Studies of a Parallel Force Balance Breaking Instability in a Stellarator

An instability has been observed in non‐neutral plasmas confined on magnetic surfaces in the presence of a finite ion fraction [Phys. Rev. Letters 100, 065002 (2008)]. The dependence of the frequency and amplitude of the instability on neutral pressure, magnetic field strength, and ion species show that the mode consists of interacting perturbations of ions and electrons. In the Columbia Non‐neutral Torus (CNT) the instability has a poloidal mode number of m = 1. This does not correspond to a rational surface, implying that the parallel force balance of the electron fluid is broken. Here the diagnostic methods used to study this instability are described in detail, and some key results are shown.