Showing papers by "Patrick Diamond published in 2010"
TL;DR: A systematic, constructive and self-consistent procedure to quantify nonlocal, nondiffusive action at a distance in plasma turbulence is exposed and applied to turbulent heat fluxes computed from the state-of-the-art full- f, flux-driven gyrokinetic GYSELA and XGC1 codes.
Abstract: A systematic, constructive and self-consistent procedure to quantify nonlocal, nondiffusive action at a distance in plasma turbulence is exposed and applied to turbulent heat fluxes computed from the state-of-the-art full- f, flux-driven gyrokinetic GYSELA and XGC1 codes. A striking commonality is found: heat transport below a dynamically selected mesoscale has the structure of a Levy distribution, is strongly nonlocal, nondiffusive, scale-free, and avalanche mediated; at larger scales, we report the observation of a self-organized flow structure which we call the " E × B staircase" after its planetary analog.
163 citations
TL;DR: In this paper, a novel mechanism for driving residual stress in tokamak plasmas based on k∥ symmetry breaking by the turbulence intensity gradient is proposed, and its connection to the wave kinetic equation and the wave-momentum flux is described.
Abstract: A novel mechanism for driving residual stress in tokamak plasmas based on k∥ symmetry breaking by the turbulence intensity gradient is proposed. The physics of this mechanism is explained and its connection to the wave kinetic equation and the wave-momentum flux is described. Applications to the H-mode pedestal in particular to internal transport barriers, are discussed. Also, the effect of heat transport on the momentum flux is discussed.
104 citations
TL;DR: In this paper, a nonlinear residual stress generation by both fluctuation intensity and the intensity gradient in the presence of broken symmetry in the parallel wavenumber spectrum is identified for the first time.
Abstract: Global gyrokinetic simulations have revealed an important nonlinear flow generation process due to the residual stress produced by electrostatic turbulence of ion temperature gradient (ITG) modes and trapped electron modes (TEMs). In collisionless TEM (CTEM) turbulence, nonlinear residual stress generation by both the fluctuation intensity and the intensity gradient in the presence of broken symmetry in the parallel wavenumber spectrum is identified for the first time. Concerning the origin of the symmetry breaking, turbulence self-generated low frequency zonal flow shear has been identified to be a key, universal mechanism in various turbulence regimes. Simulations reported here also indicate the existence of other mechanisms beyond E×B shear. The ITG turbulence driven “intrinsic” torque associated with residual stress is shown to increase close to linearly with the ion temperature gradient, in qualitative agreement with experimental observations in various devices. In CTEM dominated regimes, a net toroi...
87 citations
TL;DR: Ono et al. as discussed by the authors used nonresonant magnetic fields for spinning the plasma and found that the torque from these fields can be enhanced at low rotation, which assists in spinning the plasmas from rest, and offers increased resistance against plasma slowing.
Abstract: Recent experiments on DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] and National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] have focused on investigating mechanisms of driving rotation in fusion plasmas. The so-called intrinsic rotation is generated by an effective torque, driven by residual stresses in the plasma, which appears to originate in the plasma edge. A clear scaling of this intrinsic drive with the H-mode pressure gradient is observed. Coupled with the experimentally inferred pinch of angular momentum, such an edge source is capable of producing sheared rotation profiles. Intrinsic drive is also possible directly in the core, although the physics mechanisms are much more complex. Another option which is being explored is the use of nonresonant magnetic fields for spinning the plasma. It is found beneficially that the torque from these fields can be enhanced at low rotation, which assists in spinning the plasma from rest, and offers increased resistance against plasma slowing.
76 citations
TL;DR: In this article, the authors proposed a mechanism for producing a weak and narrow cosmic ray (CR) beam which operates en route to the observer, where the strongest pitch-angle scattering occurs for particles moving almost precisely along the field line.
