Observation of Critical-Gradient Behavior in Alfvén-Eigenmode-Induced Fast-Ion Transport.

Area of Science:

• Plasma physics
• Fusion energy research
• Tokamak experiments

Background:

• Fast-ion transport is critical for heating and stability in magnetic confinement fusion devices.
• Alfvén eigenmodes (AEs) can interact with fast ions, potentially leading to anomalous transport and losses.
• Understanding fast-ion behavior is essential for designing future burning plasma devices.

Purpose of the Study:

• To investigate the phenomenon of fast-ion transport becoming stiff in the DIII-D tokamak.
• To identify the conditions and mechanisms leading to stiff fast-ion transport.
• To assess the implications of stiff transport for predicting fast-ion profiles and losses.

Main Methods:

• Experiments conducted in the DIII-D tokamak.
• Analysis of fast-ion behavior in the presence of overlapping Alfvén eigenmodes (AEs).
• Utilizing Fast-ion D-alpha ( கரு ) spectroscopy to diagnose fast-ion transport.

Main Results:

• A critical threshold was identified above which fast-ion transport becomes suddenly stiff.
• The threshold is dependent on phase space and linked to stochastic particle orbits due to AE resonances.
• Equilibrium fast-ion density profiles remained unchanged above the threshold, despite increased drive.
• Intermittent fast-ion losses were observed.
• Radially localized transport of copassing fast ions was detected at radii corresponding to midcore AEs.

Conclusions:

• Stiff fast-ion transport occurs above a critical threshold driven by overlapping AEs.
• This phenomenon impacts fast-ion density profiles and leads to intermittent losses.
• Fast-ion Dα spectroscopy confirmed localized transport linked to AE locations.
• Reduced models incorporating stiff transport can aid in designing optimized scenarios for future burning plasma devices.