REVERSED-FIELD PINCH STUDIES IN THE MADISON-SYMMETRICAL-TORUS

Studies of large-size (R=1.5 m, a=0.5 m), moderate current (I < 750 kA ) reversed-field pinch (RFP) plasmas are carried out in the Madison Sy mmetric Torus in order to evaluate and improve RFP confinement, study general toroidal plasma MHD issues, determine the mechanism of the RFP dynamo, and measure fluctuation-induced transport and anomalous ion h eating. MST confinement scaling falls short of the RFP scaling trends observed in smaller RFPs, although the plasma resistance is classical. MHD tearing modes with poloidal mode number m=1 and toroidal mode num bers n=5-7 are prevalent and nonlinearly couple to produce sudden rela xations akin to tokamak sawteeth. Edge fluctuation-induced transport h as been measured with a variety of insertable probes. Ions exhibit ano malous heating, with increases of ion temperature occurring during str ong MHD relaxation. The anomalous heating fraction decreases with incr easing density, such that ion temperatures approach the lower limit gi ven by electron-ion friction. The RFP dynamo has been studied with att ention to various possible mechanisms, including motion-EMF drive, the Hall effect, and superthermal electrons. The toroidal field capacity of MST will be upgraded during Summer 1993 to allow low-current tokama k operation as well as improved RFP operation.