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

cross-field transport in the pedestal/sol has to be better characterized to understand the physics
controlling pedestal structure and its stability, and also to be able to scale PWi at the main
walls or at the divertor plates for plasma being lost from the pedestal/sol region. With regard
to wall erosion, the problem can be unwanted material being transported to the divertor rather
than excessive erosion of the divertor material. Wall particle and power fluxes need to be measured
systematically in three dimensions (toroidally, poloidally, temporally) as functions of plasma
parameters to identify the scaling parameters. The underlying pedestal/sol turbulent plasma
transport mechanisms responsible for these spatial distributions need to be established through
turbulence measurements, including 2-d imaging, and power spectra (both frequency and wavelength-resolved)
of density, temperature, potential and magnetic field — ideally measured at the
same time so that their phase relationships can be understood.
Real time, in situ surface diagnosis of h-uptake, erosion, deposition, and co-deposition is required.
such surface studies are seriously compromised today because most of the experimental
information is global: post mortem analysis provides spatial resolution but integrates over entire
campaigns, while other studies are for single shots but integrate over all internal surfaces. it
is difficult or impossible to extract the controlling physics from such integral experiments. Real
time, or at least between shot, in situ surface diagnosis is required, e.g., using an on-site accelerator
whose probing ion beam can be brought to various locations inside the vessel using the tokamak
coils.
2. Validate existing codes and extend models
basic understanding and predictive capability for guiding device operation and designing improved
plasma facing components and new machines require detailed models that include the relevant
physics. modeling the boundary plasma can be divided into describing how (a) the plasma
power and particles coming from the core region are distributed to PFcs, (b) the material is modified
by such bombardment (e.g., sputtered or recycled particles, tritium transport and retention),
and (c) the erosion products in turn modify the sol and core plasmas. While important progress
has been made, each area needs a substantial new effort in model validation and model development.
as more experimental data becomes available, new and likely unexpected features will be
revealed that will further motivate upgraded models. simulation tools in these areas are all ready
to advance, but are budget-constrained.
a. Pedestal and SOL models: simulation models of the edge and sol include transport codes, which
give the slow evolution of the plasma profiles and fluxes in a complex environment, and turbulence
codes, which model the unstable, strongly fluctuating plasma state to (ideally) provide transport coefficients
to the transport codes. This latter connection has only been made for a limited set of simulations.
The edge transport codes are either 2-d fluid or now emerging 4-d (2 spatial coordinates, 2 velocity
coordinates) kinetic. The turbulence codes are 2-d and 3-d fluid and emerging 5-d gyrokinetic
codes.
a focused effort is required to better model the often dominant plasma turbulence in the pedestal/sol
region, using existing fluid and developing kinetic codes. The associated understanding
and quantification of underlying edge transport mechanisms need to be brought to at least the
level of that for core transport. Given the edge’s importance and complexity, this will require a
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