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Magnetic Turbulence and Current Drive during Local Helicity Injection.

N J Richner1, G M Bodner1, M W Bongard1

  • 1Department of Engineering Physics, University of Wisconsin-Madison, 1500 Engineering Drive, Madison, Wisconsin 53706, USA.

Physical Review Letters
|March 25, 2022
PubMed
Summary
This summary is machine-generated.

Alfvénic turbulence in tokamak startup current drives plasma current through magnetic relaxation. Super-Alfvénic electrons and dynamo effects are key drivers, offering insights for future fusion devices.

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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.