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

• design and implement innovations of the boundary magnetic geometry in existing
devices to demonstrate optimized plasma heat exhaust that is within material limits, and
design and implement such a configuration in a future fusion device.
Summary
This Thrust focuses on the physics of the boundary layer, an essential area for fusion development
where understanding is incomplete. advancement for this area requires development and
implementation of both a substantial set of new edge diagnostics and increasingly realistic models,
which together can lead to identification of the dominant governing physics and to predictive
capability. as peak heat fluxes to materials projected for demo are unacceptable, we propose implementation
and testing of new boundary magnetic field shapes that may reduce peak heat fluxes
and also improve edge-plasma characteristics.
introduction
a thin boundary layer surrounds the hot core of all magnetically confined plasmas. The layer encompasses
the interface between adjacent regions where magnetic field (b) lines are closed (the
“pedestal”) or open (the scrape-off layer [sol]), and it mediates interactions between the hot confined
plasma and material surfaces that receive the plasma exhaust and generate wall impurities.
Furthermore, the plasma pressure that can be maintained at the interface with the core plasma
(the pedestal) has a strong impact on fusion gain. despite the layer’s importance, the basic processes
that determine its spatial gradient lengths, and the heat and particle transport within the layer,
are complex and not adequately understood. While substantial progress has been made, with more
than a dozen important new facets of boundary plasma behavior having been discovered over the
past decade, our understanding of the physics is significantly incomplete.
The complexity of the boundary layer arises from features that make one-dimensional (1-d) diagnosis
inadequate and direct application of many core-plasma models inappropriate:
• interacting gas (plasma + neutrals), photons, solids, and sometimes liquids.
• steep gradients, increased collisionality, and transition from closed to open magnetic
field lines yield strong 2-d (sometimes 3-d) profile variations. in contrast, core profiles
typically only vary radially (1-d) owing to rapid transport along the b-field.
• The boundary magnetic field exhibits strong shearing as the field lines change from a
closed to open topology that is characterized by very long, thin and twisted geometry.
• Plasma fluctuations, often intermittent, exceed 10% of the background, and can approach
unity, which invalidates the small-amplitude diffusive transport model.
• large electric fields near radiofrequency antennas and launchers, and on surfaces
connected to antennas along the b-field, can result in greatly amplified plasma sheath
potentials and enhanced ion sputtering.
boundary Characteristics and issues
Plasma boundary layer physics involves the interaction of a number of key phenomena that can
bridge adjoining subregions. The boundary layer begins in the closed field line region where the
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