Confinement of Nonneutral Plasmas in a Trap Composed of a Cusped Magnetic Field and an Electrostatic Octapole Field

Abstract

The field configuration formed by the superposition of a cusped magnetic field and an electrostatic octapole field provides a closed system of confinement for a charged particle. In a cusped magnetic field, the Störmer region which constrains a charged particle is open, but it is closed by adding a potential barrier made by the octapole field. One-component plasmas are thus expected to be confined in this configuration, preserving superior characteristics of the cusp field for plasma stability. A preliminary experiment was performed on the confinement of electrons in this field configuration. An electron plasma was confined for 3 s in a magnetic field as weak as B=50 G at the circular line cusp. The confinement time was roughly proportional to B2, suggesting that the confinement would be improved substantially in a higher magnetic field.