Visualization of Fast Ion Phase-Space Flow Driven by Alfvén Instabilities.

Area of Science:

• Plasma physics
• Fusion energy research
• Tokamak diagnostics

Background:

• Fast ions are crucial for heating and sustaining plasma in fusion devices like tokamaks.
• Alfvén eigenmodes (AEs) are plasma instabilities that can interact with and affect fast ions.
• Understanding fast ion behavior is essential for achieving controlled nuclear fusion.

Purpose of the Study:

• To measure and characterize the phase-space flow of fast ions driven by AEs in the DIII-D tokamak.
• To investigate the impact of this flow on fast ion thermalization and transport.
• To compare experimental measurements with theoretical models and simulations.

Main Methods:

• Utilizing an imaging neutral particle analyzer for in-situ measurement of fast ion phase-space flow.
• Analyzing ion trajectories in phase space based on energy, canonical toroidal momentum, and AE properties (frequency and toroidal mode number).
• Employing nonlinear hybrid kinetic-magnetohydrodynamics simulations for comparison.

Main Results:

• Fast ion phase-space flow was observed near the minimum safety factor, with trajectories influenced by energy changes.
• Strong interactions with AEs led to destructive flow, preventing fast ion thermalization in affected regions.
• Measured flow patterns were consistent with theoretical predictions and simulation results.

Conclusions:

• The observed phase-space flow significantly alters fast ion dynamics, hindering thermalization.
• Fast ions must transition between different flow trajectories for large-scale phase-space transport, as indicated by narrow phase-space islands.
• Experimental findings validate the accuracy of nonlinear hybrid kinetic-MHD simulations in describing these phenomena.