PIC modeling of negative ion sources for fusion

Abstract

This work represents the first attempt to model the full-size ITER negative ion source prototype including expansion, extraction and part of the acceleration regions keeping the resolution fine enough to resolve every single aperture of the extraction grid. The model consists of a 2.5-dimensional Particle-in-Cell/Monte Carlo Collision representation of the plane perpendicular to the filter field lines. Both the magnetic filter and electron deflection fields have been included. A negative ion current density of produced by neutral conversion from the plasma grid is used as fixed parameter, while negative ions produced by electron dissociative attachment of vibrationally excited molecules and by ionic conversion on plasma grid are self-consistently simulated. Results show the non-ambipolar character of the transport in the expansion region driven by electron magnetic drifts in the plane perpendicular to the filter field. It induces a top-bottom asymmetry detected up to the extraction grid which in turn leads to a tilted positive ion flow hitting the plasma grid and a tilted negative ion flow emitted from the plasma grid. As a consequence, the plasma structure is not uniform around the single aperture: the meniscus assumes a form of asymmetric lobe and a deeper potential well is detected from one side of the aperture relative to the other side. Therefore, the surface-produced contribution to the negative ion extraction is not equally distributed between both the sides around the aperture but it come mainly from the lower side of the grid giving an asymmetrical current distribution in the single beamlet.