Model of the boundary layer of a vacuum-arc magnetic filter

F. MINOTTI; L. GIULIANI,; D. GRONDONA; H KELLY

Santiago

Conferencia; ICPP International Congress on Plasma Physics LAWPP XIII LatinAmerican Workshop on Plasma Physics; 2010

CCHEN

A model is developed to describe the electrostatic boundary layer in a positively biased, straight magnetic filter to explain the current-voltage characteristics observed in the experiments in filtered vacuum arcs. In order to explain the observations Bohm diffusion across the magnetic field for the electrons is used. The set of equations used includes the electron momentum equation, including anomalous collisions due to micro-instabilities leading to Bohm diffusion, electron mass conservation, and Poisson equation. Assuming a Boltzmann distribution for ions, and using as independent variable the electrostatic potential, a reduced set of two equations is obtained, which can be numerically integrated with a single eigenvalue related to the electric field in the pre-sheath. This value is obtained by the condition of vanishing electron density at the filter wall. In this way a single value of the potential drop in the boundary layer is obtained for each set of plasma density, ion and electron temperatures, and magnetic field value, from which the electric current collected by the filter can be evaluated. Comparison is made with the voltage-current characteristic measured in a straight magnetic filter used with a pulsed copper vacuum arc with an annular anode.