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

interaction with plasma boundary and material interfaces
Present tokamaks have clearly shown the importance of the plasma edge, the scrape-off layer (region
between the plasma’s magnetic boundary and solid walls) and plasma-material interfaces to
the core plasma performance. considerable effort is spent on “conditioning” the plasma-material
interfaces for the highest performance discharges. How will self-heated plasmas interact
with their material interfaces, and what is the self-consistent core/scrape-off layer/divertor
plasma state?
The time scale for the plasma facing components to come into equilibrium with respect to many
processes is not clear. yet it is an important issue for the viability of fusion power production,
where the device would need to operate continuously for periods of about one year. The generation
of debris from solid materials in the vacuum region, such as dust, must be kept to levels that
will not affect the steady operation of the plasma. since transients, ranging from elms to disruptions,
aggravate all plasma facing component issues, they must be reduced to tolerable levels
as the high fusion power regime is reached. The core-edge coupling is broken into three primary
areas, i) heat loads, ii) particle transport, and iii) material evolution. The self-consistent core and
edge plasma demonstration in the high fusion power regime would require better understanding
of the following issues.
The heat load issue concerns the consistency of the high-performance core and pedestal with divertor
and first-wall power handling. The plasma density and temperature in the divertor are coupled
through the scrape-off layer to the edge of the core plasma, and ultimately to the core plasma. The
conventional approach to a divertor plasma region that radiates a large fraction of power requires
high plasma density, and the compatibility of this with a high-performance core plasma is unclear.
Finding solutions to the core, pedestal, and divertor will require the understanding of
significantly different plasma physics regimes, ranging from the hot, low collisionality core
plasma, to the open magnetic field line dominated scrape-off layer, and the high plasma and
high neutral density divertor. The power conducted through the plasma boundary to the scrapeoff
layer and finally to the divertor is concentrated in a narrow layer near the plasma boundary.
our understanding of this layer is quite limited, but of critical importance to finding solutions for
the survival of divertors at high fusion power. transients like edge localized modes create pulses
of high power, which are intolerable at high fusion power. The balance of power received by various
material surfaces is strongly influenced by radiation, with the core plasma radiating some fraction
of its power (P rad,core ), and the remaining power transported to the divertor, where an additional
fraction is radiated (P rad,div ). The tolerable levels of radiation from these regions must be consistent
with the underlying physics in these regions.
Particle transport issues that relate to the core-edge coupling include the removal of helium
ash from the core plasma through the pumped divertor, the cycling of impurities (both intentional
and unintentional) and fuel between the core plasma and plasma material interfaces,
particle retention in the solid materials, and the particle behavior in the presence of
high power transfer to the solid materials. The solid materials are expected to operate at much
higher temperatures than in present tokamak experiments, which may significantly change the retention
of fuel in these materials. The physics of the high-density limit, when the plasma density
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