Topic/Type: 1.5 Low-temperature, dusty and nano-plasmas, Oral

Ion collection by a sphere in ExB fields

L. Patacchini, I.H. Hutchinson

Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA, USA

We carry out 3D Particle-In-Cell (PIC) simulations accounting for the full ion
distribution function, Boltzmann electrons, and the self-consistent potential
profiles in the neighborhood of a sphere in a flowing magnetized plasma. This
can be considered as the 'spherical Mach-probe' problem, establishing how the
ion flux to the surface varies with orientation, and with parallel and
perpendicular external velocity. Alternatively the sphere can be seen as a dust
particle, for which we wish to calculate the floating potential and the ion drag
force.

We use SCEPTIC3D, a recent evolution of the parallel PIC code SCEPTIC, which
includes arbitrary uniform magnetic field, external velocity magnitude and
direction, ion temperature and electron Debye length. The simulation is
structured on a spherical grid centered on the collecting sphere, enabling easy
and accurate resolution of the plasma-boundary interaction. The code can operate
either in the 'zero Debye length' limit, when the plasma region of interest
is quasineutral and the Debye sheath at the sphere surface infinitesimally
thin, or in the 'finite Debye length' regime, when the potential profiles
are governed by Poisson equation that we solve with a fully parallelized
conjugate gradient algorithm.

Sample results in the quasineutral regime include successful comparison with
independent semi-analytic treatments of strongly magnetized ion collection by
oblique surfaces, as well as a theoretical calibration for transverse Mach
probes with four electrodes oriented at to the magnetic field in a plane
of flow and magnetic field, valid for arbitrary temperature and ion
magnetization. Accounting for finite Debye length changes the potential
profiles around the sphere; in particular for conducting dust particles, a
dipole-like field oriented parallel to the convective electric field appears,
drastically changing the ion flow and enhancing the vxB force on the particle.