Magnetic confinement and instability in partially magnetized plasma

Abstract

Discharge with an external magnetic field is promising for various applications of low-temperature plasmas from electric propulsion to semiconductor processes owing to high plasma density. It is essential to understand plasma transport across the magnetic field because plasma confinement under the field is based on strong magnetization of light electrons, maintaining quasi-neutrality through the inertial response of unmagnetized ions. In such a partially magnetized plasma, different degrees of magnetization between electrons and ions can create instability and make the confinement and transport mechanisms more complex. Theoretical studies have suggested a link between the instability of various frequency ranges and plasma confinement, whereas experimental work has not been done so far. Here, we experimentally study the magnetic confinement properties of a partially magnetized plasma considering instability. The plasma properties show non-uniform characteristics as the magnetic field increases, indicating enhanced magnetic confinement. However, the strengthened electric field at the edge of the plasma column gives rise to the Simon-Hoh instability, limiting the plasma confinement. The variation of the edge-to-center plasma density ratio (h-factor) with the magnetic field clearly reveals the transition of the transport regime through triggering of the instability. Eventually, the h-factor reaches an asymptotic value, indicating saturation of magnetic confinement.