Influence of plasma parameters on the effectiveness of multi-cusp magnetic field confinement in negative ion sources

Abstract

Cusp-shaped magnetic fields are widely used to confine plasmas in various applications. This field configuration allows to localise plasma losses: the width of such loss cone, usually called leak width, was found to be proportional to the geometric mean of the ion and electron Larmor radii, so that it becomes smaller for increasing magnetic field intensity. At the same time, plasma diffusion towards the walls is reduced in the regions where the field lines are parallel to the surfaces, leading to the formation of a Plasma Exclusion Zone (PEZ) whose characteristic dimension was found to be proportional to the distance between the magnets. Besides field intensity and geometry, the confinement effectiveness might also be affected by plasma properties such as electron temperature, plasma potential across the sheath and collisionality. Specifically, this contribution describes a numerical analysis performed by means of a 2D-3V Particle-In-Cell code of the dependencies of both the PEZ size and the leak width on magnetic field intensity and plasma potential, focusing on typical conditions found in plasma sources for negative ion beams. The leak width was found to be larger than its theoretical prediction regardless of the specific plasma parameters. The PEZ was found to be affected not only by the distance between the magnets. In particular, the plasma potential was found to strongly affect the plasma behaviour within the cusp region.