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

improved measurements of alfvén mode damping are needed, especially continuum and radiative
damping, coupled with validation. alfvén mode excitation by external antennas and beatwave
sources in the ion cyclotron range of frequencies are promising techniques for damping
studies. Recent rapid progress in internal mode structure measurements should enable better
quantitative understanding. The diagnostic challenges in this area are to measure the short-scale
structures that are predicted by theory and to identify the spatial structure and dissipation of medium-n
modes. The experimental challenge is to validate the measured unstable and stable mode
structure and compare the damping with calculated damping rates.
With respect to the nonlinear regime of alfvén instabilities, the theory for their behavior near
threshold has been developed. This theory requires extensive verification and validation; a variety
of nonlinear phenomena are observed and predicted, including chirping, bursting, avalanches,
and frequency splitting. The diagnostic challenge in this area is to measure phase space structures
and fast ion transport or loss. The experimental challenge is to explore the threshold requirement
for mode overlap and find methods for preventing its occurrence.
Controls for alpha Physics
successful validation of simulation tools with advanced diagnostics will form the knowledge base
for the final and more exploratory aspect of this Thrust: the direct control of alpha particle effects
to optimize fusion power performance. control of the alpha heating source in iteR and demo
will have high leverage in influencing plasma stability and behavior. heating source controls are
used extensively in existing devices to access improved confinement regimes, drive currents, stabilize
mhd modes, and optimize plasma transport. however, unlike the heating power in existing
experiments, alpha heating is self-generated and not as easily amenable to external control.
new techniques will have to be developed and tested to accomplish such control. experiments
on diii-d have indicated that focused electron cyclotron resonance heating can control fast iondriven
fluctuation levels. beat-wave generation, with two ion cyclotron heating sources whose
frequencies are closely spaced, has also been suggested as a way to deposit power in the range of
alfvén frequency fluctuations and modify alpha profiles. control of the ion density profile (e.g.,
with pellets), q-profile, magnetic shear, and flow shear can be expected to have an impact on alpha-driven
fluctuations. enhanced loss via externally driven stochastic resonances (phase-space
engineering) has been suggested for alpha ash removal and burn control. damping of alpha-driven
instabilities on other fast ion populations (created by neutral beam injection or ion cyclotron
resonance heating) could be used for suppression. For example, injection of medium-energy positive-ion
neutral beams near the iteR edge region could enhance landau damping of alpha-driven
instabilities and drive plasma rotation. These methods will need to be thoroughly assessed with
simulation tools and studies in existing experiments prior to d-t operation in iteR.
a second area for alpha control studies, referred to as alpha channeling, involves methods for
directly influencing the alpha particle heating feedback loops. The classical collisional slowingdown
process for fusion-born alpha particles results in most of the alpha energy being transferred
first to electrons, leading to electron temperatures that exceed ion temperatures. Fusion reactivity
could significantly be improved if hot-ion regimes (t ion > t electron ) could be sustained. ideally,
the goal of alpha channeling is to transfer the alpha particle birth energy to an appropriate set of
waves, which could then directly heat fuel ions and simultaneously cause ejection of low-energy
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