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

H-mode access: access to the h-mode is essential for iteR to fulfill its experimental mission.
a high-priority issue is the determination of the threshold heating power required for attaining
several regimes: (1) the standard h-mode, (2) steady h-mode plasmas with “good” confinement,
as determined by standard scaling relations, and (3) h-mode access during the plasma current
ramp-up/down phases. These studies need to be done in iteR-like plasma conditions, and strategies
should be developed for minimizing the heating power. how the required heating power for
these three regimes scales with isotope mass and species (e.g., hydrogen and helium plasmas) is
also of high value for the nonnuclear phase of iteR.
Research should focus on determining a physical basis for extrapolating h-mode power thresholds
accurately. experimental work would require emphasis on edge diagnostics, particularly
those that measure profiles of relevant quantities such as density, temperature, and flows. characterization
of edge fluctuations leading up to l-h transitions is also important. ample auxiliary
power should be available in multiple forms (neutral beams, radiofrequency waves, etc.). efforts
should be made to resolve differences in power threshold results that may arise with different
heating schemes. a major goal of the theoretical and computational efforts should be to simulate
the trigger mechanisms and dynamics of an l-to-h transition and to validate results against experimental
data. to facilitate comparisons across machines as well as between experiment and
theory, a database containing full profile information should be assembled and made freely accessible
to researchers.
transient Conditions: to develop plasma current ramp-up and ramp-down scenarios for iteR,
it is important to have a reliable prediction for the thermal electron diffusivity (which governs the
electron temperature profile). several itPa groups have recognized this as a high-priority topic.
Uncertainty concerning thermal transport during current ramp-up and ramp-down can be greatly
reduced by coordinated research on this topic in available tokamaks. to be most relevant to
iteR, it is desirable to use radiofrequency heating in place of neutral beam heating during the current
ramp-up and ramp-down. sawtooth “mixing” and temperature recovery between sawtooth
crashes constitute another transient condition that may be of more importance in iteR than in
present devices. to study transport within the sawtooth mixing region, high temporal resolution
is required for measurements of magnetic pitch angle, ion temperature, and flow speed. experiments
need to vary between thermal (e.g., ohmic or electron cyclotron heating) and fast ion heating
to validate models of the effect of supra-thermal sawtooth stabilization. like sawteeth, edge
localized modes (elms) will effectively produce significant radial transport in the periphery, and,
by affecting the outer “boundary condition,” the influence of elms can propagate into the plasma
core.
transport barrier Formation: The fusion gain in iteR can be enhanced by the presence of internal
transport barriers (itbs) if sources of velocity shear and magnetic field shear can be produced.
Possible rotation sources are discussed in the previous section. There are some indications
that internal transport barriers can be generated, without velocity or magnetic shear, by pellet
injection or off-axis ion cyclotron resonance heating. For future reactor-relevant regimes, it would
be useful to clarify which aspects of neutral beam injection are relevant for the formation of internal
transport barriers (for example, velocity shear versus energetic particles). Furthermore, experiments
need to demonstrate internal transport barriers induced by magnetic field shear in sta-
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