Initial results from the rebuilt EXTRAP T2R RFP device

The EXTRAP T2R thin shell reversed-field pinch (RFP) device has recentlyresumed operation after a major rebuild including the replacement of thegraphite armour with molybdenum limiters, a fourfold increase of theshell time constant, and the replacement of the helical coil used forthe toroidal field with a conventional solenoid-type coil.Wall-conditioning using hydrogen glow discharge cleaning wasinstrumental for successful RFP operation. Carbon was permanentlyremoved from the walls during the first week of operation. The initialresults from RFP operation with relatively low plasma currents in therange Ip = 70-100 kA are reported. RFP dischargesare sustained for more than three shell times. Significant improvementsin plasma parameters are observed, compared to operation before therebuild. There is a substantial reduction in the carbon impurity level.The electron density behaviour is more shot-to-shot reproducible. Thetypical density is ne = 0.5-1×1019 m-3. Monitors of Hα line radiation indicate thatthe plasma wall interaction is more toroidally symmetric and that thereis less transient gas release from the wall. The minimum loop voltage isin the range Vt = 28-35 V, corresponding to areduction by a factor of two to three compared to the value before therebuild.