Experimental equilibrium and stability studies of a linear high-beta I = 1 stellarator

A high-beta linear l = 1 stellarator plasma is produced in a low-compression 3-m theta pinch (the High-Beta Q Machine) whose compression coils are modified to have a helical offset of 2 cm, and an axial helical period of 40 cm. Detailed internal magnetic-probe measurements, in conjunction with axial interferometry and excluded flux measurements, were used to measure the spatial structure and temporal behavior of field profiles, plasma pressures, current, and magnetic-axis location, which correspond to the predictions of ideal MHD theory. It is found that the higher-temperature, lower-density quasi equilibria are perturbed by a stable m = 1, k ≅ O oscillatory mode for which magnetic-probe measurements allow characterization of the internal-mode structure. This is compared with a plasma-fluid model which includes finite ion-Larmor-radius effects. Cooler, higher-density plasmas, having near-sharp-boundary profiles are also produced and are observed to be exponentially unstable (growth time of approximately 0.5 μsec) to an m = 1, k ≅ O mode. The stability behavior of these two types of plasmas is understood from the theory with finite-Larmor radius effects. The higher-temperature diffuse profiles provide stabilization. At the lower temperatures, the profiles are near-sharp-boundary with β ≅ 1, and the mode becomes MHD unstable