404 research outputs found
Experimentally inferred thermal diffusivities in the edge pedestal between edge-localized modes in DIII-D
© 2007 American Institute of Physics. The electronic version of this article is the complete one and can be found online at:http://link.aip.org/link/PHPAEN/v14/i12/p122504/s1DOI: 10.1063/1.2817969Using temperature and density profiles averaged over the same subinterval of several successive inter-edge-localized-mode (ELM) periods, the ion and electron thermal diffusivities in the edge pedestal were inferred between ELMs for two DIII-D [ J. Luxon, Nucl. Fusion 42, 614 (2002) ] discharges. The inference procedure took into account the effects of plasma reheating and density buildup between ELMs, radiation and atomic physics cooling, neutral beam heating and ion-electron equilibration, and recycling neutral and beam ionization particle sources in determining the conductive heat flux profiles used to infer the thermal diffusivities in the edge pedestal. Comparison of the inferred thermal diffusivities with theoretical formulas based on various transport mechanisms was inconclusive insofar as identifying likely transport mechanisms
A model for microinstability destabilization and enhanced transport in the presence of shielded 3-D magnetic perturbations
A mechanism is presented that suggests shielded 3-D magnetic perturbations
can destabilize microinstabilities and enhance the associated anomalous
transport. Using local 3-D equilibrium theory, shaped tokamak equilibria with
small 3-D deformations are constructed. In the vicinity of rational magnetic
surfaces, the infinite-n ideal MHD ballooning stability boundary is strongly
perturbed by the 3-D modulations of the local magnetic shear associated with
the presence of nearresonant Pfirsch-Schluter currents. These currents are
driven by 3-D components of the magnetic field spectrum even when there is no
resonant radial component. The infinite-n ideal ballooning stability boundary
is often used as a proxy for the onset of virulent kinetic ballooning modes
(KBM) and associated stiff transport. These results suggest that the achievable
pressure gradient may be lowered in the vicinity of low order rational surfaces
when 3-D magnetic perturbations are applied. This mechanism may provide an
explanation for the observed reduction in the peak pressure gradient at the top
of the edge pedestal during experiments where edge localized modes have been
completely suppressed by applied 3-D magnetic fields
Modeling electron temperature profiles in the pedestal with simple formulas for ETG transport
This paper reports on the refinement (building on Ref.~\cite{hatch_22}) and
application of simple formulas for electron heat transport from electron
temperature gradient (ETG) driven turbulence in the pedestal. The formulas are
improved by (1) improving the parameterization for certain key parameters and
(2) carefully accounting for the impact of geometry and shaping in the
underlying gyrokinetic simulation database. Comparisons with nonlinear
gyrokinetic simulations of ETG transport in the MAST pedestal demonstrate the
model's applicability to spherical tokamaks in addition to standard aspect
ratio tokamaks. We identify bounds for model applicability: the model is
accurate in the steep gradient region, where the ETG turbulence is largely
slab-like, but accuracy decreases as the temperature gradient becomes weaker in
the pedestal top and the instabilities become increasingly toroidal in nature.
We use the formula to model the electron temperature profile in the pedestal
for four experimental scenarios while extensively varying input parameters to
represent uncertainties. In all cases, the predicted electron temperature
pedestal exhibits extreme sensitivity to separatrix temperature and density,
which has implications for core-edge integration. The model reproduces the
electron temperature profile for high scenarios but
not for low scenarios in which microtearing modes have been
identified. We develop a proof-of-concept model for MTM transport and explore
the relative roles of ETG and MTM in setting the electron temperature profile.
