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Open Access Publications from the University of California

Intrinsic plasma flows in straight magnetic fields: generation, frictionless saturation, and interaction

  • Author(s): Li, Jiacong
  • Advisor(s): Diamond, Patrick H
  • et al.
Abstract

We develop a simple model for the generation and amplification of intrinsic axial flow in a linear device, Controlled Shear Decorrelation Experiment (CSDX). This model develops a novel dynamical symmetry breaking mechanism in drift wave turbulence, which does not require complex magnetic field structure, such as shear. Thus, the model is applicable to both tokamaks and linear devices. This mechanism is, essentially, a form of negative viscosity phenomenon.

Negative compressibility ITG turbulence can also induce a negative viscosity increment.

However, we show that no intrinsic axial flow can be generated by pure ITG turbulence in a straight magnetic field. When the flow gradient is steepened by any drive mechanism, the flow profile saturates at a level close to the value above which parallel shear flow instability (PSFI) becomes dominant over the ITG instability. This saturated flow gradient exceeds the PSFI linear threshold, and grows with $\nabla T_{i0}$ as $|\nabla V_\parallel| / |k_\parallel c_s| \sim|\nabla T_{i0}|^{2/3} / (k_\parallel T_{i0})^{2/3}$.

The coupling of azimuthal and axial flows in CSDX--in absence of magnetic shear--is investigated.

In particular, we focus on the apportionment of turbulence energy between azimuthal and axial flows, and how the azimuthal flow shear affects axial flow generation and saturation by drift wave turbulence.

Detailed measurements of intrinsic axial flow parallel to the magnetic field are performed on CSDX, with no axial momentum input.

The results present a direct demonstration that the broken spectral symmetry of drift wave turbulence causes the development of axial mean flows in cylindrical magnetized plasmas.

Measurements suggest the axial flow is parasitic to the drift wave--zonal flow system.

Besides, we show that consideration of wave--flow resonance resolves the long-standing problem of how zonal flows (ZFs) saturate in the limit of weak or zero frictional drag and also determines the ZF scale directly from analysis.

We show that resonant vorticity mixing, which conserves potential enstrophy, enables ZF saturation in the absence of drag, and so is effective at regulating the Dimits up-shift regime.

Vorticity mixing is incorporated as a nonlinear, self-regulation effect in an extended 0D predator--prey model of drift--ZF turbulence.

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