Magnetic confinement and instability in E×B Penning source: Discovery of high-magnetic-confinement mode

Understanding instability is essential to effectively generate high-density plasma and deliver a stable ion beam current in several applications of E×B sources, such as the semiconductor process to electric propulsion. Gradient-drift driven instabilities originating from ambipolar electric fields aligned with density gradients perpendicular to the magnetic-field line have recently received renewed attention in linear instability studies, and comprehensive considerations have been proposed as a part of the theory of the gradient-drift instability. Notably, partially magnetized plasmas have a chamber dimension comparable to the Larmor radius of their ions, implying that boundary effects are important in the formation of instabilities. We focused on the importance of the boundary effect and proposed the breaking of spatially symmetric non-ambipolar flow. By using a positively biased additional electrode, we effectively broke the spatial symmetry of the global flow of electrons and ions. Finally, in the steady-state, the plasma density tends to peak in the plasma core, approaching plasma densities that are four times larger than those observed in the case where the instability is the strongest. We have verified the transition into a strong-magnetic-confinement mode by measuring the edge-to-center density ratio (h-factor) that estimates the degree of plasma loss to the boundary. A high-magnetic-confinement mode with a reduced h-factor of 0.16 is observed, which demonstrates that the saturation of magnetic confinement due to the gradient-drift driven instability can be prevented by an asymmetric nonambipolar flow.