Nonlinear collisionless plasma wakes of small particles

Abstract

The wake behind a spherical particle smaller than the Debye length (λ De ) in flowing plasma is calculated using a particle-in-cell code. The results with different magnitudes of charge reveal substantial non-linear effects down to values that for a floating particle would correspond to a particle radius approximately 10 −2 λ De . The peak potential in the oscillatory wake structure is strongly suppressed by non-linearity, never exceeding approximately 0.4 times the unperturbed ion energy. By contrast, the density peak arising from ion focusing can be many times the ambient. Strong heating of the ions occurs in the non-linear regime. Direct ion absorption by the particle is not important for the far wake unless the radius exceeds 10 −1 λ De , and is therefore never significant (for the far wake) in the linear regime. Reasonable agreement with full-scale linear-response calculations are obtained in the linear regime. The wake wavelength is confirmed and an explanation, in terms of the conical potential structure, is proposed for experimentallyobserved oblique alignment of different-sized grains.

Impact of collisions on the dust wake potential with Maxwellian and non-Maxwellian ions

“…Winske et al 17 , however, found a nonlinear relationship between charge and wake potential utilizing one-and two-dimensional PIC-simulations. Hutchinson et al 20 also reported the nonlinear suppression of the peak potential in the oscillatory wake structure and a reasonable agreement with linear response calculations in the linear regime.…”

“…COPTIC is a hybrid particle-in-cell code in a sense that electron dynamics are governed by the Boltzmann description, n e = n e∞ exp(eφ/T e ), whereas ion dynamics are considered in six-dimensional phase space in the presence of the self-consistent electric field and an optional external field. Length scale, velocity and other normalizations are adopted from the paper by Hutchinson et al 20 . In this work, we have considered only point-charge grains using the particle-particle-particle-mesh (PPPM) scheme 29 .…”

“…In particular, we present a wide-ranging numerical exploration of the role of charge-exchange collisions and the ion distribution function on the wake formation around a grain in a uniform streaming plasma using the threedimensional Particle-in-Cell (PIC) simulation code COP-TIC 20 . In contrast to linear response calculations 26,27 , this allows us to fully account for nonlinear effects, which are expected to play an important role in the close vicinity of the grain.…”

“…For our simulations, we have used the threedimensional Cartesian mesh, oblique boundary, particles and thermals in cell (COPTIC) code 20 . COPTIC is a hybrid particle-in-cell code in a sense that electron dynamics are governed by the Boltzmann description, n e = n e∞ exp(eφ/T e ), whereas ion dynamics are considered in six-dimensional phase space in the presence of the self-consistent electric field and an optional external field.…”

The electron forewake: Shadowing and drift-energization as flowing magnetized plasma encounters an obstacle

“…Hybrid methods are a useful way to relax those limitations and study large multi-dimensional systems, but can only include certain classes of (typically fluid) electron phenomena. Exploring the importance of true kinetic electron effects thus requires a fully kinetic treatment, and the present simulations have been enabled by an upgrade of the COPTIC code 35 to allow for kinetic (rather than Boltzmann) electrons.…”