E × B electron drift current across the aperture of an ion source surrounded by a cusped magnetic field profile

Abstract

In negative ion sources, a cusped magnetic field is generated by magnets placed around each aperture of the extraction grid in order to limit the co-extracted electron current. In spite of this suppression magnetic field, the co-extracted electron current is large, on the same order as the negative ion current extracted from the plasma. In this paper we study the mechanisms of electron extraction from the plasma through a cusped aperture in a simplified situation, in the absence of negative ions, with the help of a three-dimensional Particle-In-Cell Monte Carlo Collisions (PIC-MCC) model. The calculation results show that the electron current extracted from the plasma is small for an infinite slit aperture with suppression (cusped) magnetic field and significantly increases in the case of finite slit or circular grid apertures. We find that E × B electron drift plays an important role in the extraction of electrons through a finite slit grid aperture and that current driven micro instabilities are present in the aperture region. This work is relevant to negative ion sources and micro-ECR neutralizers designed for space propulsion.

Tutorial: Modeling of the extraction and acceleration of negative ions from plasma sources using particle-based methods

“…11(b), while it is asymmetric close to the aperture in the xy plane, which is a telltale sign of a cross B drift induced by the magnetic field of the suppression magnets. This mechanism drives the electron current outward toward the accelerator 76,147 . The difference in the plasma density on the PG surface leaves an imprint on the depth of the virtual cathode, which as a consequence, is asymmetric as well as shown in Fig.…”

“…From this position and up to the PG, the Larmor radius of the negative ions is smaller than 9 mm, which is by a factor 2 smaller than the simulation domain width. Thus, tron transport 147 , which seems to be attenuated when negative ions are present 145 ; further investigations are needed.…”

Latest experimental and theoretical advances in the production of negative ions in caesium-free plasmas

“…Drift waves rotating in the transverse direction will possibly occur in the magnetized region increasing electron transport across the filter (but still the axial electron flux reduction would be substantial). Note that the co-extracted electron current can in principle be rendered negligible with an infinite slit (or similar to the shape of a race track) surrounded by a cusp magnetic field across the aperture length [149].…”