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

the controlling instabilities and saturation mechanisms. in contrast, core turbulence simulations,
theory and experiment have standard benchmarks (e.g., the so-called cyclone test case for turbulence-driven
transport), agree on many aspects of the big picture of the dominant ion transport,
and have made good progress on electron transport.
in addition to radiative dissipation and detached divertor operation, further reduction of the
peak heat flux to divertor surfaces may be obtainable by modification of the details of the boundary
magnetic configuration. one recent idea of this approach is the super-X divertor where the
sol magnetic flux tube is extended to substantially larger major radius to increase the area available
for heat deposition and increase the effective distance between the core edge and the divertor
plate. a second idea, the snowflake divertor, expands the flux tube locally to spread the heat
flux, expand the effective core/divertor distance, and increases magnetic shear that can affect
elm stability. initial modeling results are encouraging for both of these ideas, but detailed experimental
tests are required. There is clearly a range of configurations intermediate to the super-X/
snowflake that should be examined, including careful evaluation of the minimum practical angle
between the divertor surface and b that would also improve conventional divertors. another
method for increasing divertor heat-load capabilities involves various PFc materials and is discussed
in Thrust 10.
as the plasma has an impact on the material surface, it causes heating, absorption and recycling
of hydrogen, and sputtering of surface material; in some circumstances, melting of the surface can
occur. The recycled and sputtering neutrals enter the plasma, where they are ionized and become a
plasma ion species responding to the electromagnetic fields. many of the ions are returned to the
surface in a process called redeposition and can thus build up layers of new surface material that
may be an elemental mixture of different portions if the walls are made of several materials (e.g.,
carbon, beryllium, and tungsten mixing in iteR is a poorly understood issue). establishing the basic
science of these processes as verified in linear plasma simulator devices and beam-particle test
stands is being proposed through Thrust 10. First-principle molecular dynamics codes can treat
mixed materials, but the key input of the inter-atomic potential is largely unknown even for carefully
prepared samples, let alone the mix that develops in an operating tokamak (see Thrust 14). in
this Thrust, the near-surface plasma modeling needs to be combined with comprehensive sol modeling
in a time-dependent manner to properly model the erosion and buildup of new material layers.
a special plasma-wall interaction that requires careful evaluation and integration is the interaction
of the edge plasma with radiofrequency antennas and launchers — devices used extensively
to inject auxiliary power into fusion devices to heat and control the core plasma. large electric
fields and sheaths can be driven at the antenna and launcher or where the b-field line touching
the antenna also comes in contact with a remote material boundary. The sheath potential can accelerate
ions into the materials, yielding undesirable sputtering of metallic impurities and power
loss at the antenna. other parasitic radiofrequency losses in this region include edge modes (shear
alfvén and cavity modes), parametric instabilities, and nonlinear wave-particle interactions. The
radiofrequency sheath potential can also drive radial e×b convection in front of the antenna,
which increases the radial flux of plasma to the wall. models of the radiofrequency sheath require
treatment of both the ion and electron debye length space scales, either explicitly, or by a sheath
boundary condition, which is nonlinear.
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