59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017;
Milwaukee, Wisconsin
Session VI2: Transport
3:00 PM–5:00 PM,
Thursday, October 26, 2017
Room: 102ABC
Chair: Orso Meneghini, General Atomics
Abstract ID: BAPS.2017.DPP.VI2.1
Abstract: VI2.00001 : Experimental Challenges to Stiffness as a Transport Paradigm*
3:00 PM–3:30 PM
Preview Abstract
Abstract
Author:
T.C. Luce
(General Atomics)
Transport in plasmas is treated experimentally as a relationship between
gradients and fluxes in analogy to the random-walk problem. Gyrokinetic
models often predict strong increases in local flux for small increases in
local gradient when above a threshold, holding all other parameters fixed.
This has been named `stiffness'. The radial scalelength is then expected to
vary little with source strength as a result of high stiffness. To probe the
role of ExB shearing on stiffness in the DIII-D tokamak, two neutral beam
injection power scans in H-mode plasmas were specially crafted---one with
constant, low torque and one with increasing torque. The ion heat, electron
heat, and ion toroidal momentum transport do not show expected signatures of
stiffness, while the ion particle transport does. The ion heat transport
shows the clearest discrepancy; the normalized heat flux drops with
increasing inverse ion temperature scalelength. ExB shearing affects the
transport magnitude, but not the scalelength dependence. Linear gyrofluid
(TGLF) and nonlinear gyrokinetic (GYRO) predictions show stiff ion heat
transport around the experimental profiles. The ion temperature gradient
required to match the ion heat flux with increasing auxiliary power is not
correctly described by TGLF, even when parameters are varied within the
experimental uncertainties. TGLF also underpredicts transport at smaller
radii, but overpredicts transport at larger radii. Independent of the
theory/experiment comparison, it is not clear that the theoretical
definition of stiffness yields any prediction about parameter scans such as
the power scans here, because the quantities that must be held fixed to
quantify stiffness are varied. A survey of recent literature indicated that
profile resilience is routinely attributed to stiffness, but simple model
calculations show profile resilience does not imply stiffness. Taken
together, these observations challenge the use of local stiffness as a
paradigm for explaining global transport behavior.
*Work supported by US DOE under DE-FC02-04ER54698.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2017.DPP.VI2.1