Experimental Equilibrium and Stability Studies of a Linear High-Beta L = 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 pressure, current, and magnetic axis location, which correspond to the predictions of ideal MHD theory. It is found that the higher temperature, lower density quasiequilibria are perturbed by a stable m = 1, k (DBLTURN) 0 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 (mu)sec) to an m = 1, k (DBLTURN) 0 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 (beta) (DBLTURN) 1, and the mode becomes MHD unstable.
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- Physics: Fluid and Plasma