Long Term Vortex Flow Evolution around a Magnetic Island in Tokamaks

Area of Science:

• Plasma Physics
• Fusion Energy Research
• Fluid Dynamics

Background:

• Magnetic islands are crucial in tokamak plasmas, influencing confinement and stability.
• Understanding vortex flow dynamics within these islands is key to predicting plasma behavior.

Purpose of the Study:

• To analyze the evolution of vortex flow within magnetic islands in collisionless tokamak plasmas.
• To investigate the impact of finite island width and toroidicity on flow structures.

Main Methods:

• Gyrokinetic analysis of plasma behavior.
• Examination of flow evolution over short (ωD t < 1) and long (ωD t > 1) timescales.

Main Results:

• Short-term: Monopolar vortex flow relaxes to a neoclassical residual level, enhanced by finite island width.
• Long-term: Toroidicity breaks helical symmetry, evolving flow to a dipolar zonal-vortex mixture.
• This mixture creates flow shear layers, enabling turbulence advection across the magnetic island.

Conclusions:

• Small magnetic islands (w≲qρTi/s) favor the formation of the dipolar zonal-vortex mixture.
• Larger islands tend to maintain the simpler monopolar vortex flow.
• The dipolar flow mixture facilitates cross-island turbulence transport, impacting plasma confinement.