Ion collection by a sphere in a flowing plasma: 3. Floating potential and drag force

Abstract

The interaction of an ion-collecting sphere at floating potential with a flowing collisionless plasma is investigated using the particle in cell code SCEPTIC. The dependence of the floating potential on the flow velocity for a conducting sphere is found to agree very well with the orbital motion limited approximation, which ignores the asymmetry in the plasma potential. But the charge, even on conducting spheres and at zero flow, is not well represented by using the standard expression for capacitance. Insulating spheres become asymmetrically charged because of ion collection asymmetry, and their total (negative) charge is considerably increased by flow. The collection flux asymmetry is documented for both conducting and insulating spheres and is not greatly different between them. The drag force upon the sphere is obtained from the code calculations. It shows reasonable agreement with appropriate analytic approximations. However, numerical discrepancies up to 20% are found, which are attributed to uncertainties in the analytical values.