. Core and divertor radiation Asakura results are promising, consistent with ARIES assumptions on impurity radiation & detachment. ◦ 9 MW/m 2 peak heat flux in SlimCS ◦ Modeling shows strong non-uniformity of radiation in the divertor slot, implications for a power plant? ◦ SOLDOR/NEUT2D is like B2-Eirene. What is the US combo, UEDGE/DEGAS2….? Are very high radiation fractions (90%) realistic? What might prevent it? ◦ It is known that high radiation fractions can be achieved with attachment or partial attachment in divertor, but this can not be relied on out to 100%; conditions in the divertor can lead to collapse thru MARFEs or other high density phenonema. So is there a clear criteria for this, or does it require a B2-Eirene run for every case?

2. Core and divertor radiation Recommendations for ARIES Perform similar analysis in ARIES. Explore the limits of radiation. ◦ ITER has set partial-detachment as the required operating regime, which results in significant radiation in the divertor (70%). Attached operation has unacceptably lower radiation (20%). Are these from consistent B2-Eirene runs or something else? ◦ In ARIES we are often looking at 75-90% radiation in the divertor.

Topic 3. Erosion/redep and particle fluxes High density in divertor leads to low net erosion. Low density on FW leads to little redeposition. Are we missing something? ◦ In a reactor, is the SOL density is high relative to present day and ITER? Maybe FW erosion is low, not high. ◦ “Little re-deposition” means it went somewhere else! The divertor or where? Codes are available to study this. Can we use them? ◦ Is it sufficient for ARIES to perform scans in isolation of other effects, SOL density/FW particle flux/erosion magnitude, and similar for the divertor. Other effects means impurities liberated ending up in core plasma, do we have all the atomic processes correct in the very high divertor density region? What are the implications of high SOL density on tritium recycling?

P. Stangeby

3. Erosion/redep and particle fluxes Recommendations for ARIES ARIES typically does not try to analyze erosion/redeposition. Should we start doing this in more detail? The engineering group is currently quantifying the impact of a reduction in divertor armor thickness. It may not have a big effect (up to a point). For the FW, erosion is a topic that deserves more attention. We should promote R&D to fill the particle flux knowledge gap.

Topic 4. ELM’s Numerous ELM regimes exist. Some are far more benign than ITER. What is the ARIES reference scenario?

4. ELM’s Recommendations for ARIES Determine ELM regime(s) for ARIES and define heat flux scenarios for engineering analyses. ◦ Develop small-ELM and no-ELM scenarios and consider trade-offs. ◦ This probably requires a scan from worst Type-I ELM downward in  W ELM /W ELM as an artificial scan first, find where structure/coolant and q nominal,div margins exist. ◦ Examine criteria for specific ELM regimes that appear promising. The main problem with these regimes are that they often have restrictive windows. ◦ Both the divertor pulse and the FW pulse must be considered.

Topic 5. Disruptions EM loads in a reactor are similar to ITER. Thermal loads may be 10 times worse. Wesley: Predict, Avoid, Mitigate ◦ We need a PAM strategy for power plants, especially if the plasma is different than ITER. ◦ Demo must be more robust than commercial, to support ongoing PAM improvements.

5. Disruptions Recommendations for ARIES Not clear we need to change our 10th-of-a-kind strategy (very low frequency). But next-step machines should help evolve PAM for Demo, and Demo for power plant. As with ELM’s, we need some reference scenarios for engineering analysis. ◦ Kessel is charged to do this

Topic 6. Innovative concepts 2 main thrusts 1.Flux expansion configurations (SuperX, snowflake) 2.Materials advances (quaternary W alloy, flow-through C, liquid metals) Community interest exists for concept integration ◦ Configuration impacts, physics impacts, maintenance, …

6. Innovative concepts Recommendations for ARIES W coating and flow-through C ideas probably too speculative for us to implement in our designs – we should track progress. LM divertors already explored in ARIES. Very low recycling presents problems for reactors. Not clear there is anything new for us to do. Community interest exists for concept integration of heat-flux spreading techniques ◦ Configuration impacts, physics impacts, maintenance, … ◦ Further exploration of heat flux spreading techniques would be useful, and was mentioned in our proposal. Do we have the resources needed for serious integrated studies?

Topic 7. ITER vs. Power Plant ITER is often used as a well-characterized point of reference ◦ In some cases, ITER is used as a reference point for further improvements. ◦ In some cases, people mistakenly assume power plants must be worse than ITER. ◦ In any case, ARIES needs a story to explain our assumptions, e.g.  FW heat flux, SOL width  ELM's and disruptions  SN v. DN (can DN become SN?)  precision of wall shaping

7. ITER vs. Power Plant Recommendations for ARIES This is a problem in communication: we need to let the community know how power plants are (or are not) different than ITER. ◦ A power plant is not an experiment, it will make ONE flattop plasma, and some fractional power plasmas. ◦ Are there any other aspects that make the situation better? Document either as a section of the Town Meeting journal article, or as a standalone journal article.