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

Recent accomplishments and progress
considerable progress has been made in understanding alpha particle effects through closely coupled
efforts in experiments, diagnostics, theory, and simulations. highlights include:
• alfvén instabilities excited by energetic ions have been theoretically predicted and
observed in a range of magnetic configurations (e.g., small to large aspect ratio tokamaks,
also stellarators).
• linear alfvén stability thresholds have been predicted and validated with diagnostics
capable of measuring the internal spatial and spectral structure of multiple fluctuating
fields and with external antennas capable of exciting stable alfvén modes to measure
their in-situ damping rates.
• enhanced transport and direct loss of energetic ions associated with alfvén instabilities
have been observed.
• The potential for control of the fast ion instabilities has been demonstrated via localized
radiofrequency wave heating (at both the ion and electron cyclotron frequencies).
• Fast ion instabilities with spatially localized, high-mode-number eigenmodes (similar to
those expected in iteR) have been observed experimentally and modeled theoretically.
• analysis of recently observed instabilities associated with acoustic (sound wave) coupling
to alfvén modes suggests a potential means for efficiently transferring alpha particle
energy to the thermal plasma directly.
in addition, significant progress has been made in modeling energetic particle instabilities and
their associated effects. new numerical codes based on extended-mhd theory are being developed
for realistic nonlinear simulation of fast ion instabilities in burning plasmas. already they
have been used to successfully model various energetic particle-driven instabilities in existing experiments.
sophisticated gyrokinetic codes, with self-consistent treatment of the fast ion populations
and also the thermal plasma fluctuations, are also under development and have been used
to simulate alfvén eigenmodes.
SCiEnCE CHaLLEngES, OPPORtunitiES, anD RESEaRCH nEEDS
The primary alpha physics issues revolve around energetic particle instabilities and their diagnosis,
alpha confinement, and self-heating. Recent observations with new diagnostic techniques
(see Figures 3 and 4) indicate that significant fast ion transport induced by multiple alfvénic instabilities
may be present in the core of reactor-relevant regimes. This underscores the urgent
need to develop validated predictive models and effective control tools for alfvénic instabilities.
however, the existing theoretical, simulation, diagnostic, and control tools are not yet fully adequate
to accurately predict the impact of fast ion-driven instabilities in present devices and in
iteR. The research needs in the various research categories are listed.
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