Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session TI2: Turbulence and Thermal Transport |
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Chair: Anne White, Massachusetts Institute of Technology Room: Ballroom BD |
Thursday, November 17, 2011 9:30AM - 10:00AM |
TI2.00001: Finding the Latent Structure in Non-local Electron Heat Transport Event Invited Speaker: Latent structures in electron heat transport in stationary, magnetically confined plasma are unmasked by dynamic transport experiment in LHD. An edge perturbation induced by a trace impurity pellet (TESPEL) injection in LHD evokes a non-local transport phenomenon (large scale transport event, LSTE, such as a core electron temperature rise in response to edge cooling). The LSTEs are not peculiar to helical plasmas nor to plasmas with an impurity pellet injection, but are a very common characteristic. At the onset of the LSTE, a first-order phase transition of the electron heat transport, which is characterized by a discontinuity of the electron temperature gradient, is found to take place over a wide region (at least 6 cm wide) in the periphery of the plasma. At about the same time, over a wide region (about 10 cm wide) in the plasma core, a second-order phase transition of the electron heat transport, which is characterized by a discontinuity of the time derivative of the electron temperature gradient, appears. Both transitions involve coherent structures of a scale much larger than a typical micro-turbulent eddy size (a few mm in this case). In order to evaluate how the local heat transport properties change during an LSTE in LHD, a new transit time distribution analysis is applied to the temporal behavior of the electron temperature gradient. The analysis results show that two large-scale coherent structures in the electron heat transport exist, and are qualitatively different from each other. Recently, we found a long distance correlation of electron temperature fluctuation of the order of 30 eV, with a size corresponding to the plasma minor radius. Therefore the non-local transport phenomenon observed in LHD is evoked by the interaction of those structures via a long distance radial correlation of electron temperature fluctuations. [Preview Abstract] |
Thursday, November 17, 2011 10:00AM - 10:30AM |
TI2.00002: Experimental Study of Parametric Dependence of Electron-gyro Scale Turbulence on NSTX Invited Speaker: Electron-gyro scale turbulence, e.g. driven by Electron Temperature Gradient (ETG), has been proposed as a potential candidate for driving anomalous electron thermal transport in toroidal confinement devices. However, experimental studies of ETG turbulence are still in the early stages. In order to characterize electron-gyro scale turbulence and clarify its role in transport, experiments were carried out to study its parametric dependence on the National Spherical Torus eXperiment (NSTX), using a unique microwave scattering diagnostic which measures the radial wavenumber spectrum with high radial localization. Here, we present the first direct experimental demonstration of density gradient stabilization of electron-gyro scale turbulence. The experimental observation is in quantitative agreement with linear numerical simulations and supports the conclusion that the observed density fluctuations are driven by ETG. Furthermore, it is observed that longer wavelength modes, with normalized perpendicular wavenumber less than 10 (normalized using ion gyro-radius with electron temperature), are most stabilized by density gradient, and the stabilization is accompanied by about a factor of 2 decrease in the plasma effective thermal diffusivity, suggesting ETG turbulence may play a role in driving anomalous transport. Motivated by the observed strong inverse dependence of NSTX confinement time on electron collisionality, a study of the collisionality dependence of the electron-gyro scale turbulence was also carried out. The measured wavenumber spectral power was found to decrease as collisionality increased by more than a factor of two, with electron gyroradius, electron beta and $q_ {95}$ kept approximately constant. This result suggests that ETG may not be the only mechanism driving anomalous transport. Comparisons with non-linear gyrokinetic simulations will also be presented. [Preview Abstract] |
Thursday, November 17, 2011 10:30AM - 11:00AM |
TI2.00003: Suppressing Electron Turbulence and Triggering Internal Transport Barriers with Reversed Magnetic Shear in the National Spherical Torus Experiment Invited Speaker: Observations in the National Spherical Torus Experiment (NSTX)\footnote{M. Ono et al., Nucl. Fusion {\bf 40}, 557 (2000).} have found electron temperature gradients that greatly exceed the linear threshold for the onset for electron temperature gradient-driven (ETG) turbulence. These discharges, deemed electron internal transport barriers (e-ITBs), coincide with a reversal in the shear of the magnetic field and with a reduction in electron-scale density fluctuations, qualitatively consistent with earlier gyrokinetic predictions.\footnote{H. Y. Yuh et al., Phys. Rev. Lett. {\bf 106}, 055003 (2011); F. Jenko and W. Dorland, Phys. Rev. Lett {\bf 89}, 225001 (2002)} To investigate this phenomenon further, we numerically model electron turbulence in NSTX reversed-shear plasmas using the gyrokinetic turbulence code GYRO.\footnote{J. Candy and R. E. Waltz, J. Comput. Phys. {\bf 186}, 545 (2003).} These first-of-a-kind nonlinear gyrokinetic simulations of NSTX e-ITBs confirm that reversing the magnetic shear can allow the plasma to reach electron temperature gradients well beyond the critical gradient for the linear onset of instability. This effect is very strong, with the nonlinear threshold for significant transport approaching three times the linear critical gradient in some cases, in contrast with moderate shear cases, which can drive significant ETG turbulence at much lower gradients. In addition to the experimental implications of this upshifted nonlinear critical gradient, we explore the behavior of ETG turbulence during reversed shear discharges. This work is supported by the SciDAC Center for the Study of Plasma Microturbulence, DOE Contract DE-AC02-09CH11466, and used the resources of NCCS at ORNL and NERSC at LBNL. [Preview Abstract] |
