Showing papers by "Patrick Diamond published in 2009"
TL;DR: In this paper, a systematic decomposition of the physical processes which drive intrinsic rotation, a calculation of the critical external torque necessary to hold the plasma stationary against the intrinsic residual stress, a simple model of net velocity scaling which recovers the salient features of the experimental trends and the elucidation of the impact of the particle flux on the net toroidal velocity pinch are offered.
Abstract: Recent results in the theory of turbulent momentum transport and the origins of intrinsic rotation are summarized. Special attention is focused on aspects of momentum transport critical to intrinsic rotation, namely the residual stress and the edge toroidal flow velocity pinch. Novel results include a systematic decomposition of the physical processes which drive intrinsic rotation, a calculation of the critical external torque necessary to hold the plasma stationary against the intrinsic residual stress, a simple model of net velocity scaling which recovers the salient features of the experimental trends and the elucidation of the impact of the particle flux on the net toroidal velocity pinch. Specific suggestions for future experiments are offered.
179 citations
TL;DR: In this article, an adiabatic electron model was used to simulate global electrostatic ITG turbulence physics, together with background dynamics, in a realistic tokamak core geometry using XGC1, a full-function 5D gyrokinetic particle code.
Abstract: Global electrostatic ITG turbulence physics, together with background dynamics, has been simulated in a realistic tokamak core geometry using XGC1, a full-function 5D gyrokinetic particle code. An adiabatic electron model has been used. Some verification exercises of XGC1 have been presented. The simulation volume extends from the magnetic axis to the pedestal top inside the magnetic separatrix. Central heating is applied, and a number, momentum and energy conserving linearized Monte Carlo Coulomb collision is used. In the turbulent region, the ion temperature gradient profile self-organizes globally around R/LT = (Rd logT/dr = major radius on the magnetic axis/temperature gradient length) 6.5–7, which is somewhat above the conventional nonlinear criticality of 6. The self-organized ion temperature gradient profile is approximately stiff against variation of heat source magnitude. Results indicate that the relaxation to a self-organized state proceeds in two phases, namely, a transient phase of excessively bursty transport followed by a 1/f avalanching phase. The bursty types of behaviour are allowed by the quasi-periodic collapse of local E × B shearing barriers.
173 citations
TL;DR: In this article, the authors show that there is ion temperature gradient turbulence across an entire L-mode-like edge density pedestal in a diverted tokamak plasma in which the ion thermal flux is on the order of experimentally inferred values, which strongly suggests that a localized estimate of the ITG-driven χi will not be valid due to the nonlocal dynamics of the compressed turbulence in an Lmode-type density slope.
Abstract: It is found from a heat-flux-driven full-f gyrokinetic particle simulation that there is ion temperature gradient (ITG) turbulence across an entire L-mode-like edge density pedestal in a diverted tokamak plasma in which the ion temperature gradient is mild without a pedestal structure, hence the normalized ion temperature gradient parameter ηi=(d log Ti/dr)/(d log n/dr) varies strongly from high (>4 at density pedestal top/shoulder) to low (<2 in the density slope) values. Variation of density and ηi is in the same scale as the turbulence correlation length, compressing the turbulence in the density slope region. The resulting ion thermal flux is on the order of experimentally inferred values. The present study strongly suggests that a localized estimate of the ITG-driven χi will not be valid due to the nonlocal dynamics of the compressed turbulence in an L-mode-type density slope. While the thermal transport and the temperature profile saturate quickly, the E×B rotation shows a longer time damping during...
111 citations
TL;DR: Gurcan et al. as discussed by the authors derived two physically distinct mechanisms by which microturbulence may drive intrinsic rotation, one emanates from E×B convection of parallel momentum, and the other emerges due to charge separation induced by the polarization drift.
Abstract: Beginning from a phase space conserving gyrokinetic formulation, a systematic derivation of parallel momentum conservation uncovers two physically distinct mechanisms by which microturbulence may drive intrinsic rotation. The first mechanism, which emanates from E×B convection of parallel momentum, has already been analyzed [O. D. Gurcan et al., Phys. Plasmas 14, 042306 (2007); R. R. Dominguez and G. M. Staebler, Phys. Fluids B 5, 3876 (1993)] and was shown to follow from radial electric field shear induced symmetry breaking of the spectrally averaged parallel wave number. Thus, this mechanism is most likely active in regions with steep pressure gradients or strong poloidal flow shear. The second mechanism uncovered, which appears in the gyrokinetic formulation through the parallel nonlinearity, emerges due to charge separation induced by the polarization drift. This novel means of driving intrinsic rotation, while nominally higher order in an expansion of the mode frequency divided by the ion cyclotron f...
