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pueschel_epr14.pdf2014-08-14 17:51:19M.J. Pueschel

An Extremely Stiff Beta Limit in High-Gradient ITG Turbulence: The Non-Zonal Transition

Author: M.J. Pueschel
Requested Type: Consider for Invited
Submitted: 2014-05-28 16:17:56

Co-authors: P.W.Terry

Contact Info:
1150 University Ave
Madison, Wisconsin   53706

Abstract Text:
Recently, the non-zonal transition has been identified
as a possible strict beta limit for toroidal fusion
devices exhibiting ion temperature gradient mode driven
turbulence. Such devices include tokamaks, stellarators,
and reversed-field pinches. For sufficiently high
background pressure gradients, magnetic field lines
start to decorrelate well below the kinetic ballooning
limit. In that case, electrons are provided a means of
moving radially on the parallel transit time scale. This
allows them to react very efficiently to radial
differences in the electrostatic potential, causing them
to short out zonal flows. Therefore, turbulent
saturation can no longer rely on those zonal flows to
keep heat fluxes at moderate levels. As a result, heat
is transported at a much-accelerated rate, leading to
quasilinear flattening of the temperature profile,
almost regardless of how much heat is injected into the
system. Candidates for this scenario are certain Ohmic
confinement regime transitions, as well as the increased
transport stiffness in high-gradient regions found, for
instance, in H-mode pedestals.

From a database of simulations of plasma microturbulence,
an expression is derived which can be used to replace
the radial displacement of magnetic field lines after
traversing half a poloidal circle. Equating this
quantity with the correlation length of the magnetic
field perturbation leads to an estimate of the beta
threshold where field lines first decorrelate and may
therefore cause a non-zonal transition.

Characterization: 4.0

Category 4.0 is likely the most suitable, but the organizers may determine that 1.2 may also apply.

Workshop on Exploratory Topics in Plasma and Fusion Research (EPR) and US-Japan Compact Torus (CT) Workshop
August 5-8, 2014
Madison, Wisconsin

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