We propose that pedestal scenarios predicted for future devices should be
tested for compatibility with ETG transport
Gyrokinetic Simulations Compared with Magnetic Fluctuations Diagnosed with a Faraday-Effect Radial Interferometer-Polarimeter in the DIII-D pedestal
Experimental data on electromagnetic fluctuations in DIII-D, made available
by the Faraday-effect Radial Interferometer-Polarimeter (RIP) diagnostic, is
examined in comparison with detailed gyrokinetic simulations using Gyrokinetic
Electromagnetic Numerical Experiment (GENE). The diagnostic has the unique
capability of making internal measurements of fluctuating magnetic fields
. Local linear simulations identify
microtearing modes (MTMs) over a substantial range of toroidal mode numbers
(peaking at ) with frequencies in good agreement with the experimental
data. Local nonlinear simulations reinforce this result by producing a magnetic
frequency spectrum in good agreement with that diagnosed by RIP. Simulated heat
fluxes are in the range of experimental expectations. However, magnetic
fluctuation amplitudes are substantially lower than the experimental
expectations. Possible sources of this discrepancy are discussed, notably the
fact that the diagnostics are localized at the mid-plane -- the poloidal
location where the simulations predict the fluctuation amplitudes to be
smallest. Despite some discrepancies, several connections between simulations
and experiments, combined with general criteria discriminating between
potential pedestal instabilities, strongly point to MTMs as the source of the
observed magnetic fluctuations
Zonal flows and long-distance correlations during the formation of the edge shear layer in the TJ-II stellarator
A theoretical interpretation is given for the observed long-distance
correlations in potential fluctuations in TJ-II. The value of the correlation
increases above the critical point of the transition for the emergence of the
plasma edge shear flow layer. Mean (i.e. surface averaged, zero-frequency)
sheared flows cannot account for the experimental results. A model consisting
of four envelope equations for the fluctuation level, the mean flow shear, the
zonal flow amplitude shear, and the averaged pressure gradient is proposed. It
is shown that the presence of zonal flows is essential to reproduce the main
features of the experimental observations.Comment: 19 pages, 7 figure
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Comparison of Sawtooth Phenomenology on TFTR and DIII-D
An experiment to study sawtooth phenomena and to find the threshold for sawtooth stabilization with neutral beam injection heating, as was commonly observed on TFTR, has been done on DIII-D. In the experiments, with co-tangential neutral beam injection at powers of up to 13MW, the sawtooth period was observed to increase to of order 250 msec. Stabilization of the sawteeth for the length of the high power NBI (0.5-0.8 sec) was not observed. The sawtooth characteristics were studied with fast electron temperature (ECE) and soft x-ray diagnostics. Fast, 2 msec interval, measurements were made of the ion temperature evolution following the sawtooth to document the ion heat pulse characteristics. These data show that the ion heat pulse does not exhibit the very fast, ''ballistic'' behavior seen for the electrons. The current profile and other equilibrium profiles were measured on slower time scales. These results are compared to the data from similar studies carried out on TFTR
Gyrokinetic analysis and simulation of pedestals, to identify the culprits for energy losses using fingerprints
Fusion performance in tokamaks hinges critically on the efficacy of the Edge
Transport Barrier (ETB) at suppressing energy losses. The new concept of
fingerprints is introduced to identify the instabilities that cause the
transport losses in the ETB of many of today's experiments, from widely posited
candidates. Analysis of the Gyrokinetic-Maxwell equations, and gyrokinetic
simulations of experiments, find that each mode type produces characteristic
ratios of transport in the various channels: density, heat and impurities.
This, together with experimental observations of transport in some channel, or,
of the relative size of the driving sources of channels, can identify or
determine the dominant modes causing energy transport. In multiple ELMy H-mode
cases that are examined, these fingerprints indicate that MHD-like modes are
apparently not the dominant agent of energy transport; rather, this role is
played by Micro-Tearing Modes (MTM) and Electron Temperature Gradient (ETG)
modes, and in addition, possibly Ion Temperature Gradient (ITG)/Trapped
Electron Modes (ITG/TEM) on JET. MHD-like modes may dominate the electron
particle losses. Fluctuation frequency can also be an important means of
identification, and is often closely related to the transport fingerprint. The
analytical arguments unify and explain previously disparate experimental
observations on multiple devices, including DIII-D, JET and ASDEX-U, and
detailed simulations of two DIII-D ETBs also demonstrate and corroborate this
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