Abstract: Both the acceleration of cosmic rays (CRs) in supernova remnant shocks and their subsequent propagation through the random magnetic field of the Galaxy are deemed to result in an almost isotropic CR spectrum. However, the MILAGRO TeV observatory discovered sharp (~10°) arrival anisotropies of CR nuclei. We suggest a mechanism for producing a weak and narrow CR beam which operates en route to the observer. The key assumption is that CRs are scattered by a strongly anisotropic Alfven wave spectrum formed by the turbulent cascade across the local field direction. The strongest pitch-angle scattering occurs for particles moving almost precisely along the field line. Partly because this direction is also the direction of the minimum of the large-scale CR angular distribution, the enhanced scattering results in a weak but narrow particle excess. The width, the fractional excess, and the maximum momentum of the beam are calculated from a systematic transport theory depending on a single scale l which can be associated with the longest Alfven wave, which efficiently scatters the beam. The best match to all three characteristics of the beam is achieved at l ~ 1 pc. The distance to a possible source of the beam is estimated to be within a few 100 pc. Possible approaches to the determination of the scale l from the characteristics of the source are discussed. Alternative scenarios of drawing the beam from the galactic CR background are considered. The beam-related large-scale anisotropic CR component is found to be energy independent, which is also consistent with the observations.
56 citations
TL;DR: In this article, the authors proposed a mechanism for producing a weak and narrow cosmic ray (CR) beam which operates en route to the observer by using a strongly anisotropic Alfven wave spectrum formed by the turbulent cascade across the local field direction.
Abstract: Both the acceleration of cosmic rays (CR) in supernova remnant shocks and their subsequent propagation through the random magnetic field of the Galaxy deem to result in an almost isotropic CR spectrum. Yet the MILAGRO TeV observatory discovered a sharp ($\sim10^{\circ})$ arrival anisotropy of CR nuclei. We suggest a mechanism for producing a weak and narrow CR beam which operates en route to the observer. The key assumption is that CRs are scattered by a strongly anisotropic Alfven wave spectrum formed by the turbulent cascade across the local field direction. The strongest pitch-angle scattering occurs for particles moving almost precisely along the field line. Partly because this direction is also the direction of minimum of the large scale CR angular distribution, the enhanced scattering results in a weak but narrow particle excess. The width, the fractional excess and the maximum momentum of the beam are calculated from a systematic transport theory depending on a single scale $l$ which can be associated with the longest Alfven wave, efficiently scattering the beam. The best match to all the three characteristics of the beam is achieved at $l\sim1$pc. The distance to a possible source of the beam is estimated to be within a few 100pc. Possible approaches to determination of the scale $l$ from the characteristics of the source are discussed. Alternative scenarios of drawing the beam from the galactic CR background are considered. The beam related large scale anisotropic CR component is found to be energy independent which is also consistent with the observations.
56 citations
TL;DR: In this paper, a theory of the efficiency of the plasma flow generation process is presented, where the efficiency is defined as the ratio of the entropy destruction rate due to flow generation to the entropy production rates due to ∇T relaxation (i.e., related to turbulent heat flux).
Abstract: A theory of the efficiency of the plasma flow generation process is presented. A measure of the efficiency of plasma self-acceleration of mesoscale and mean flows from the heat flux is introduced by analogy with engines, using the entropy budget defined by thermal relaxation and flow generation. The efficiency is defined as the ratio of the entropy destruction rate due to flow generation to the entropy production rate due to ∇T relaxation (i.e., related to turbulent heat flux). The efficiencies for two different cases, i.e., for the generation of turbulent driven E×B shear flow (zonal flow) and for toroidal intrinsic rotation, are considered for a stationary state, achieved by balancing entropy production rate and destruction rate order by order in O(k∥/k⊥), where k is the wave number. The efficiency of intrinsic toroidal rotation is derived and shown to be eIR∼(Mach)th2∼0.01. The scaling of the efficiency of intrinsic rotation generation is also derived and shown to be ρ∗2(q2/s2)(R2/LT2)=ρ∗2(Ls2/LT2), w...
45 citations
Journal Article•
TL;DR: In this article, a theory of the efficiency of the plasma flow generation process is presented, where the efficiency is defined as the ratio of the entropy destruction rate due to flow generation to the entropy production rates due to ∇T relaxation (i.e., related to turbulent heat flux).
