Comparison of particle trajectories and collision operators for collisional transport in nonaxisymmetric plasmas

In this work, we examine the validity of several common simplifying assumptions used in numerical neoclassical calculations for nonaxisymmetric plasmas, both by using a new continuum drift-kinetic code and by considering analytic properties of the kinetic equation. First, neoclassical phenomena are computed for the LHD and W7-X stellarators using several versions of the drift-kinetic equation, including the commonly used incompressible-ExB-drift approximation and two other variants, corresponding to different effective particle trajectories. It is found that for electric fields below roughly one third of the resonant value, the different formulations give nearly identical results, demonstrating the incompressible ExB-drift approximation is quite accurate in this regime. However, near the electric field resonance, the models yield substantially different results. We also compare results for various collision operators, including the full linearized Fokker-Planck operator. At low collisionality, the radial transport driven by radial gradients is nearly identical for the different operators, while in other cases it is found to be important that collisions conserve momentum

Collisional damping rates for plasma waves

The distinction between the plasma dynamics dominated by collisional transport versus collective processes has never been rigorously addressed until recently. A recent paper [Yoon et al., Phys. Rev. E 93, 033203 (2016)] formulates for the first time, a unified kinetic theory in which collective processes and collisional dynamics are systematically incorporated from first principles. One of the outcomes of such a formalism is the rigorous derivation of collisional damping rates for Langmuir and ion-acoustic waves, which can be contrasted to the heuristic customary approach. However, the results are given only in formal mathematical expressions. The present Brief Communication numerically evaluates the rigorous collisional damping rates by considering the case of plasma particles with Maxwellian velocity distribution function so as to assess the consequence of the rigorous formalism in a quantitative manner. Comparison with the heuristic ("Spitzer") formula shows that the accurate damping rates are much lower in magnitude than the conventional expression, which implies that the traditional approach over-estimates the importance of attenuation of plasma waves by collisional relaxation process. Such a finding may have a wide applicability ranging from laboratory to space and astrophysical plasmas.Comment: 5 pages, 2 figures; Published in Physics of Plasmas, volume/Issue 23/6. Publisher: AIP Publishing LLC. Date: Jun 1, 2016. URL: http://aip.scitation.org/doi/10.1063/1.4953802 Rights managed by AIP Publishing LL

From Bare Metal Powders to Colloidally Stable TCO Dispersions and Transparent Nanoporous Conducting Metal Oxide Thin Films

Cataloged from PDF version of article.A simple, green, robust, widely applicable, multi-gram and cost-effective 'one-pot' synthesis of aqueous dispersions of colloidally stable 3-6 nm TCO NPs using bare metal powder precursors is described, and their utilization for making TCO high surface area nanoporous films is also demonstrated, which speaks well for their usage in a wide range of possible processes and devices. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Linearized model Fokker-Planck collision operators for gyrokinetic simulations. I. Theory

A new analytically and numerically manageable model collision operator is developed specifically for turbulence simulations. The like-particle collision operator includes both pitch-angle scattering and energy diffusion and satisfies the physical constraints required for collision operators: it conserves particles, momentum and energy, obeys Boltzmann's H-theorem (collisions cannot decrease entropy), vanishes on a Maxwellian, and efficiently dissipates small-scale structure in the velocity space. The process of transforming this collision operator into the gyroaveraged form for use in gyrokinetic simulations is detailed. The gyroaveraged model operator is shown to have more suitable behavior at small scales in phase space than previously suggested models. A model operator for electron-ion collisions is also presented.Comment: revtex, 12 page

Linearized model Fokker-Planck collision operators for gyrokinetic simulations. II. Numerical implementation and tests

A set of key properties for an ideal dissipation scheme in gyrokinetic simulations is proposed, and implementation of a model collision operator satisfying these properties is described. This operator is based on the exact linearized test-particle collision operator, with approximations to the field-particle terms that preserve conservation laws and an H-Theorem. It includes energy diffusion, pitch-angle scattering, and finite Larmor radius effects corresponding to classical (real-space) diffusion. The numerical implementation in the continuum gyrokinetic code GS2 is fully implicit and guarantees exact satisfaction of conservation properties. Numerical results are presented showing that the correct physics is captured over the entire range of collisionalities, from the collisionless to the strongly collisional regimes, without recourse to artificial dissipation.Comment: 13 pages, 8 figures, submitted to Physics of Plasmas; typos fixe

Understanding the effect of sheared flow on microinstabilities

The competition between the drive and stabilization of plasma microinstabilities by sheared flow is investigated, focusing on the ion temperature gradient mode. Using a twisting mode representation in sheared slab geometry, the characteristic equations have been formulated for a dissipative fluid model, developed rigorously from the gyrokinetic equation. They clearly show that perpendicular flow shear convects perturbations along the field at a speed we denote by $Mc_s$ (where $c_s$ is the sound speed), whilst parallel flow shear enters as an instability driving term analogous to the usual temperature and density gradient effects. For sufficiently strong perpendicular flow shear, $M >1$, the propagation of the system characteristics is unidirectional and no unstable eigenmodes may form. Perturbations are swept along the field, to be ultimately dissipated as they are sheared ever more strongly. Numerical studies of the equations also reveal the existence of stable regions when $M < 1$, where the driving terms conflict. However, in both cases transitory perturbations exist, which could attain substantial amplitudes before decaying. Indeed, for $M \gg 1$, they are shown to exponentiate $\sqrt{M}$ times. This may provide a subcritical route to turbulence in tokamaks.Comment: minor revisions; accepted to PPC

Non-destructive controlled single-particle light scattering measurement

We present a set of light scattering data measured from a millimeter-sized extraterrestrial rock sample. The data were acquired by our novel scatterometer, which enables accurate multi-wavelength measure- ments of single-particle samples whose position and orientation are controlled by ultrasonic levitation. The measurements demonstrate a non-destructive approach to derive optical properties of small mineral samples. This enables research on valuable materials, such as those returned from space missions or rare meteorites.Peer reviewe

Solar Flares as Cascades of Reconnecting Magnetic Loops

A model for the solar coronal magnetic field is proposed where multiple directed loops evolve in space and time. Loops injected at small scales are anchored by footpoints of opposite polarity moving randomly on a surface. Nearby footpoints of the same polarity aggregate, and loops can reconnect when they collide. This may trigger a cascade of further reconnection, representing a solar flare. Numerical simulations show that a power law distribution of flare energies emerges, associated with a scale free network of loops, indicating self-organized criticality.Comment: 4 pages, 4 figures, To be published in Phys. Rev. Let