Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

tHEME 3: taMiNG tHE PLaSMa-MatERiaL iNtERFacE
introduction
ScopE aNd FocuS
The hot plasma in the core of a fusion energy device must interact with low-temperature material
walls. That interaction is mediated by a thin plasma layer called the scrape-off layer (sol). many
of the recent improvements in plasma core performance have been enabled by either improved
understanding of the sol or improved plasma facing components (PFcs). The report of the Priorities,
Gaps and opportunities Panel, or “Greenwald Report” 1 , identified three major issues at
the plasma-material interface. We must “understand and control all of the processes which couple
plasma and nearby materials” (issue 8). We must “understand the materials and processes that
can be used to design replaceable components which can survive the enormous heat, plasma and
neutron fluxes without degrading the performance of the plasma or compromising the fuel cycle”
(issue 9). We must “establish the necessary understanding of plasma interactions, neutron loading
and materials to allow design of RF [radiofrequency] antennas and launchers, control coils, final
optics and any other diagnostic equipment which can survive and function within the plasma
vessel” (issue 10). a total of nineteen gaps in knowledge was identified for the three issues.
at least a dozen new facets of scrape-off layer behavior have been uncovered over the past decade.
This improvement in understanding has been accomplished through a combination of new diagnostics
(edge Thomson scattering, fast camera imaging), improved edge models (coupled fluid and
neutral codes), and more dedicated plasma operation for edge diagnosis. several of these new facets
have caused a major alteration to the design of iteR. The remaining uncertainty in the scaling
of the power scale-length in the scrape-off layer has placed very tight constraints on the design
of iteR components.
Plasma facing components have increased significantly in heat flux capability in the last decade
due to new materials (carbon fiber composite), improved design (protection of leading edges and
better shaping relative to plasma flux surfaces), surface conditioning (boronization, helium glow
cleaning, high-temperature baking), and in some cases active cooling. laboratory studies and
limited application to fusion devices have proven the capability of water-cooled plasma facing
components to achieve the heat removal required for iteR. There is an ongoing debate about the
choice of plasma facing materials for iteR with some advocating an all tungsten option while others
argue for all graphite, but the baseline PFcs for deuterium-tritium (d-t) operation have only
beryllium and tungsten.
long pulse radiofrequency antennas and launchers have been operated at high power on the tore
supra device where pulse lengths are measured in minutes. improvements in feedback control
have yielded more stable radiofrequency heating in existing devices. such operation has revealed
interesting new physics in the plasma edge. electron cyclotron resonance heating has been used
1 “Priorities, Gaps and Opportunities: Towards A Long-Range Strategic Plan For Magnetic Fusion
Energy” Report to FESAC Oct 2007, M. Greenwald, Chairman.
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