Orbital Motion Theory of the Shielding Potential Around an Electron Emitting Body

Abstract

Summary form only given. The problem of the shielding potential around a body immersed in a collisionless plasma is investigated for the case of a body emitting electrons due to thermionic emission, photoemission and secondary emission. The system is investigated by means of an analytic, kinetic theory as well as particle-in-cell (PIC) simulations. The kinetic theory takes into account self-consistently plasma particles collection by the body as well as electron emission. The theory is derived under the assumption of spherical symmetry so that conservation of energy and angular momentum can be used to calculate the plasma distribution functions at any given point in phase space. Far away from the body, the plasma is assumed unperturbed and described by a Maxwellian distribution function at rest. Thus, the unperturbed plasma acts as a source of particles balancing the absorptions from the body and a steady state is eventually reached. Furthermore, the theory is formulated for positively charged bodies and for radial profiles of the shielding potential presenting an attractive well, as found with PIC simulations by Delzanno et al. for the case of thermionic emission. This assumption is needed in order to be able to describe correctly potential barriers to the particle motion. The theory is shown to be in good agreement with PIC simulations. Furthermore, several cases (focusing on parameters typical of laboratory experiments) are presented for the three different emission mechanisms, showing that shielding potentials having an attractive well are possible for all emission mechanisms. The influence of physical parameters such as ion to electron mass ratio, temperature of the emitted electrons, work function of the body, etc is also shown