MAGNETIC-CONFIGURATION STUDIES IN A COMPACT-TORSATRON

An overview is presented of an experimental program of magnetic field line mapping on the research-grade Compact Auburn Torsatron (CAT). The vacuum magnetic flux surfaces of the CAT device have been experimenta lly mapped in a variety of magnetic configurations. The results are co mpared with an extensive computer model in order to validate the coil design. In initial field mapping experiments, an up-down asymmetry was identified in the vacuum magnetic surfaces, and was corrected with th e use of a radial trim field. Magnetic islands are observed and their size has been reduced, also through the use of auxiliary trim coils. T he Compact Auburn Torsatron is equipped with two pairs of large Helmho ltz coils producing mutually orthogonal magnetic fields in the horizon tal plane, and two pairs of helical saddle coils wound directly on the toroidal vacuum vessel. These trim coils are used to control the size and phase of the t = 1/2 magnetic island. Through a systematic variat ion of trim field components, we demonstrate a reduction of the inhere nt t = 1/2 magnetic island size by a factor of three. The technique is applicable to correcting small error fields in larger helical confine ment devices. The measurements of island size are compared with measur ements of magnetic field line rotation within the island, and are foun d to be in good agreement with first-order perturbation theory.