Magnetic Turbulence and Current Drive during Local Helicity Injection

Area of Science:

• Plasma Physics
• Fusion Energy Research
• Magnetohydrodynamics

Background:

• Tokamak startup relies on efficient plasma current generation.
• Direct current (DC) helicity injection is a method to drive plasma currents.
• Understanding the underlying physics of current drive is crucial for fusion energy.

Purpose of the Study:

• To investigate the role of Alfvénic turbulence in DC helicity injection during tokamak startup.
• To identify the mechanisms responsible for magnetic relaxation and plasma current drive.
• To determine the influence of super-Alfvénic electrons and dynamo effects on turbulence.

Main Methods:

• Performing magnetic measurements during DC helicity injection.
• Analyzing the localization and onset dependence of turbulent activity.
• Utilizing higher-order spectral analysis to characterize electron behavior.
• Measuring fluctuation helicity to assess dynamo activity.

Main Results:

• Alfvénic turbulence in injected current streams mediates magnetic relaxation and drives macroscopic plasma current.
• Turbulence is localized to current streams and depends on bias voltage.
• Super-Alfvénic electrons excite instabilities driving the observed turbulence.
• Measured fluctuation helicity aligns with an alpha-dynamo electromotive force, driving current comparable to equilibrium levels.

Conclusions:

• DC helicity injection utilizes Alfvénic turbulence for plasma current drive.
• Super-Alfvénic electrons and dynamo mechanisms are critical for this process.
• Findings provide constraints for scaling helicity injection to larger fusion devices.