Parallel electron force balance and the L-H transition
Authors: Timothy Stoltzfus-Dueck
Abstract: In one popular paradigm for the L-H
transition, energy transfer to the mean flows directly depletes
turbulence fluctuation energy, resulting in suppression of the
turbulence and a corresponding transport bifurcation. To
quantitatively
evaluate this mechanism, one must remember that electron parallel
force balance couples nonzonal velocity fluctuations with electron
pressure fluctuations on rapid timescales, comparable with the
electron transit time. For this reason, energy in the nonzonal
velocity stays in a fairly fixed ratio to the free energy in
electron density fluctuations, at least for frequency scales much
slower than electron transit. In order for direct depletion of the
energy in turbulent fluctuations to cause the L-H transition,
energy transfer via Reynolds stress must therefore drain enough
energy to significantly reduce the sum of the free energy in
nonzonal velocities and electron pressure fluctuations. At low k⊥,
the electron thermal free energy is much larger than the energy in
nonzonal velocities, posing a stark challenge for this model of
the L-H transition.
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