Resistive wall mode instability at intermediate plasma rotation.

Area of Science:

• Plasma physics
• Fusion energy research
• Magnetohydrodynamics

Background:

• Resistive wall modes (RWMs) are plasma instabilities that can degrade confinement in fusion devices.
• Existing models often rely on simplified critical rotation thresholds that may not capture complex plasma behaviors.
• Understanding RWMs is crucial for achieving stable plasma conditions necessary for fusion energy generation.

Purpose of the Study:

• To investigate the insufficient predictive power of low critical rotation threshold models for RWM instability.
• To explore the impact of kinetic effects on the relationship between plasma rotation and RWM stability.
• To compare experimental RWM observations with theoretical calculations incorporating kinetic physics.

Main Methods:

• Experimental observations of RWM instability in the National Spherical Torus Experiment (NSTX).
• Theoretical calculations using the MISK code, incorporating kinetic modifications to ideal stability criteria.
• Self-similar scaling of experimental plasma rotation profiles and collisionality to analyze mode resonances.

Main Results:

• Experimental RWM instability observed at intermediate plasma rotation levels, challenging simple models.
• MISK code calculations with kinetic effects show good agreement with experimental RWM marginal points.
• Resonances between the RWM and ion precession drift/bounce frequencies were explored through profile scaling.
• RWMs experimentally become unstable when plasma rotation is between stabilizing precession drift and bounce resonances.

Conclusions:

• Low critical rotation threshold models are insufficient for explaining RWM behavior at intermediate rotation speeds.
• Kinetic effects significantly influence RWM stability, leading to a more complex rotation-stability relationship.
• The MISK code, including kinetic physics, successfully predicts experimental RWM behavior, validating theoretical advancements.