This thesis presents the first detailed study of an instability driven by the presence of a finite ion fraction in an electron-rich non-neutral plasma, confined on magnetic surfaces. The mode appears when the ion density exceeds approximately 10% of the electron density, and the measured frequency dependences imply that the mode consists of resonantly interacting perturbations of electrons and ions. In the Columbia Non-neutral Torus (CNT), the instability has a poloidal mode number m=1, which does not correspond to a rational surface. This implies that parallel force balance of the electron fluid is broken. A global instability that does not resonate with a rational surface has never before been observed in a stellarator.

The experiments show that the mode persists even for large magnetic field strengths, except when the ion species is hydrogen. For the case of hydrogen, substantial suppression of the mode is seen at high magnetic fields. A numerical code was written which integrates ion trajectories in the stable equilibrium of CNT. Comparing the code results with the observed mode behavior, it was found that the plasma is unstable as long as the ion Larmor radius is not much smaller than the size of the plasma. When the Larmor radius is small the ion density profiles are very hollow in CNT. The code results also indicate that the ion rotation frequency ω_i in CNT had a broad spectrum; this is an important difference from the ion motion in a simple cylindrically symmetric electron plasma, where the ions all have a single rotation frequency. This code result is consistent with experimental observations in CNT—the mode can be exited over a broad range of frequencies.