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

Do the alpha particles couple to the dynamical behavior of the background plasma and cause a
new set of instabilities and associated transport?
The strong self-heating of future burning plasmas will cause energetic particle behavior to be coupled
with core stability and turbulence to a degree not observable on existing experiments. For
example, it is expected that sawtooth crashes could redistribute fast ions and create conditions
that are favorable to alfvén “avalanche” modes. a related issue is the evolution of the core plasma
into a high-pressure-gradient, energetic-particle-stabilized state that can rapidly become sawtooth/ballooning
unstable as the stabilizing alphas are transported by energetic particle modes.
at smaller scales, the question of how energetic particle instabilities interact with plasma microturbulence
will become increasingly important in evaluating the damping rates of the shorterwavelength
alfvén instabilities that will characterize iteR. The simultaneous investigation of the
full spectrum of relevant instabilities and their interaction is complicated by the disparate time
scales involved, from acoustic to ion cyclotron; new techniques will have to be developed to efficiently
deal with this scale separation.
Can the interaction of the alpha particle population with the background plasma be controlled for
beneficial purposes?
The successful operation of sustained flattop regimes in burning plasmas will require new techniques
for active control of the alpha population. For example, methods for temporarily inducing
enhanced alpha losses over limited energy ranges could provide instability suppression, burn control,
or alpha ash removal. such losses could be driven by external radiofrequency sources or timevarying
ripple fields or by the destabilization of modes that resonantly transport alphas. other
control methods based upon rotation control, q-profile control, and use of radiofrequency and
electron cyclotron resonance heating are expected to be useful for controlling alpha-driven fluctuation
levels. alpha channeling refers to the identification of mechanisms for directly transferring
the alpha population energy to the deuterium and tritium ions without having to first pass
through the electron channel. if successful, this could vastly improve the self-heating efficiency of
the burning plasma. other forms of channeling might involve using the alpha particles to assist
with current drive, enhance plasma rotation, or reduce the alpha particle partial pressure.
Finally, it should be noted that existing alpha physics simulation tools have typically focused on
time scales that are short compared to those that characterize core plasma transport and magnetic
flux evolution. For the strongly self-heated regimes of iteR and demo, alpha physics and
core plasma simulations must be fully integrated, using quasi-linear theory and other techniques,
so that the impact of alpha transport and turbulence on the core plasma and vice versa can be selfconsistently
assessed. For example, the question of whether true steady-state profiles can be sustained
in burning plasmas will be crucial for extrapolations to demo. also, such a fully integrated
model could be applied to better evaluate the stability of the alpha population, using realistic
sources and sinks rather than arbitrarily imposed profiles and boundary conditions.
COnCLuSiOn
The goal of alpha physics research is to develop predictive understanding and control methods for
strongly self-heated regimes in burning plasmas. This will involve making significant improvements
in understanding alpha-driven alfvénic instabilities through advanced integrated diag-
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