53 citations
TL;DR: In this paper, a parametric-modulational interaction between the drift-wave fluctuation (7-8 kHz) and azimuthally symmetric sheared radial electric field structure (∼0.4 kHz) in a cylindrical laboratory plasma is presented.
Abstract: Observation of the parametric-modulational interaction between the drift-wave fluctuation (7–8 kHz) and azimuthally symmetric sheared radial electric field structure (∼0.4 kHz) in a cylindrical laboratory plasma is presented. Oscillation of the sheared radial electric field is synchronized at modulations of the radial wave number and Reynolds stress per mass density of the drift-wave spectrum. Bispectral analysis at the location where the sheared radial electric field has finite radial wave numbers shows that nonlinear energy transfers from the drift wave to the sheared radial electric field occur. Nonlocal energy transfers of fluctuations via “channel of the azimuthally symmetric sheared radial electric field” in spectral space as well as real space are discovered.
51 citations
TL;DR: The residual turbulence found in the presence of strong E x B flow shear may account for neoclassical-level ion heat and anomalous momentum transport widely observed in experiments.
Abstract: A significant inward flux of toroidal momentum is found in global gyrokinetic simulations of ion temperature gradient turbulence, leading to core plasma rotation spin-up. The underlying mechanism is identified to be the generation of residual stress due to the k parallel symmetry breaking induced by global quasistationary zonal flow shear. Simulations also show a significant off-diagonal element associated with the ion temperature gradient in the neoclassical momentum flux, while the overall neoclassical flux is small. In addition, the residual turbulence found in the presence of strong E x B flow shear may account for neoclassical-level ion heat and anomalous momentum transport widely observed in experiments.
49 citations
TL;DR: A systematic derivation of parallel momentum conservation uncovers a novel mechanism by which microturbulence may drive intrinsic rotation due to charge separation induced by the polarization drift.
Abstract: Starting from a phase space conserving gyrokinetic formulation, a systematic derivation of parallel momentum conservation uncovers a novel mechanism by which microturbulence may drive intrinsic rotation. This mechanism, which appears in the gyrokinetic formulation through the parallel nonlinearity, emerges due to charge separation induced by the polarization drift. The derivation and physical discussion of this mechanism will be pursued throughout this Letter.
46 citations
TL;DR: A simple model for the evolution of turbulence fluctuation spectra, which includes neighboring interactions leading to the usual dual cascade as well as disparate scale interactions corresponding to refraction by large scale structures, is derived and recovers the usual Kraichnan-Kolmogorov picture.
Abstract: A simple model for the evolution of turbulence fluctuation spectra, which includes neighboring interactions leading to the usual dual cascade as well as disparate scale interactions corresponding to refraction by large scale structures, is derived. The model recovers the usual Kraichnan-Kolmogorov picture in the case of exclusively local interactions and midrange drive. On the other hand, when disparate scale interactions are dominant, a simple spectrum for the density fluctuations of the form |nk|2 proportional to k(-3)/(1+k2)2 is obtained. This simple prediction is then compared to, and found to be in fair agreement with, Tore Supra CO2 laser scattering data.
45 citations
TL;DR: In this article, an acoustic instability of a shock precursor driven by the pressure gradient of accelerated particles is studied in the nonlinear regime, and the instability develops out of cyclotron unstable seed magnetohydrodynamic (MHD) waves.