Thursday, November 17, 2011 11:00AM - 11:30AM |
TI2.00004: Production of Internal Transport Barriers via self-generated flows in Alcator C-Mod Invited Speaker: New results suggest that changes observed in the intrinsic toroidal rotation influence ITB formation in Alcator C-Mod that arise when the resonance for ICRF minority heating is positioned off-axis at or outside of the plasma half-radius. These ITBs form in a reactor relevant regime, without particle or momentum injection, with Ti$\approx $Te, and with monotonic q profiles (q$_{min}<$ 1). C-Mod H-mode plasmas exhibit strong intrinsic co-current rotation that increases with increasing stored energy without external drive. With the resonance position off-axis, the rotation decreases in the center of the plasma resulting in a radial rotation profile with a central well which deepens and moves farther off-axis when the ICRF resonance is at the plasma half-radius. This profile results in strong ExB shear ($>$1.5x10$^{5}$ Rad/sec) in the region where the ITB foot is observed. The self generated ExB shearing increases rapidly after the H-mode transition in off-axis ICRF heated discharges, before other profile changes are observed. Gyrokinetic analyses indicate that this spontaneous shearing rate is comparable to the linear ITG growth rate at the ITB location and may be responsible for stabilizing the underlying turbulence. Detailed measurement of the ion temperature demonstrates that the radial profile also flattens as the ICRF resonance position moves off axis. This decreases R/L$_{Ti}$ in the barrier region, lessening the drive for the ITG turbulence and the resulting particle transport. The reduction in particle transport resulting from increase in core stability allows the neoclassical pinch to peak the density and pressure on axis. This suggests that spontaneous rotation is a potential tool for plasma profile control in reactor plasmas. The experimental results and corresponding gyrokinetic study will be presented. [Preview Abstract] |
Thursday, November 17, 2011 11:30AM - 12:00PM |
TI2.00005: Gyrokinetic predictions of tearing mode turbulence in standard tokamaks Invited Speaker: For several years, it has been assumed that microtearing turbulence can destroy rational magnetic surfaces and contribute to the turbulent heat transport in spherical tokamaks. Recently, however, microtearing modes have also been found in various linear gyrokinetic studies for standard tokamaks, in contrast to conventional wisdom. In the present work, microtearing turbulence in such devices is studied for the first time by means of nonlinear gyrokinetic simulations, using the GENE code. The relevant case of weakly to moderately collisional plasmas in model geometries as well as real tokamak geometries is investigated, with a particular emphasis on the character of the nonlinear saturation process which determines the saturation levels. The resulting heat transport is dominated by the electron magnetic component, and the transport levels are found to be comparable with typical experimental fluxes. Microtearing modes are thus a candidate for explaining turbulent transport in standard tokamaks. In this context, pioneering studies of the nonlinear interplay between microtearing modes and other microinstabilities (like ITG modes) are presented, corresponding to experimentally relevant situations. Moreover, it is shown that field-line stochastization can also be caused by the nonlinear excitation (by ITG or trapped electron modes) of linearly stable modes with tearing parity, yielding significant magnetic transport levels for high performance discharges. The expected relevance of both types of tearing modes for ITER is discussed. [Preview Abstract] |
Thursday, November 17, 2011 12:00PM - 12:30PM |
TI2.00006: Simulation of microtearing turbulence in NTSX and scaling with collisionality Invited Speaker: Thermal energy confinement times in NSTX dimensionless parameter scans increase with decreasing collisionality, B$_{T}\tau _{E}\sim \nu _{\ast }^{-0.95}$ [1]. While ion thermal transport is neoclassical, the source of anomalous electron thermal transport in these discharges remains unclear, leading to considerable uncertainty when extrapolating to future ST devices at much lower collisionality. Linear gyrokinetic simulations find microtearing modes to be unstable in high collisionality discharges. First non-linear gyrokinetic simulations of microtearing turbulence in NSTX have recently been reported [2], showing from first principles they can yield experimental levels of transport. Magnetic flutter is responsible for almost all the transport ($\sim $98{\%}), perturbed field line trajectories are globally stochastic, and a test particle stochastic transport model [3] agrees to within 25{\%} of the simulated transport. Most significantly, microtearing transport is predicted to increase with electron collisionality, consistent with the observed NSTX confinement scaling. While this suggests microtearing modes may be the source of electron thermal transport, the predictions are also very sensitive to electron temperature gradient, indicating the scaling of the instability threshold is important. In addition, microtearing turbulence is susceptible to suppression via sheared E$\times $B flows, as experimental values of E$\times $B shear (comparable to the linear growth rates) dramatically reduce the transport below experimental values. This work is supported by US DOE contract DE-AC02-09CH11466.\\[4pt] [1] S.M. Kaye et al., Nucl. Fusion \textbf{47}, 499 (2007).\\[0pt] [2] W. Guttenfelder et al., Phys. Rev. Lett. \textbf{106}, 155004 (2011).\\[0pt] [3] W.M. Nevins et al., Phys. Rev. Lett. \textbf{106}, 065003 (2011); E. Wang et al., Phys. Plasmas \textbf{18}, 056111 (2011). [Preview Abstract] |
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