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

longer than the (solid) thermal equilibration time in these systems. Thus, much longer pulses will
be required in a future device designed to understand and control the dynamics of high recycling
and detachment. it is also anticipated that the heat flux in a ctF or demo will far exceed what
can currently be studied for long pulses in tokamak experiments. The temperature of the plasma
facing components will also be much higher than in current experiments. together these will
strongly change the dynamics of plasma recycling. Thus, the physics, particularly of high recycling
and detachment, may be quite different from what is encountered in current experiments.
Therefore, an intermediate-scale experiment capable of providing the hot walls, long pulse, and
high-power plasma-material interface of ctF and demo, with high flexibility to test innovations
and with excellent diagnostic access, is needed to validate the understanding of plasma-wall interactions
at a level that supports a major long-pulse, high-power d-t initiative.
improVed models and Code ComponenTs
Can basic scrape-off layer plasma models and code components be improved to validate predictions for future
devices?
Plasma transport in the edge/sol has a direct impact on plasma energy and particle fluxes to
PFcs and on impurity transport and redeposition. The observed nature of this transport has long
been differentiated from that in the core plasma, with historical measurements of large density
fluctuations relative to the time-average values, which can approach unity in the sol. more recent
measurements have shown additional effects such as strong intermittency, filamentation,
toroidal asymmetry, and large flows.
two-dimensional plasma-fluid transport codes (e.g., UedGe, b2-eirene, edGe2d, osm-eirene)
are the primary tools used to model particle and heat fluxes in a tokamak boundary layer. Up to
now such codes have not provided full predictive capability for the scrape-off layer profiles, but
typically fit data at one location and compare at remote locations. yet even with this modest goal,
it has been difficult to reproduce the observed plasma phenomenology, such as very strong (nearsonic)
plasma flow around the plasma periphery, without postulating beforehand strong spatial
dependencies of the transport coefficients. moreover, experiments clearly show that simple diffusive
and/or convective transport descriptions are not appropriate. large amplitude fluctuations,
critical-gradient transport dynamics and intermittent transport events involving quasi-coherent
structures (blobs, elms) dominate the edge plasma. accurate descriptions of divertor detachment
are also lacking. likewise, the present generation of edge plasma turbulence simulation
codes (e.g., boUt, dalFti, esel) appears to reproduce much of the edge plasma phenomenology
(e.g., blob-like propagation, intermittency), but there are only limited and approximate comparisons
of wave number spectra, radial fluxes, and blob speeds.
our present deficiencies in boundary layer physics understanding are a major obstacle for accurately
projecting to edge plasma conditions in iteR and demo – devices which will have combinations
of power densities, plasma collisionalities and neutral opacities that are inaccessible in today’s
devices. current tokamaks will not be able to access the high-power output per unit surface
area of ~1 mW/m 2 that is required for a demo-class reactor, nor will they be able to simultaneously
attain the divertor collisionality and neutral opacity of a demo. Thus, to understand edge
plasma behavior in regimes that more closely match those of a demo and to validate correspond-
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