Abstract: An acoustic instability of a shock precursor driven by the pressure gradient of accelerated particles is studied in the nonlinear regime. The nonlinearity steepens unstable acoustic waves and turns them into shocks. The shocks form a "shocktrain" but they may merge into each other. Traveling wave solutions are obtained analytically in two different cases. In the first case, only acoustic instability is included and the characteristic scale (distance between the shocks) is limited only by the system size (while shocks merge). In the second case, the instability develops out of cyclotron unstable seed magnetohydrodynamic (MHD) waves. The spatial distance between the MHD wave packets sets the scale of the acoustic shocktrain. The internal structure of the individual shocks is presumably determined by the ion skin depth c/ω pi and by the relaxation length of slightly superthermal particle distributions near these shocks. The shocks are assumed to be arbitrarily thin compared with the distance between them which is ensured by a small viscous term. Both types of solutions are dynamically verified by numerical calculations. The hydromagnetic flow in the shock precursor emerging from the acoustic instability is crucial for two recently suggested phenomena in diffusive shock acceleration. One phenomenon is the enhancement of the acceleration rate well above its standard (Bohm) value due to the narrowing of the shock precursor. The second phenomenon is the amplification of the long-scale magnetic field by an inverse cascade of Alfven waves generated by accelerated particles and scattered in k-space on the acoustic perturbations.
36 citations
TL;DR: In this paper, a simple one-field L-H transition model is studied in detail, analytically and numerically, and it is shown that by shaping the heating rate temporal profile, one can reduce the average power required for the transition to the QH-mode.
Abstract: A simple one-field L-H transition model is studied in detail, analytically and numerically. The dynamical system consists of three equations coupling the drift wave turbulence level, zonal flow speed, and the pressure gradient. The fourth component, i.e., the mean shear velocity, is slaved to the pressure gradient. Bursting behavior, characteristic for predator-prey models of the drift wave - zonal flow interaction, is recovered near the transition to the quiescent H-mode (QH) and occurs as strongly nonlinear relaxation oscillations. The latter, in turn, arise as a result of Hopf bifurcation (limit cycle) of an intermediate fixed point (between the L- and H-modes). The system is shown to remain at the QH-mode fixed point even after the heating rate is decreased below the bifurcation point (i.e., hysteresis, subcritical bifurcation), but the basin of attraction of the QH-mode shrinks rapidly with decreasing power. This suggests that the hysteresis in the H-L transition may be less than that expected from S-curve models. Nevertheless, it is demonstrated that by shaping the heating rate temporal profile, one can reduce the average power required for the transition to the QH-mode.
33 citations
Courant Institute of Mathematical Sciences1, University of California, San Diego2, Columbia University3, Oak Ridge National Laboratory4, University of Colorado Boulder5, California Institute of Technology6, Princeton Plasma Physics Laboratory7, University of California, Irvine8, Georgia Institute of Technology9, Lawrence Berkeley National Laboratory10, Rutgers University11
TL;DR: In this article, a multi-scale turbulence-neoclassical simulation of integrated edge-core plasma in a realistic diverted geometry is presented, where the ion-temperature-gradient driven turbulence in realistic diverted tokamak geometry for the first time is found.
Abstract: Performance prediction for ITER is based upon the ubiquitous experimental observation that the plasma energy confinement in the device core is strongly coupled to the edge confinement for an unknown reason. The coupling time-scale is much shorter than the plasma transport time-scale. In order to understand this critical observation, a multi-scale turbulence-neoclassical simulation of integrated edge-core plasma in a realistic diverted geometry is a necessity, but has been a formidable task. Thanks to the recent development in high performance computing, we have succeeded in the integrated multiscale gyrokinetic simulation of the ion-temperature-gradient driven turbulence in realistic diverted tokamak geometry for the first time. It is found that modification of the self-organized criticality in the core plasma by nonlocal core-edge coupling of ITG turbulence can be responsible for the core-edge confinement coupling.
TL;DR: In this paper, an electromagnetic gyrokinetic formulation was used to calculate the turbulent radial flux of parallel momentum for a strongly magnetized plasma in the large aspect ratio limit, and the level of momentum transport induced by microturbulence was found to be well described within the electrostatic approximation.
Abstract: An electromagnetic gyrokinetic formulation is utilized to calculate the turbulent radial flux of parallel momentum for a strongly magnetized plasma in the large aspect ratio limit. For low-β plasmas, excluding regions of steep density gradients, the level of momentum transport induced by microturbulence is found to be well described within the electrostatic approximation. However, near regions of steep equilibrium profile gradients, strong electromagnetic contributions to the momentum flux are predicted. In particular, for sufficiently steep density gradient, the magnitude of transport induced by the off-diagonal residual stress component of the momentum flux induced by drift wave turbulence can be quenched. This quenching mechanism, which results from shielding of the parallel electric field by the inductive term, is distinct from E×B shear decorrelation, since it allows for the level of off-diagonal turbulent transport to be strongly reduced without extinguishing the underlying microturbulence. In contr...
