3-DIMENSIONAL NUMERICAL-SIMULATION OF CURRENT COLLECTION BY A PROBE IN A MAGNETIZED PLASMA

A three-dimensional numerical model for current collection in a magnet ized plasma is reported. The model is based on an electrostatic partic le-in-cell code. The model yields self-consistent sheath structure inc luding distributions of plasma and the electric potential around the b ody and the collection of electrons. The analytical theory of current collection by a body in a magnetized plasma yields an upper bound for the collected current determined by the conservation of energy and can onical angular momentum. The theory shows that the collected charged p articles come from a cylindrical volume aligned with the magnetic shad ow of the body; the maximum radius r(o) of this volume is determined b y the body size, body potential, and the ambient magnetic field. This theory does not deal with the sheath structure around the body. The co ndition for the actual current to approach the upper-bound value has b een a matter of debate. Our simulations reveal when and why the collec ted current becomes equal to its upper-bound value. Sheath size in the radial direction perpendicular to the axial ambient magnetic field is determined by the current-limiting radius r(o). Our simulation yields time-average current in good agreement with its upper bound. This fea ture of the current collection is explained as follows: Once electrons enter the sheath, some of them are freely accelerated perpendicular t o the magnetic field because they are demagnetized by the large gradie nts in the perpendicular electric fields. Simulations show a large per pendicular acceleration, producing perpendicular energy as large as th at determined by the potential on the body, especially in the region w here perpendicular electric fields (E(perpendicular-to) are the strong est. An analysis shows that the demagnetization of electrons occurs ab ove a threshold potential on the body. This threshold condition follow s from the breakdown of the adiabaticity of the electron dynamics insi de the sheath.