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

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interaction with plasma boundary and material interfaces<br />

Present tokamaks have clearly shown the importance of the plasma edge, the scrape-off layer (region<br />

between the plasma’s magnetic boundary and solid walls) and plasma-material interfaces to<br />

the core plasma per<strong>for</strong>mance. considerable ef<strong>for</strong>t is spent on “conditioning” the plasma-material<br />

interfaces <strong>for</strong> the highest per<strong>for</strong>mance discharges. How will self-heated plasmas interact<br />

with their material interfaces, and what is the self-consistent core/scrape-off layer/divertor<br />

plasma state?<br />

The time scale <strong>for</strong> the plasma facing components to come into equilibrium with respect to many<br />

processes is not clear. yet it is an important issue <strong>for</strong> the viability of fusion power production,<br />

where the device would need to operate continuously <strong>for</strong> periods of about one year. The generation<br />

of debris from solid materials in the vacuum region, such as dust, must be kept to levels that<br />

will not affect the steady operation of the plasma. since transients, ranging from elms to disruptions,<br />

aggravate all plasma facing component issues, they must be reduced to tolerable levels<br />

as the high fusion power regime is reached. The core-edge coupling is broken into three primary<br />

areas, i) heat loads, ii) particle transport, and iii) material evolution. The self-consistent core and<br />

edge plasma demonstration in the high fusion power regime would require better understanding<br />

of the following issues.<br />

The heat load issue concerns the consistency of the high-per<strong>for</strong>mance core and pedestal with divertor<br />

and first-wall power handling. The plasma density and temperature in the divertor are coupled<br />

through the scrape-off layer to the edge of the core plasma, and ultimately to the core plasma. The<br />

conventional approach to a divertor plasma region that radiates a large fraction of power requires<br />

high plasma density, and the compatibility of this with a high-per<strong>for</strong>mance core plasma is unclear.<br />

Finding solutions to the core, pedestal, and divertor will require the understanding of<br />

significantly different plasma physics regimes, ranging from the hot, low collisionality core<br />

plasma, to the open magnetic field line dominated scrape-off layer, and the high plasma and<br />

high neutral density divertor. The power conducted through the plasma boundary to the scrapeoff<br />

layer and finally to the divertor is concentrated in a narrow layer near the plasma boundary.<br />

our understanding of this layer is quite limited, but of critical importance to finding solutions <strong>for</strong><br />

the survival of divertors at high fusion power. transients like edge localized modes create pulses<br />

of high power, which are intolerable at high fusion power. The balance of power received by various<br />

material surfaces is strongly influenced by radiation, with the core plasma radiating some fraction<br />

of its power (P rad,core ), and the remaining power transported to the divertor, where an additional<br />

fraction is radiated (P rad,div ). The tolerable levels of radiation from these regions must be consistent<br />

with the underlying physics in these regions.<br />

Particle transport issues that relate to the core-edge coupling include the removal of helium<br />

ash from the core plasma through the pumped divertor, the cycling of impurities (both intentional<br />

and unintentional) and fuel between the core plasma and plasma material interfaces,<br />

particle retention in the solid materials, and the particle behavior in the presence of<br />

high power transfer to the solid materials. The solid materials are expected to operate at much<br />

higher temperatures than in present tokamak experiments, which may significantly change the retention<br />

of fuel in these materials. The physics of the high-density limit, when the plasma density<br />

86

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