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

as a final note, the overall importance of the boundary layer is underscored by the european tokamak
program’s fundamental reorientation toward boundary research over the past decade. an
outcome of this shift is the decision to dedicate europe’s two most important tokamaks to exploring
materials other than carbon; Jet is committed to studying a beryllium-wall plus tungsten-divertor
PFc system, while aUG is focusing on an all-tungsten PFc system. Thus they have decided
that enhanced tokamak core physics priorities be subordinated to learning how to tame the plasma-materials
interface. The europeans have also been steadily growing their boundary modeling
and diagnostic efforts. The present Us situation stands in significant contrast: staffing levels in
Us tokamak scrape-off layer diagnosis and physics research have decreased over the past decade
from about 40 to about 20 people working at the major devices. The Us fusion program cannot
afford to fall behind in this critical area, recognized by the Greenwald Report as being at the top
of the magnetic Fusion energy priority list. at a minimum the heavy investment in the existing
major Us devices needs to be better exploited for addressing critical boundary issues. This will require
restoring, at least, the earlier level of effort as well as major investment in edge diagnostics.
Proposed actions:
1. Develop and deploy new diagnostics
Understanding complex systems requires extensive diagnostics. For example, exploiting aerodynamic
lift and drag for aircraft design would have been impossible if the air velocity were measured
at just one or two locations around airfoils; such measurements are typically made at dozens
of locations. The pedestal/sol is much more complicated than air flow around an object and
one can hardly expect to identify edge controlling physics with the spatially sparse diagnostic deployments
available today. in addition to the diagnostics sets described below, other techniques
should be assessed.
extensive sets of pop-up (or similar drive) probes should be used to map out the 4-d (spatially
and temporally) distributions of density, electron temperature, and electric potential (langmuir
probes); parallel flow velocity (mach probes); cross-field transport (turbulence probes); and ion
temperature and energy distributions of electrons and ions (gridded energy analyzers). magnetically
activated probe drives of various types have been developed, including swing action and reciprocation,
achieving viable penetration to the separatrix. deployments of such probes are needed
in significant numbers (e.g., > 10 systems), around the periphery of tokamaks. While materials
costs will be relatively modest, the additional staffing requirements will be significant.
Thomson scattering is complicated in edge plasmas by high background light, but it has been successfully
used on diii-d to reconstruct the 2-d profiles of electron density and temperature (n e and
t e ) by sweeping of the magnetic X-point relative to the Thomson channels. With the addition of a
high-dispersion instrument such as a Fabry-Perot interferometer, measurements could be made
of electron/ion energy distributions and z eff . in addition, a multi-laser Thomson system (firing
laser 1, 2, 3 in rapid succession) could make images that reveal the detailed time evolution of socalled
“blobs” and elms. charge-exchange recombination (ceR) spectroscopy using new powerful
pulsed ion diode neutral beams (10 6 a/m 2 , 1 μs, 100 kev/amu) may permit measurement of t i and
v || (along b) in to the separatrix. advances in ultrasoft X-ray spectroscopy would allow reconstruction
of the t e profiles with time resolution higher than possible from Thomson scattering alone.
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