TL;DR: In this paper, nonlinear turbulent-shear flow interactions are directly measured in a cylindrical helicon plasma device and found to lead to the development of an azimuthally symmetric radially sheared flow by the action of a turbulent Reynolds stress which transfers kinetic energy into the large-scale shear flow.
Abstract: Nonlinear turbulent-shear flow interactions are directly measured in a cylindrical helicon plasma device and found to lead to the development of an azimuthally symmetric radially sheared azimuthal flow by the action of a turbulent Reynolds stress which transfers kinetic energy into the large-scale shear flow. Radially resolved measurements of the nonlinear kinetic energy transfer show that the flow is driven at the plasma boundary; temporally resolved flow measurements show a penetration of the resulting azimuthal flow into the central plasma region. The results provide an initial qualitatively test of theoretical predictions, and suggest how similar studies might be carried out in confinement devices.
TL;DR: Hahm et al. as discussed by the authors showed that the comment by Peeters et al contains an incorrect and misleading interpretation of our paper regarding the density gradient dependence of momentum pinch and the turbulent equipartition theory.
Abstract: This response demonstrates that the comment by Peeters et al. contains an incorrect and misleading interpretation of our paper [T. S. Hahm et al., Phys. Plasmas 15, 055902 (2008)] regarding the density gradient dependence of momentum pinch and the turbulent equipartition theory.
01 Dec 2009
TL;DR: Magnetic Relaxation and Self-Organization in Astrophysical and Laboratory Plasmas: Mean Field Dynamo Theory Origin, Structure and Stability ofAstrophysical MHD Jets Turbulence and Turbulent Transport - The Agents of Relaxation.
Abstract: Magnetic Relaxation and Self-Organization in Astrophysical and Laboratory Plasmas: Mean Field Dynamo Theory Origin, Structure and Stability of Astrophysical MHD Jets Turbulence and Turbulent Transport - The Agents of Relaxation and Structure Formation: A Tutorial on Basic Concepts in MHD Turbulence and Turbulent Transport Intermittency Like Phenomena in Plasma Turbulence Nonlinear Cascades and Spatial Structure of MHD Turbulence Scale Covariance and Scale-Ratio Covariance in Turbulent Front Propagation Transport Bifurcations and Relaxation: Transport Barrier Relaxations in Tokamak Edge Plasmas Dynamics of Edge Localized Modes On the Onset of Collapse Events in Toroidal Plasmas - Turbulence Trigger.
01 Jan 2009
TL;DR: Yoshihiko NAGASHIMA, Sanae-I.ITOH, Kimitaka ITOH, Akihide FUJISAWA, Shigeru INAGAKI, Yoshinobu KAWAI, Shunjiro SHINOHARA, Masayuki FUKAO, Takuma YAMADA, Kenichiro TERASAKA, Takashi MARUTA, Kunihiro KAMATAK, Hiroyuki ARAKAWA and Yuichi Takase The University of Tokyo, Kashiwa, Ch
Abstract: Yoshihiko NAGASHIMA, Sanae-I. ITOH, Kimitaka ITOH, Akihide FUJISAWA, Shigeru INAGAKI, Yoshinobu KAWAI, Shunjiro SHINOHARA, Masayuki FUKAO, Takuma YAMADA, Kenichiro TERASAKA, Takashi MARUTA, Kunihiro KAMATAK, Hiroyuki ARAKAWA, Masatoshi YAGI, Naohiro KASUYA, George R. TYNAN, Patrick H. DIAMOND, and Yuichi Takase The University of Tokyo, Kashiwa, Chiba 277-8561, Japan Kyushu University, Kasuga, Fukuoka 816-8580, Japan National Institute for Fusion Science, Toki, Gifu 509-5292, Japan Myojocho, Uji, Kyoto 611-0014, Japan University of California San Diego, La Jolla, CA 92093, USA
23 Feb 2009
TL;DR: A brief survey of concepts and themes in confinement physics can be found in this article, which summarizes past history and the current state of our conceptual picture of tokamak transport and turbulence.
Abstract: This brief paper surveys concepts and themes in confinement physics It summarizes past history and the current state of our conceptual picture of tokamak transport and turbulence It emphasizes ideas, not details The article concludes with a look ahead to possible future research topics in confinement physics