Observations of an Ion-Driven Instability in Non-Neutral Plasmas Confined on Magnetic Surfaces

The first detailed experimental study of an instability driven by the presence of a finite ion fraction in an electron-rich non-neutral plasma confined on magnetic surfaces is presented. The instability has a poloidal mode number $m=1$, implying that the parallel force balance of the electron fluid is broken and that the instability involves rotation of the entire plasma, equivalent to ion-resonant instabilities in Penning traps and toroidal field traps. The mode appears when the ion density exceeds approximately 10% of the electron density. The measured frequency decreases with increasing magnetic field strength, and increases with increasing radial electric field, showing that the instability is linked to the $E\ifmmode\times\else\texttimes\fi{}B$ flow of the electron plasma. The frequency does not, however, scale exactly with $E/B$, and it depends on the ion species that is introduced, implying that the instability consists of interacting perturbations of ions and electrons.