Initial results of the PBX-M experiment

The Modified Princeton Beta Experiment (PBX-M) is designed to provide access to the second stability regime by utilizing the stabilizing properties of operation with minimum B and strong indentation. The distinctive features of PBX-M as compared to its predecessor device, PBX, are the ability to operate at higher indentations (up to 30%) and the presence of conducting passive stabilizer plates almost completely surrounding the plasma surface. The purpose of the plates is to stabilize ideal n=1 surface modes which are believed to be partially responsible for the first regime β-limit. In addition, PBX-M can operate in a closed divertor mode, thus allowing for H-mode operation with enhanced confinement properties that will aid in the goal of second stability. Plasmas with indentations of up to 25% have been successfully formed during the initial period of operation. Furthermore, disruption-free operation could be carried out to qmhd values down to 2.0 even with indentations > 15%. Distinctive observed MHD phenomena included long period sawteeth in Ohmic and NBI plasmas without strong current ramps, and a lower power threshold for transition into the H-mode. These preliminary results suggest some beneficial effects of operation with higher indentation and passive stabilizers. (author). 5 refs, 8 figs