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

substantial investment. This work should include the nonlinear saturation of elms and how they
eject plasma from the pedestal, through the sol, and onto PFcs. There should be a major effort
to verify (code-theory and code-code) and validate existing fluid turbulence codes with the existing
and extended diagnostics. as the kinetic codes become more capable, they should be used to
quantify kinetic corrections. important basic theory work that should be done includes unstable
modes in the complex edge geometry, development of validated reduced models, and a set of gyrokinetic
equations that are implementable in numerical codes. consideration should also be given
to full three-velocity simulations that don’t require a gyrokinetic approximation.
boundary transport models that describe both pedestal structure and heat-loads to PFcs need
to be enhanced to realistically include time-dependent, large-amplitude filamentary structures
from short wavelength turbulence-driven “blobs” to large elms. options to be examined further
are efficient reduced-dimension models (3-d to >2-d), a time-averaged coupling to turbulence
code, and direct turbulence and transport simulations for long-time transport. impurities should
be included to understand their transport through the time-dependent sol. kinetic edge codes
have begun to quantify neoclassical ion transport in the edge more carefully, a key addition that
should be supplemented by reduced models that can provide faster fluid transport models. as
with turbulence models, an extensive verification and validation program should be undertaken
for transport models (see also Thrust 6).
b. Near-surface models and utilizing wall response models: The plasma and neutral energy and particle
fluxes incident on PFcs cause sputtering, recycling, and in some extreme cases, melting of the materials.
The removed material is ionized within the plasma and often flows back to a nearby surface location
in the process of redeposition, which can have major impact in fusion devices, including tritium
retention. The wall response is incorporated at a basic level in the sol transport models through
recycling and sputtering boundary conditions, but these now lack kinetic detail and time-dependent
coefficients.
Presently, these recycling and sputtering processes are treated in a simplified manner by whole
edge transport codes, while detailed guiding-center or full 3-d orbit monte-carlo codes are used
in the near-surface region. although some limited file-based coupling of these two models (whole
edge and near-surface) has been undertaken, a more vigorous program is warranted. The coupling
should be more tightly integrated (no human intervention) to allow iteration for self-consistency
and time-dependence. This would describe the nonlinear sputtering and recycling responses to
blobs and elms, as well as long-time changes associated with wall temperature changes and particle
saturation. The details of the sheath can be calculated from a 3-d Pic code, but again needs
integration into other components.
There is a strong connection to Thrust 10 aimed at developing models of the surface response
and validating these models in linear plasma “divertor simulator” devices and beam-particle test
stands. These models are needed here to provide the basic data on particle recycling and sputtering.
Furthermore, changes in the surface-material properties are closely related to the response
data needed here and that aspect of the issue is included in Thrust 14.
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