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

diagnosTiC inVesTmenTs
Can diagnostics be improved to identify the missing physics that control plasma-wall interactions?
Present edge/sol diagnostic capability is seriously inadequate and is the main impediment to the
identification of missing edge physics. on most tokamaks today, only ne and te (without energy
distribution information and usually only time-averaged) are measured regularly — at a few locations.
a spatially extensive set of edge measurements is required. Understanding aerodynamic lift
would have been impossible if the air velocity was measured at just one or two locations around airfoils;
the sol is much more complicated and present spatially sparse diagnostic sets cannot identify
edge controlling physics. extensive sets of pop-up probes could be used to map out the 4-d
distributions of n e , t e , f ,v || , ⊥
v , t i and electron and ion energy distributions. Thomson scattering
with a high dispersion instrument such as a Fabry-Perot could measure electron and ion energy
distributions and z eff . a multi-laser Thomson system (firing laser 1, 2, 3 in rapid succession) could
make images that follow the time evolution of “blobs” and edge localized modes (elms). charge exchange
recombination (ceR) spectroscopy using new powerful pulsed ion diode neutral beams (10 6
a/m 2 , 1 μs, 125 kev/amu) may permit measurement of t i and v || into the separatrix. Real time, in
situ surface diagnosis of hydrogen-uptake, erosion, deposition, co-deposition, etc., is likewise required,
using various proposed concepts to perform surface analysis inside the vessel. better characterization
of sol cross-field transport is needed. since most of these techniques are labor-intensive,
a significant increase in the number of edge diagnosticians will be required.
off-line facilities are needed to provide in-situ surface dynamic diagnosis of innovative materials
to quickly test fundamental properties that can be accelerated for further testing in more
complex environments (e.g., linear plasma device and fusion device). We must also close a gap between
materials modeling and experimental techniques to elucidate the fundamental damage
mechanisms in the complex plasma edge environment and the particle-induced material evolution
during steady-state and transient plasma events. in particular, we must study how the PWi
at length scales less than a micron couples to structural effects at depths of a few microns or
more. off-line facilities can also introduce appropriate energetic particle beams to study the bulk
and surface interface to understand effects on retention, diffusion, segregation, permeation and
phase transformation, among other mechanisms.
dediCaTed eXperimenTal Time
Can more dedicated experimental time on existing facilities or a dedicated new facility provide the edge
measurements required to sort out key plasma-wall interaction issues?
There is a strong need for greater predictability of the plasma scrape-off layer’s basic parameters
and PWi issues. We critically need data to make design choices for both a d-t component test
Facility (ctF) and demo. The most fundamental requirement is for a predictive-quality scaling
of the power scrape-off width. This has a strong impact on the heat-flux handling capability required
of the divertor in future devices. carefully planned experiments on existing facilities are
needed, with controls over radiative power fraction in the plasma and in the sol, as well as monitoring
the degree of divertor recycling and/or detachment. other high-level issues that require
more thorough study include the dependencies on key parameters (major/minor radii, magnetic
field, and discharge current and power) of the density and/or fueling rate at which the transitions
occur from sheath-limited to high recycling conduction-limited, and then to detached operation.
it is well known that the particle equilibration time in the sol and material surfaces can be much
125