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 1 = 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 of2.7 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 i near the magnetic axis can be varied from 0.47 to 0.59> so that the presence and the location of 1 = 112 surface can be controlled. The vertical field experimentally required to obtain a specified 1 value on the axis agrees with that from numerical calculation including the error field produced by feeder bar currents. The eddy currents induced in the vacuum chamber and the coil supports are found to cause the vertical and horizontal shift of magnetic surfaces. The influence of error fields due to various causes on magnetic island formation is investigated by the numerical calculations. The earth field and the error fields caused by feeder bar currents and by coil misalignment produce large islands around the I = 1/2 surface when it is present.