Abstract: A theory of the efficiency of the plasma flow generation process is presented. A measure of the efficiency of plasma self-acceleration of mesoscale and mean flows from the heat flux is introduced by analogy with engines, using the entropy budget defined by thermal relaxation and flow generation. The efficiency is defined as the ratio of the entropy destruction rate due to flow generation to the entropy production rate due to ∇T relaxation (i.e., related to turbulent heat flux). The efficiencies for two different cases, i.e., for the generation of turbulent driven E×B shear flow (zonal flow) and for toroidal intrinsic rotation, are considered for a stationary state, achieved by balancing entropy production rate and destruction rate order by order in O(k∥/k⊥), where k is the wave number. The efficiency of intrinsic toroidal rotation is derived and shown to be eIR∼(Mach)th2∼0.01. The scaling of the efficiency of intrinsic rotation generation is also derived and shown to be ρ∗2(q2/s2)(R2/LT2)=ρ∗2(Ls2/LT2), w...
43 citations
TL;DR: A no-slip condition, imposed by ion-neutral flow damping outside the annular region, combined with a diffusive stress arising from turbulent and collisional viscous damping in the central plasma region, leads to net plasma rotation in the absence of momentum input.
Abstract: An azimuthally symmetric radially sheared azimuthal flow is driven by a nondiffusive, or residual, turbulent stress localized to a narrow annular region at the boundary of a cylindrical magnetized helicon plasma device A no-slip condition, imposed by ion-neutral flow damping outside the annular region, combined with a diffusive stress arising from turbulent and collisional viscous damping in the central plasma region, leads to net plasma rotation in the absence of momentum input
40 citations
TL;DR: In this article, the effect of the geodesic acoustic mode (GAM) on turbulence and its role in the L-H transition has been investigated using wave-kinetic modulational analysis.
Abstract: A theory of the effect of the geodesic acoustic mode (GAM) on turbulence is presented. Two synergistic issues are elucidated: namely, the physics of the zonal flow modulation and its role in the L-H transition, and the role of the GAM wave group propagation in turbulence spreading. Using a wavekinetic modulational analysis, the response of the turbulence intensity field to the GAM is calculated. This analysis differs from previous studies of zero-frequency zonal flows since it accounts for resonance between the drift wave group speed and the GAM strain field, which induces secularity. This mechanism is referred to as secular stochastic shearing. Finite real frequency and radial group velocity are intrinsic to the GAM, so its propagation can induce nonlocal phenomena at the edge and pedestal regions. To understand the effect of the GAM on turbulence and transition dynamics, a predator-prey model incorporating the dynamics of both turbulence and the GAMs is constructed and analyzed for stability around fixed points. Three possible states are identified, namely, an L-modelike stationary state, a reduced turbulence state, and a GAM limit-cycle state. The system is attracted to the state with the minimum turbulence level.
32 citations
TL;DR: In this paper, a simple unified model of intrinsic rotation and momentum transport in high-constraint regime (H-mode) tokamak plasmas is presented, which combines E×B shear-driven residual stress in the pedestal with a turbulent equipartition pinch to yield rotation profiles.
Abstract: A simple unified model of intrinsic rotation and momentum transport in high confinement regime (H-mode) tokamak plasmas is presented. Motivated by the common dynamics of the onset of intrinsic rotation and the L-H transition, this simple model combines E×B shear-driven residual stress in the pedestal with a turbulent equipartition pinch to yield rotation profiles. The residual stress is the primary mechanism for buildup of intrinsic rotation in the H-mode pedestal, while the pinch drives on-axis peaking of rotation profiles. Analytical estimates for pedestal flow velocities are given in terms of the pedestal width, the pedestal height, and various model parameters. The predicted scaling of the toroidal flow speed with pedestal width is found to be consistent with the International Tokamak Physics Activity database global scaling of the flow speed on-axis with the total plasma stored energy.
TL;DR: In this paper, a kinetic generalization of a Taylor identity appropriate to strongly magnetized plasma is derived, which provides an explicit link between the radial mixing of a four-dimensional (4D) gyrocenter fluid and the poloidal Reynolds stress.
