One of the reasons improved confinement in MST can be described as “tokamak-like” is the increase of the total beta (plasma pressure/magnetic field energy density) to ~15% with Ohmic heating alone, which is as large (or larger) than the total beta demonstrated in advanced tokamak plasmas with powerful auxiliary heating. A beta limit for the RFP remains to be identified experimentally, and that achieved is thought to be transport limited. Plasma beta in the RFP may be limited by several effects. The limit set by the ideal MHD interchange instability is rather high (varying from about 50% at reversal parameter F = -1 to about 15% at F = 0). At high beta, the pressure gradient can be a significant contributor to resistive tearing instability. Initial results from nonlinear MHD computation indicate that at beta of about 25% the pressure can be the dominant drive for tearing modes. In addition, resistive interchange modes are expected to be unstable in the RFP, although their nonlinear behavior is relatively unstudied. We are initiating a study of pressure effects on both large-scale and small-scale fluctuations through nonlinear MHD.
The experimental effort to determine the beta limit on MST will employ external heating of the plasma, primarily neutral beam injection . Results of a recent experiment with a short-pulse neutral beam demonstrated that the beam-generated fast ions slow down and deposit most of their energy in the plasma prior to being transported to the wall, despite the magnetic stochasticity present in a standard RFP plasma. This implies that neutral beam heating will be effective in heating an RFP plasma, perhaps allowing us to experimentally reach the beta limit and observe the resulting instability. Heating will also allow us to vary key dimensionless parameters such as the Lundquist number, magnetic Prandtl number (normalized viscosity), and collisionality.