Avalanches triggered by Kelvin-Helmholtz instability in a cylindrical plasma device

Area of Science:

• Plasma Physics
• Fluid Dynamics
• Computational Physics

Background:

• Understanding plasma confinement and transport is crucial for fusion energy.
• The Controlled Shear Decorrelation Experiment (CSDX) investigates plasma turbulence and transport in linear devices.
• Previous studies have observed intermittent events and profile evolution in CSDX, but the underlying mechanisms require further elucidation.

Purpose of the Study:

• To simulate the profile-evolving plasma dynamics in the CSDX linear device.
• To identify the mechanisms driving plasma density avalanches and their associated phenomena.
• To validate simulation results against experimental observations of plasma profiles and fluctuations.

Main Methods:

• Development and application of a new profile-evolving simulation code.
• Comparison of simulation outputs with experimental data from CSDX standard discharges (B=1000 G).
• Analysis of plasma density, electric potential, instability transitions, vorticity, and convective structures.

Main Results:

• Simulation results show excellent agreement with experimental observations of plasma density and electric potential profiles and fluctuations.
• Plasma density avalanches are triggered by the transition from adiabatic collisional drift waves to non-adiabatic Kelvin-Helmholtz instability.
• Avalanches initiate at locations of maximum local vorticity, with a critical vorticity threshold identified.
• Intermittent particle and heat convective structures are ejected as 'avaloids' when zonal flow intensity is weak.

Conclusions:

• The developed simulation code accurately reproduces CSDX experimental observations, validating its predictive capability.
• The study elucidates the mechanism of plasma density avalanches, linking them to specific instability transitions and vorticity dynamics.
• The findings reveal the formation and ejection of convective structures (avaloids) as a consequence of these avalanches, particularly under weak zonal flow conditions.