Vlasov simulation in multiple spatial dimensions

A long-standing challenge encountered in modeling plasma dynamics is achieving practical Vlasov equation simulation in multiple spatial dimensions over large length and time scales. While direct multi-dimension Vlasov simulation methods using adaptive mesh methods [J. W. Banks et al., Physics of Plasmas 18, no. 5 (2011): 052102; B. I. Cohen et al., November 10, 2010, http://meetings.aps.org/link/BAPS.2010.DPP.NP9.142] have recently shown promising results, in this paper we present an alternative, the Vlasov Multi Dimensional (VMD) model, that is specifically designed to take advantage of solution properties in regimes when plasma waves are confined to a narrow cone, as may be the case for stimulated Raman scatter in large optic f# laser beams. Perpendicular grid spacing large compared to a Debye length is then possible without instability, enabling an order 10 decrease in required computational resources compared to standard particle in cell (PIC) methods in 2D, with another reduction of that order in 3D. Further advantage compared to PIC methods accrues in regimes where particle noise is an issue. VMD and PIC results in a 2D model of localized Langmuir waves are in qualitative agreement

Simulation study of the interaction between large-amplitude HF radio waves and the ionosphere

The time evolution of a large-amplitude electromagnetic (EM) wave injected vertically into the overhead ionosphere is studied numerically. The EM wave has a carrier frequency of 5 MHz and is modulated as a Gaussian pulse with a width of approximately 0.1 milliseconds and a vacuum amplitude of 1.5 V/m at 50 km. This is a fair representation of a modulated radio wave transmitted from a typical high-power HF broadcast station on the ground. The pulse is propagated through the neutral atmosphere to the critical points of the ionosphere, where the L-O and R-X modes are reflected, and back to the neutral atmosphere. We observe mode conversion of the L-O mode to electrostatic waves, as well as harmonic generation at the turning points of both the R-X and L-O modes, where their amplitudes rise to several times the original ones. The study has relevance for ionospheric interaction experiments in combination with ground-based and satellite or rocket observations.Comment: 6 pages, 7 figure

Electron cyclotron resonance near the axis of the gas-dynamic trap

Propagation of an extraordinary electromagnetic wave in the vicinity of electron cyclotron resonance surface in an open linear trap is studied analytically, taking into account inhomogeneity of the magnetic field in paraxial approximation. Ray trajectories are derived from a reduced dispersion equation that makes it possible to avoid the difficulty associated with a transition from large propagation angles to the case of strictly longitudinal propagation. Our approach is based on the theory, originally developed by the Zvonkov and Timofeev [1], who used the paraxial approximation for the magnetic field strength, but did not consider the slope of the magnetic field lines, which led to considerable error, as has been recently noted by Gospodchikov and Smolyakova [2]. We have found ray trajectories in analytic form and demonstrated that the inhomogeneity of both the magnetic field strength and the field direction can qualitatively change the picture of wave propagation and significantly affect the efficiency of electron cyclotron heating of a plasma in a linear magnetic trap. Analysis of the ray trajectories has revealed a criterion for the resonance point on the axis of the trap to be an attractor for the ray trajectories. It is also shown that a family of ray trajectories can still reach the resonance point on the axis if the latter generally repels the ray trajectories. As an example, results of general theory are applied to the electron cyclotron resonance heating experiment which is under preparation on the Gas Dynamic Trap in the Budker Institute of Nuclear Physics [3]

Dynamical friction force exerted on spherical bodies

We present a rigorous calculation of the dynamical friction force exerted on a spherical massive perturber moving through an infinite homogenous system of field stars. By calculating the shape and mass of the polarization cloud induced by the perturber in the background system, which decelerates the motion of the perturber, we recover Chandrasekhar's drag force law with a modified Coulomb logarithm. As concrete examples we calculate the drag force exerted on a Plummer sphere or a sphere with the density distribution of a Hernquist profile. It is shown that the shape of the perturber affects only the exact form of the Coulomb logarithm. The latter converges on small scales, because encounters of the test and field stars with impact parameters less than the size of the massive perturber become inefficient. We confirm this way earlier results based on the impulse approximation of small angle scatterings.Comment: 5 pages, 2 figures, accepted in MNRA

Collisionless Magnetic Reconnection via Alfven Eigenmodes

We propose an analytic approach to the problem of collisionless magnetic reconnection formulated as a process of Alfven eigenmodes' generation and dissipation. Alfven eigenmodes are confined by the current sheet in the same way that quantum mechanical waves are confined by the tanh^2 potential. The dynamical time scale of reconnection is the system scale divided by the eigenvalue propagation velocity of the n=1 mode. The prediction of the n=1 mode shows good agreement with the in situ measurement of the reconnection-associated Hall fields

