Global confinement and edge transport measurements in the MST reversed-field pinch

Several approaches have been used to improve global confinement in the MST reversed-field pinch (R = 1.5 m, a = 0.5 m, I < 750 kA) including solid-target boronization, circuit modifications and hydrogen pellet injection. Of these, boronization has been the most dramatic, resulting in long periods free of major sawteeth in many discharges during which global energy confinement improves by over a factor of two. The sawtooth-free periods usually terminate in strong relaxation events. A large confinement database confirms that β increases with density at fixed current up to a maximum of 10% near the radiation limit. An extensive series of edge fluctuation-induced transport measurements have been made in low-current discharges. Electrostatic fluctuations can account for much of the edge particle flux with Deff ∼ 50 m2/s, but account for less than 10% of the energy flux (χeff ∼ 60 m2/s); however, the latter measurement is insensitive to ions and fast electrons. Magnetic fluctuations are found to result in small, ambipolar particle flux and corresponding small energy flux carried by fast electrons. (author) 7 refs., 2 figs., 2 tabs