Construction and vacuum-magnetic-surface measurement of modular-coil torsatron/heliotron device, SHATLET-M

Abstract

A small modular-coil torsatron/heliotron device, SHATLET-M, has been designed and constructed, and its vacuum magnetic surfaces were measured. The machine has an l=2, m=12 equivalent helical winding with a major radius of 42 cm and a minor radius of 9.6 cm. It also has vertical field coils. The helical winding comprises twelve identical one-turn modular coils designed on the basis of the windback method. These coils have been energized by pulse currents with a rise time of 0.74 or 1.8 ms and a decay time constant of 2.7 or 22 ms, respectively, allowing the effects of eddy currents in the surrounding metallic bodies to be investigated. By the magnetic surface measurement with electron-beam probes, the average radius of the outermost magnetic surface observed is found to be about 3 cm. The rotational transform l near the magnetic axis can be varied from 0.47 to 0.59, so that the presence and the location of l=1/2 surface can be controlled. The vertical field experimentally required to obtain a specified l value on the axis agrees with that from numerical calculation including the error field produced by feeder bar currents.