Magnetic cusps have been used for more than 50 years to limit charged particle losses to the walls and confine the plasma in a large variety of plasma sources or ion sources. Quantification of the effective loss area has been the subject of many experimental as well as theoretical investigations in the 1970-1990's. In spite of these efforts there is no fully reliable expression of the effective wall loss as a function of cusp magnetic field, electron temperature, ion mass, gas pressure, etc… We describe in this paper a first attempt at obtaining scaling laws for the effective loss width of magnetic cusps, based on two-dimensional PIC MCC (Particle-In-Cell Monte Carlo Collisions) simulations. The results show that the calculated leak width follows a 1/B scaling in the collisionless low B limit, is approximately proportional to the hybrid gyroradius with an ion velocity equal to the Bohm velocity and is proportional to the square root of gas pressure in the collisional limit.
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.
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