Understanding the conductivity in ion propulsion devices

Abstract

Summary form only given. A SPT (stationary plasma thruster) is a type of ion source developed primarily in Russia over the past 30 years and used as an electromagnetic propulsion device in applications requiring a low to moderate thrust with a high efficiency. In the usual geometry, a SPT consists of two concentric dielectric cylinders with an anode at one end and an external hollow cathode. A magnetic field created by external coils is applied such that the B field has a large component perpendicular to the walls. A flux of gas (usually xenon) is introduced at the anode and in the hollow cathode. The ions generated in this region by electron impact ionization are accelerated by the large longitudinal electric field which forms in the plasma and provide the thrust of the device. This device is unlike other discharge devices because the magnetic field is perpendicular (or almost) to the walls. The principles of operation are far from clear. One of the outstanding issues is the identification of the mechanisms leading to the observed high conductivity in these devices. Electron-neutral and electron-ion collisions are insufficient to account for the observed conductivity across the magnetic field lines. Bohm diffusion resulting from turbulence is a possible explanation for the observed high conductivity but other effects such as electron-wall interaction seem to play a very important role. Electron collisions with the dielectric walls can enhance the conductivity in SPTs. Because the B field is perpendicular to the walls, the electron current is forced to the walls and secondary electron emission can occur for electron energies greater than about 30 eV on these surfaces. We have performed Monte Carlo calculations to study the effect of reflection and secondary emission on the calculated conductivity.