Abstract: A kinetic generalization of a Taylor identity appropriate to a strongly magnetized plasma is derived. This relation provides an explicit link between the radial mixing of a four–dimensional (4D) gyrocenter fluid and the poloidal Reynolds stress. This kinetic analog of a Taylor identity is subsequently utilized to link the turbulent transport of poloidal momentum to the mixing of potential vorticity. A quasilinear calculation of the flux of potential vorticity is carried out, yielding diffusive, turbulent equipartition, and thermoelectric convective components. Self-consistency is enforced via the quasineutrality relation, revealing that for the case of a stationary small amplitude wave population, deviations from neoclassical predictions of poloidal rotation can be closely linked to the growth/damping profiles of the underlying drift wave microturbulence.
TL;DR: In this paper, the authors show that while the CR pressure builds up ahead of the supernova, the acceleration enters into a strongly nonlinear phase in which an acoustic instability, driven by the CR-pressure gradient, dominates other instabilities (at least in the case of low β plasma).
Abstract: Strong astrophysical shocks, diffusively accelerating cosmic rays (CRs) ought to develop CR precursors. The length of such precursor Lp is believed to be set by the ratio of the CR mean free path λ to the shock speed, i.e. Lp ~ cλ/Vsh ~ crg/Vsh, which is formally independent of the CR pressure Pc. However, the x-ray observations of supernova remnant shocks suggest that the precursor scale may be significantly shorter than Lp which would question the above estimate unless the magnetic field is strongly amplified and the gyroradius rg is strongly reduced over a short (unresolved) spatial scale. We argue that while the CR pressure builds up ahead of the shock, the acceleration enters into a strongly nonlinear phase in which an acoustic instability, driven by the CR-pressure gradient, dominates other instabilities (at least in the case of low β plasma). In this regime the precursor steepens into a strongly nonlinear front whose size scales with the CR pressure as Lf ~ Lp (Ls/Lp)2(Pc/Pg)2, where Ls is the scale of the developed acoustic turbulence and Pc/Pg is the ratio of CR to gas pressure. Since Ls Lp, the precursor scale reduction may be strong in the case of even a moderate gas heating by the CRs through the acoustic and (possibly also) the other instabilities driven by the CRs.
TL;DR: An extensive study of spectral shell models with possibilities for application to fusion plasmas is discussed in this article, where a set of shell models addressing various aspects of the characteristics of fusion plasma have been derived.
Abstract: An extensive study of spectral shell models with possibilities for application to fusion plasmas is discussed. A set of shell models addressing various aspects of the characteristics of fusion plasmas have been derived. Difficulties associated with plasma medium, namely its intrinsic excitability, and importance of mescals have been discussed. The numerical implementation of shell models is discussed. It was observed that depending on the parameter regime, they may lead to steady state or display characteristics of predator–prey dynamics.
24 May 2010
TL;DR: In this article, experimental data from the Tore Supra experiments are extrapolated in the SOL and edge to investigate the Kelvin-Helmholtz instability, and linear analysis indicates that a large part of the SOL is rather unstable.
Abstract: Experimental data from the Tore Supra experiments are extrapolated in the SOL and edge to investigate the Kelvin–Helmholtz instability. The linear analysis indicates that a large part of the SOL is rather unstable. The effort is part of the setup of the Mistral base case that is organised to validate the codes and address new issues on turbulent edges, including the comparison of kinetic and fluid modelling in the edge plasma.
TL;DR: In this article, the authors investigated toroidal temperature gradient mode (ITG) affected by static magnetic field of magnetic islands by means of Landau fluid model and found that the strength of coupling is characterized by the square of island width.
Abstract: Ion temperature gradient mode (ITG) affected by static magnetic field of magnetic islands is investigated numerically by means of Landau fluid model. The ITG is localized around O-points of magnetic islands, and the localization in poloidal direction is similar to the poloidal localization of toroidal ITG. This is because the helical magnetic field of magnetic islands causes geometrical coupling, and thus Fourier modes that have the same helicity as the islands are coupled together. The strength of coupling is characterized by the square of island width, and it corresponds to the fact that the strength of mode coupling of toroidal ITG is characterized by the inverse aspect ratio of torus in reduced fluid models.
