14 Search Results

Investigating the radial structure of axisymmetric fluctuations in the TCV tokamak with local and global gyrokinetic GENE simulations
Axisymmetric (n=0) density fluctuations measured in the TCV tokamak are observed to possess a frequency f<sub>0</sub> which is either varying (radially dispersive oscillations) or a constant over a large fraction of the plasma minor radius (radially global oscillations) as reported in a companion paper [Z. Huang <i>et al.</i>, this issue]. Given that f<sub>0</sub> scales with the sound speed and given the poloidal structure of density fluctuations, these oscillations were interpreted as Geodesic Acoustic Modes, even though f<sub>0</sub> is in fact smaller than the local linear GAM frequency f<sub>GAM</sub> . In this work we employ the Eulerian gyrokinetic code GENE tomore » 
Scientific data interpolation with low dimensional manifold model
Here, we propose to apply a low dimensional manifold model to scientific data interpolation from regular and irregular samplings with a significant amount of missing information. The low dimensionality of the patch manifold for general scientific data sets has been used as a regularizer in a variational formulation. The problem is solved via alternating minimization with respect to the manifold and the data set, and the Laplace–Beltrami operator in the Euler–Lagrange equation is discretized using the weighted graph Laplacian. Various scientific data sets from different fields of study are used to illustrate the performance of the proposed algorithm on datamore » 
Hybrid simulations of a parallel collisionless shock in the large plasma device
We present twodimensional hybrid kinetic/magnetohydrodynamic simulations of planned laserablation experiments in the Large Plasma Device (LAPD). Our results, based on parameters which have been validated in previous experiments, show that a parallel collisionless shock can begin forming within the available space. Carbondebris ions that stream along the magnetic eld direction with a blowo speed of four times the Alfv en velocity excite strong magnetic uctuations, eventually transfering part of their kinetic energy to the surrounding hydrogen ions. This acceleration and compression of the background plasma creates a shock front, which satis es the Rankine{Hugoniot conditions and can therefore propagate onmore »Cited by 1 
The anisotropic redistribution of free energy for gyrokinetic plasma turbulence in a Zpinch
For a Zpinch geometry, we report on the nonlinear redistribution of free energy across scales perpendicular to the magnetic guide field, for a turbulent plasma described in the framework of gyrokinetics. The analysis is performed using a local fluxsurface approximation, in a regime dominated by electrostatic fluctuations driven by the entropy mode, with both ion and electron species being treated kinetically. To explore the anisotropic nature of the free energy redistribution caused by the emergence of zonal flows, we use a polar coordinate representation for the fieldperpendicular directions and define an angular density for the scale flux. Positive values formore » 
COSMICRAY PITCHANGLE SCATTERING IN IMBALANCED MHD TURBULENCE SIMULATIONS
Pitchangle scattering rates for cosmicray particles in MHD simulations with imbalanced turbulence are calculated for fully evolving electromagnetic turbulence. We compare with theoretical predictions derived from the quasilinear theory of cosmicray diffusion for an idealized slab spectrum and demonstrate how cross helicity affects the shape of the pitchangle diffusion coefficient. Additional simulations in evolving magnetic fields or static field configurations provide evidence that the scattering anisotropy in imbalanced turbulence is not primarily due to coherence with propagating Alfvén waves, but an effect of the spatial structure of electric fields in crosshelical MHD turbulence. 
Fusion Energy Sciences Exascale Requirements Review. An Office of Science review sponsored jointly by Advanced Scientific Computing Research and Fusion Energy Sciences, January 2729, 2016, Gaithersburg, Maryland
The additional computing power offered by the planned exascale facilities could be transformational across the spectrum of plasma and fusion research — provided that the new architectures can be efficiently applied to our problem space. The collaboration that will be required to succeed should be viewed as an opportunity to identify and exploit crossdisciplinary synergies. To assess the opportunities and requirements as part of the development of an overall strategy for computing in the exascale era, the Exascale Requirements Review meeting of the Fusion Energy Sciences (FES) community was convened January 27–29, 2016, with participation from a broad range ofmore » 
Aspects of linear Landau damping in discretized systems
Basic linear eigenmode spectra for electrostatic Langmuir waves and driftkinetic slab ion temperature gradient modes are examined in a series of scenarios. Collisions are modeled via a LenardBernstein collision operator which fundamentally alters the linear spectrum even for infinitesimal collisionality [Ng et al., Phys. Rev. Lett. 83, 1974 (1999)]. A comparison between different discretization schemes reveals that a Hermite representation is superior for accurately resolving the spectra compared to a finite differences scheme using an equidistant velocity grid. Additionally, it is shown analytically that any even power of velocity space hyperdiffusion also produces a CaseVan Kampen spectrum which, in themore » 
The energetic coupling of scales in gyrokinetic plasma turbulence
In magnetized plasma turbulence, the couplings of perpendicular spatial scales that arise due to the nonlinear interactions are analyzed from the perspective of the freeenergy exchanges. The plasmas considered here, with appropriate ion or electron adiabatic electroneutrality responses, are described by the gyrokinetic formalism in a toroidal magnetic geometry. Turbulence develops due to the electrostatic fluctuations driven by temperature gradient instabilities, either ion temperature gradient (ITG) or electron temperature gradient (ETG). The analysis consists in decomposing the system into a series of scale structures, while accounting separately for contributions made by modes possessing special symmetries (e.g., the zonal flow modes).more »