Constraining the neutrino magnetic moment with anti-neutrinos from the Sun

We discuss the impact of different solar neutrino data on the spin-flavor-precession (SFP) mechanism of neutrino conversion. We find that, although detailed solar rates and spectra allow the SFP solution as a sub-leading effect, the recent KamLAND constraint on the solar antineutrino flux places stronger constraints to this mechanism. Moreover, we show that for the case of random magnetic fields inside the Sun, one obtains a more stringent constraint on the neutrino magnetic moment down to the level of \mu_\nu \lsim few \times 10^{-12}\mu_B, similar to bounds obtained from star cooling.Comment: 4 pages, 3 figures. Final version to appear in Phys. Rev. Let

Measuring Dislocation Density in Aluminum with Resonant Ultrasound Spectroscopy

Dislocations in a material will, when present in enough numbers, change the speed of propagation of elastic waves. Consequently, two material samples, differing only in dislocation density, will have different elastic constants, a quantity that can be measured using Resonant Ultrasound Spectroscopy. Measurements of this effect on aluminum samples are reported. They compare well with the predictions of the theory.Comment: 4 pages, 2 figure

A crude model to study radio frequency induced density modification close to launchers

The interplay between radio frequency (RF) waves and the density is discussed by adopting the general framework of a 2-time-scale multi-fluid treatment, allowing to separate the dynamics on the RF time scale from that on the time scale on which macroscopic density and flows vary as a result of the presence of electromagnetic and/or electrostatic fields. The focus is on regions close to launchers where charge neutrality is incomplete and waves are commonly evanescent. The fast time scale dynamics influences the slow time scale behavior via quasilinear terms (the Ponderomotive force for the case of the equation of motion). Electrons and ions are treated on the same footing. Also, both fast and slow waves are retained in the wave description. Although this work is meant as a subtopic of a large study-the wave induced "convective cell" physics at hand is of a 2- or 3-dimensional nature while this paper limits itself to a single dimension-a few tentative examples are presented

Theoretical and numerical studies of wave-packet propagation in tokamak plasmas

Theoretical and numerical studies of wave-packet propagation are presented to analyze the time varying 2D mode structures of electrostatic fluctuations in tokamak plasmas, using general flux coordinates. Instead of solving the 2D wave equations directly, the solution of the initial value problem is used to obtain the 2D mode structure, following the propagation of wave-packets generated by a source and reconstructing the time varying field. As application, the 2D WKB method is applied to investigate the shaping effects (elongation and triangularity) of tokamak geometry on the lower hybrid wave propagation and absorbtion. Meanwhile, the Mode Structure Decomposition (MSD) method is used to handle the boundary conditions and simplify the 2D problem to two nested 1D problems. The MSD method is related to that discussed earlier by Zonca and Chen [Phys. Fluids B 5, 3668 (1993)], and reduces to the well-known "ballooning formalism" [J. W. Connor, R. J. Hastie, and J. B. Taylor, Phys. Rev. Lett. 40, 396 (1978)], when spatial scale separation applies. This method is used to investigate the time varying 2D electrostatic ITG mode structure with a mixed WKB-full-wave technique. The time varying field pattern is reconstructed and the time asymptotic structure of the wave-packet propagation gives the 2D eigenmode and the corresponding eigenvalue. As a general approach to investigate 2D mode structures in tokamak plasmas, our method also applies for electromagnetic waves with general source/sink terms, either by an internal/external antenna or nonlinear wave interaction with zonal structures.Comment: 24 pages, 14 figure

Nonlinear turbulent magnetic diffusion and effective drift velocity of large-scale magnetic field in a two-dimensional magnetohydrodynamic turbulence

We study a nonlinear quenching of turbulent magnetic diffusion and effective drift velocity of large-scale magnetic field in a developed two-dimensional MHD turbulence at large magnetic Reynolds numbers. We show that transport of the mean-square magnetic potential strongly changes quenching of turbulent magnetic diffusion. In particularly, the catastrophic quenching of turbulent magnetic diffusion does not occur for the large-scale magnetic fields $B \gg B_{\rm eq} / \sqrt{\rm Rm}$ when a divergence of the flux of the mean-square magnetic potential is not zero, where $B_{\rm eq}$ is the equipartition mean magnetic field determined by the turbulent kinetic energy and Rm is the magnetic Reynolds number. In this case the quenching of turbulent magnetic diffusion is independent of magnetic Reynolds number. The situation is similar to three-dimensional MHD turbulence at large magnetic Reynolds numbers whereby the catastrophic quenching of the alpha effect does not occur when a divergence of the flux of the small-scale magnetic helicity is not zero.Comment: 8 pages, Physical Review E, in pres