Modeling of negative ion extraction from a magnetized plasma source: Derivation of scaling laws and description of the origins of aberrations in the ion beam

• Published in: Physics of plasmas 2018-02, Vol.25 (2)
• Main Authors: Fubiani, G., Garrigues, L., Boeuf, J. P.
• Format: Article
• Language: English
• Subjects: Aberration; Chamfering; Computer simulation; Curvature; Ion beams; Ion extraction; Mathematical models; Monte Carlo simulation; Negative ions; Origins; Particle in cell technique; Perveance; Physics; Plasma; Plasma density; Plasma Physics; Positive ions; Scaling factors; Scaling laws
• ISSN: 1070-664X; 1089-7674
• DOI: 10.1063/1.4999707

We model the extraction of negative ions from a high brightness high power magnetized negative ion source. The model is a Particle-In-Cell (PIC) algorithm with Monte-Carlo Collisions. The negative ions are generated only on the plasma grid surface (which separates the plasma from the electrostatic accelerator downstream). The scope of this work is to derive scaling laws for the negative ion beam properties versus the extraction voltage (potential of the first grid of the accelerator) and plasma density and investigate the origins of aberrations on the ion beam. We show that a given value of the negative ion beam perveance correlates rather well with the beam profile on the extraction grid independent of the simulated plasma density. Furthermore, the extracted beam current may be scaled to any value of the plasma density. The scaling factor must be derived numerically but the overall gain of computational cost compared to performing a PIC simulation at the real plasma density is significant. Aberrations appear for a meniscus curvature radius of the order of the radius of the grid aperture. These aberrations cannot be cancelled out by switching to a chamfered grid aperture (as in the case of positive ions).