Non-linear collisionless plasma wakes of small particles

Abstract

The wake behind a spherical particle smaller than the Debye length ( De) in owing plasma is calculated using a particle-in-cell code. The results with di erent magnitudes of charge reveal substantial non-linear e ects down to values that for a oating particle would correspond to a particle radius approximately 10ð€€€2 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 10ð€€€1 De, and is therefore never signi cant (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 con rmed and an explanation, in terms of the conical potential structure, is proposed for experimentally- observed oblique alignment of di erent-sized grains.