R. Marchand

Kinetic simulations of electron response to shear Alfvén waves in magnetospheric plasmas

Standing and traveling shear Alfven waves contribute to electron acceleration and parallel electric field formation on auroral field lines in the Earth’s magnetosphere. In this paper, the self-consistent coupled Vlasov–Maxwell system for shear Alfven waves is solved in one dimension. The Vlasov equation is gyro-averaged in order to minimize the number of dimensions in the problem, and to avoid numerical problems with the direct evaluation of the parallel electric field, the electron distribution function is described in terms of a spatial coordinate along the field line, the magnetic moment, and the canonical parallel momentum per unit mass. Some preliminary studies of shear Alfven wave pulses propagating in a uniform magnetic field on model auroral field lines are presented with an emphasis on the kinetic electron response and the parallel electric field. It is shown that when the full parallel electron dynamics are included, wave–particle interactions result in intensification of the parallel electric field and significant heating of the electrons. It is also demonstrated the acceleration of electrons to speeds of the order of twice the wave phase speed, which has been shown by previous calculations. The results are compared with recent observations made by both the NASA Fast Auroral Snapshot and the Freja satellites.Standing and traveling shear Alfven waves contribute to electron acceleration and parallel electric field formation on auroral field lines in the Earth’s magnetosphere. In this paper, the self-consistent coupled Vlasov–Maxwell system for shear Alfven waves is solved in one dimension. The Vlasov equation is gyro-averaged in order to minimize the number of dimensions in the problem, and to avoid numerical problems with the direct evaluation of the parallel electric field, the electron distribution function is described in terms of a spatial coordinate along the field line, the magnetic moment, and the canonical parallel momentum per unit mass. Some preliminary studies of shear Alfven wave pulses propagating in a uniform magnetic field on model auroral field lines are presented with an emphasis on the kinetic electron response and the parallel electric field. It is shown that when the full parallel electron dynamics are included, wave–particle interactions result in intensification of the parallel electric f...

CARRE: a quasi-orthogonal mesh generator for 2D edge plasma modelling

A computer code is described which automatically generates a structured curvilinear quasi-orthogonal mesh of the type used in several models of transport for the edge and divertor regions in tokamak fusion experiments. The method considered works from numerically generated equilibria and from digitized parametrisations of structures and boundaries in the simulation domain. It therefore produces realistic computational meshes which allow comparisons between simulation results and experiments. The method is well adapted to the generation of meshes for a number of interesting magnetic field topologies, such as single null, connected and disconnected double null divertor geometries.

Finite element modelling of transport in a tokamak edge and divertor

A finite element approach is described, for modelling transport in tokamak edge and divertor plasma. The method discretizes all transport equations on an unstructured triangular mesh. The advantages and difficulties of this approach are discussed. Results are presented and compared with experimental measurements made on TdeV. Example results are also given for JET, using a simplified physics model (single ion species, no neutrals and no flux limits), and using a more comprehensive model with helium impurity ions and neutrals.

PTetra, a Tool to Simulate Low Orbit Satellite–Plasma Interaction

A model is presented to numerically simulate the time-dependent interaction of satellites with space plasma. The approach is based on a fully kinetic description of the plasma using particle in cell modeling for all plasma species with physical charges and masses. The model also relies on a discretization of space with an unstructured tetrahedral mesh that is capable of representing complex boundaries and realistic spacecraft geometries. The code solves for the self-consistent electric fields, given calculated volume charges and charge deposition on the various satellite components. The model is purely electrostatic, but it does account for the effects of a possible constant and uniform background magnetic field.

Spatiotemporal characteristics of ultra-low frequency dispersive scale shear Alfvén waves in the Earth’s magnetosphere

The high-latitude nightside auroral zone is threaded by geomagnetic field lines supporting localized ultra-low frequency (ULF) shear Alfven waves (SAWs). A particular class of dispersive scale ULF waves can be attributed to resonant mode conversion of global scale near-monochromatic compressional waves that are excited by high-speed solar wind flow past the magnetosphere. In the more distant magnetotail, warm plasma dispersive effects preclude the excitation of latitudinally narrow waveforms. Closer to Earth, flux tubes are loaded with colder plasma that favors inertial scale dispersive SAWs and nonlinear wave processes. The spatiotemporal characteristics of ULF shear Alfven field line resonances near midnight are analyzed, and it is demonstrated that in warm plasma, small-scale nonlinear structuring occurs on field lines where two competing wave dispersion mechanisms cancel. Over realistic time scales, this situation occurs naturally as a result of SAW ponderomotive forces that initiate up-flowing ion mo...

