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

maximum plasma pressure obtainable, and thus often core performance, is set by a combination
of plasma radial scale lengths and magnetic field shear structure. excessive pressure results in a
well-documented magnetohydrodynamic (mhd) instability — the edge localized mode (elm)
— that causes periodic, rapid local ejection of plasma into the sol. The elms thus limit core performance
and present potentially damaging impulsive heat loads to material divertors and walls.
edge localized modes are a major concern for iteR and demo, and must be strongly mitigated.
The physical understanding used to predict elm amplitude and frequency, and to devise mitigation
strategies (see Thrust 2), rely on identifying the plasma and neutral transport and turbulence
processes that determine the pedestal structure (local gradients); despite progress, no first-principles
model exists. also, shaping of the magnetic equilibrium to access more benign elm regimes
should be tested.
The boundary layer must also distribute the steady-state heat exhaust to wall components. The
power handling capability of divertor and first-wall surfaces cannot be directly scaled up to match
the corresponding scaleup in burning plasma power output. steady-state power of ~10 mW/m 2
remains the practical divertor limit with 1-5 mW/m 2 on some walls; iteR is being designed for
such limits. steady-state heat removal poses operational constraints (e.g., requiring detached divertor
plasma operation). lacking careful control of steady-state operation and elms, divertor
and first-wall components will be destroyed, and iteR’s scientific mission would not be realized.
in light of this expectation, it is not yet possible to design a credible demo-class fusion device,
with a size like iteR but power output that is ~4-5 times higher, whose material surfaces can survive
the expected heat exhaust. specifically, predictive understanding is lacking for boundary layer
heat and particle transport dynamics, which typically involve filamentary structures; yet this
physics defines not only the level of plasma-wall interaction (peak heat flux, particle fluxes, erosion
rates), but also the boundary conditions imposed on the core plasma (edge gradients, flows,
impurity levels).
The transport simulations to model plasma fluxes use ad hoc radial transport coefficients, rendering
them more in the class of “interpretive,” rather than predictive simulations. This limitation
prevents fundamental prediction for heat-flux widths and pedestal/sol transport at present,
although it does test parallel (along b) transport models. impurity transport in the pedestal/sol
is treated at the fluid level through the multi-species 2-d transport codes or by trace-impurity
monte carlo ion codes, using fluid or experimental background plasma for the hydrogenic
species. limited code validation with data often shows rough agreement with various quantities
for “attached” divertor plasma conditions, but typically only to the 30-50% level. For “detached”
conditions (planned for iteR), the disagreement is worse. There appears to be a larger-than-predicted
plasma density in the private-flux region that may have an important effect on divertor
behavior. These problems likely need refined models of neutral transport physics and radiation
trapping.
For turbulence models, there is a set of 3-d fluid codes that likely contain important electromagnetic
effects, one of which spans the separatrix. These models describe some of the dominant
characteristics of the measured fluctuation features (some spectra similarities, blob-like propagation,
intermittency), but detailed validation is sketchy (large fluctuations and transport increasing
with density). however, at a fundamental level, there is not yet a clear consensus concerning
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