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

**
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.