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

the high-n regime of iteR; the large collection of interacting alfvén modes will drive nonlinear
cross-scale couplings from meso to micro-scale dynamics. This feature will become more dominant
in iteR than in current experiments, which predominantly observe moderate-n versions of
these instabilities. This regime requires a paradigm shift to nonlinear gyrokinetic models that include
all relevant wave-particle resonances and finite larmor radius effects for both the thermal
and energetic particle species. in addition, avalanche models need further development; the presence
of many closely spaced modes could allow the energetic particle pressure gradient to progressively
relax (flatten) from the center to the edge, implying rapid fast ion transport. to realistically
model alpha transport in the edge plasma and divertor regions, improved equilibrium magnetic
field models will be required that include the 3-d effects of toroidal field ripple, ferromagnetic
materials, and coils to control resistive wall modes and edge localized modes; also, the effects
of parasitic absorption of radiofrequency current drive power on alpha confinement will need to
be assessed. The goals of such improved nonlinear models are to identify regimes that might be
dangerous to the integrity of the plasma facing components, to predict the alpha heating profile
evolution, and to evaluate the alpha particle influence on the driven current and its distribution.
Ultimately, for iteR, with a fixed energy distribution (slowing-down, isotropic) for the alpha particles,
it would also be expected that alpha transport scaling laws could be developed from simulations
and experimental measurements. These could be useful for integrated modeling and for
operational scenario selection.
The best simulation methods are in the process of being determined. Presently several approaches
are in development. They include: (1) fully kinetic 3-d simulations that include the microinstability
drive acting on the background plasma; (2) detailed 3-d hybrid mhd fluid simulations for
the energetic particles and core plasma with the effects from microinstability treated in viscous
and heat transport terms; and (3) the development of quasi-linear codes that can couple results
from large fluid-particle codes to determine the transport coefficients that apply in the reduced
1-d profile evolution for the entire system. to achieve a deeper comprehension of the consequences
of alfvénic activity driven by alpha particles, reduced nonlinear theory will be incorporated in
quasi-linear codes and then compared with the results of the larger simulations. The development
path and viability for any of the above approaches will depend on regular validation against experimental
data provided by the new diagnostics discussed in the following section. While validation
tests in fully consistent burning plasma regimes (low r/a, v a /v alfvén ≥ 1, central b a (0) ≈
1 %) may have to wait for iteR, there are many other aspects that can be checked from ongoing
experiments.
advanced Diagnostics for alpha Physics
For the above improvements in simulation to have realism, an aggressive program of advanced
energetic particle diagnostic development is needed. The goal of this effort should be to provide
validation at appropriately resolved spatial and temporal scales and to stimulate improvements
in modeling as unexpected phenomena are uncovered. The requirements for such diagnostics is
an area with important overlaps to Thrust 1 and that much of the underlying development activity
may occur under Thrust 1.
The first element of such a program is the measurement of the fluctuating electric and magnetic
field instability mode structures. a number of techniques are now available for this task, such as
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