Finite element modelling of TdeV edge and divertor with E*B drifts

Simulation results are presented from a finite element calculation of transport in the edge and divertor of TdeV. In addition to the usual transport processes, this model also includes a calculation of the electrostatic potential and the associated E*B drifts. The inclusion of E*B drifts is shown to be essential in the calculation of the parallel and toroidal flow velocities. With the inclusion of drifts, the effect of toroidal magnetic field reversal is studied, particularly with regard to in-out asymmetries in the Halpha radiation and power deposited on the plates. Relatively weak asymmetries are calculated on the basis of these drifts alone, and it is concluded that the much stronger asymmetries observed experimentally must be associated with other physical processes not yet included in the model. The transport equations are based on fluid conservation equations for particles, parallel (to the magnetic field) momentum, electron and ion energy. For simplicity, a single ion species is considered and, when modelling recycling, neutral transport is treated in the diffusion approximation. All transport equations are discretized with finite elements on a triangular unstructured mesh that is locally aligned with the flux surfaces. The use of such a mesh allows an accurate representation of the simulation geometry, with boundaries of arbitrary shapes and complexity. In particular, it is possible to account for divertor plates that are not orthogonal to the flux surfaces without any penalty for the validity of the transport equations. The analysis presented here concerns a low density attached ohmic TdeV discharge in a single null geometry, with the X point above the midplane

Self‐focusing and ion wave generation in laser‐produced plasmas

Two‐dimensional hydrodynamic simulations of laser light self‐focusing in a hydrogen plasma are presented. The simulation code includes a model for laser beam propagation which accounts for inverse bremsstrahlung absorption, refraction, diffraction, and ponderomotive forces. A Gaussian hot spot, superimposed upon a collimated, spatially uniform laser beam, is used to initiate self‐focusing. Intense filaments provide a driving source for ion waves near the axis of the laser beam. The radially propagating ion waves cause spatiotemporal modulations of the flux where it is initially uniform, as well as the more usual focusing that occurs along the axis. Some of the factors affecting the generation of the ion waves are considered. In particular, the effect of changing the amplitude and width of the imposed nonuniformity is investigated. The intensity thresholds for thermal and ponderomotively driven self‐focusing have also been determined by artificially turning the ponderomotive force on and off.

Measurement of pre-sheath flow velocities by laser-induced fluorescence

Abstract For the first time, the pre-sheath ion flow velocity has been measured using the Doppler shift of laser-induced fluorescence in singly-ionized argon ions. The velocity shows a monotonic increase, from a value of about 0.15 of the sound speed VS far from the target to 0.5 of Vs at a distance of 5 mm from the surface. The temperature, the floating potential and the density are calculated from cylindrical probe measurements taken in the same region under identical conditions. These experimental results are compared with those from a 1D isothermal single-ion fluid model of the pre-sheath and a kinetic electron/fluid ion model. Both models agree well with the density profile, but underestimate the potential change and overestimate the velocity. In addition, the bulk flow velocity has been independently determined from “Mach probe” measurements, using various candidate theories to relate the Mach number to the ratio of the upstream to downstream saturation currents. Comparison with the optical measurements indicate that the probe models which include viscosity provide reasonable agreement with our Mach probe data.

Cross-comparison of spacecraft-environment interaction model predictions applied to Solar Probe Plus near perihelion

Five spacecraft-plasma models are used to simulate the interaction of a simplified geometry Solar Probe Plus (SPP) satellite with the space environment under representative solar wind conditions ne ...

Spacecraft charging analysis with the implicit particle-in-cell code iPic3D

We present the first results on the analysis of spacecraft charging with the implicit particle-in-cell code iPic3D, designed for running on massively parallel supercomputers. The numerical algorithm is presented, highlighting the implementation of the electrostatic solver and the immersed boundary algorithm; the latter which creates the possibility to handle complex spacecraft geometries. As a first step in the verification process, a comparison is made between the floating potential obtained with iPic3D and with Orbital Motion Limited theory for a spherical particle in a uniform stationary plasma. Second, the numerical model is verified for a CubeSat benchmark by comparing simulation results with those of PTetra for space environment conditions with increasing levels of complexity. In particular, we consider spacecraft charging from plasma particle collection, photoelectron and secondary electron emission. The influence of a background magnetic field on the floating potential profile near the spacecraft is also considered. Although the numerical approaches in iPic3D and PTetra are rather different, good agreement is found between the two models, raising the level of confidence in both codes to predict and evaluate the complex plasma environment around spacecraft.