TL;DR: In this paper, the role of zonal flows in relaxation and transport in a reduced model of collisionless ITG turbulence is studied, and a fundamentally new constituent in the relaxation dynamics is revealed, namely that ion and electron guiding center motion togather necessitate a radial flux of polarization charge, which in turn exerts a dynamical friction on phase space density evolution.
Abstract: We present a study of the role of zonal flows in relaxation and transport in a reduced model of collisionless ITG turbulence. A fundamentally new constituent in the relaxation dynamics is revealed, namely that ion and electron guiding center motion togather necessitate a radial flux of polarization charge, which in turn exerts a dynamical friction on phase space density evolution. This effect then enters the evolution of 〈δ f 2〉 and the transport dynamics, as described by a Lenard-Balescu type equation. The underlying physics is similar to that which follows from conservation of potential vorticity, albeit now for a phase space fluid, and is not simple shearing or wave packet modulation. Consequences for zonal flow momentum balance are discussed.
TL;DR: In this paper, it was shown that while the CR pressure builds up ahead of the supernova, the acceleration enters into a strongly nonlinear phase in which an acoustic instability, driven by the pressure gradient, dominates other instabilities (at least in the case of low $\beta$ plasma).
Abstract: Strong astrophysical shocks, diffusively accelerating cosmic rays (CR) ought to develop CR precursors. The length of such precursor $L_{p}$ is believed to be set by the ratio of the CR mean free path $\lambda$ to the shock speed, i.e., $L_{p}\sim c\lambda/V_{sh}\sim cr_{g}/V_{sh}$, which is formally independent of the CR pressure $P_{c}$. However, the X-ray observations of supernova remnant shocks suggest that the precursor scale may be significantly shorter than $L_{p}$ which would question the above estimate unless the magnetic field is strongly amplified and the gyroradius $r_{g}$ is strongly reduced over a short (unresolved) spatial scale. We argue that while the CR pressure builds up ahead of the shock, the acceleration enters into a strongly nonlinear phase in which an acoustic instability, driven by the CR pressure gradient, dominates other instabilities (at least in the case of low $\beta$ plasma). In this regime the precursor steepens into a strongly nonlinear front whose size scales with \emph{the CR pressure}as $L_{f}\sim L_{p}\cdot(L_{s}/L_{p})^{2}(P_{c}/P_{g})^{2}$, where $L_{s}$ is the scale of the developed acoustic turbulence, and $P_{c}/P_{g}$ is the ratio of CR to gas pressure. Since $L_{s}\ll L_{p}$, the precursor scale reduction may be strong in the case of even a moderate gas heating by the CRs through the acoustic and (possibly also) the other instabilities driven by the CRs.
Book•
01 Jan 2010TL;DR: In this paper, it was shown that the intrinsic nature of turbulent heat transport in tokamaks is nonlocal and that the global structure of turbulence and transport results from a synergy between edge-driven inward propagation of turbulence intensity with outward heat transport.
Abstract: The understanding of plasma turbulent transport in tokamaks used to rely on a local process, in the sense that locally excited fluctuations exhibit short radial correlation lengths only, ultimately leading to diffusive transport. We find here that the intrinsic nature of turbulent heat transport in tokamaks is nonlocal. This nonlocality is thoroughly defined and quantified. In the same vein, it is also found that the global structure of turbulence and transport results from a synergy between edge-driven inward propagation of turbulence intensity with outward heat transport. This synergy results in inward-outward pulse scattering leading to spontaneous production of strong internal shear layers in which the turbulent transport is almost suppressed over several radial correlation lengths. These two examples represent different sides of the same coin: the turbulence-generated self-organised processes which occur at mesoscales are central to our understanding of transport processes as they govern shear generation and flow pattern formation.
01 Jun 2010
01 Jan 2010