Diamagnetic measurements on ISX-B: method and results

Abstract

A diamagnetic loop is used on the ISX-B tokamak to measure the change in toroidal magnetic flux, sigma phi, caused by finite plasma current and perpendicular pressure. From this measurement, the perpendicular poloidal beta ..beta../sub I perpendicular to/ is determined. The principal difficulty encountered is in identifying and making corrections for various noise components which appear in the measured flux. These result from coupling between the measuring loops and the toroidal and poloidal field windings, both directly and through currents induced in the vacuum vessel and coils themselves. An analysis of these couplings is made and techniques for correcting them developed. Results from the diamagnetic measurement, employing some of these correction techniques, are presented and compared with other data. The obtained values of ..beta../sub I perpendicular to/ agree with those obtained from the equilibrium magnetic analysis (..beta../sub I..delta../) in ohmically heated plasmas, indicating no anisotropy. However, with 0.3 to 2.0 MW of tangential neutral beam injection, ..beta../sub I..delta../ is consistently greater than ..beta../sub I pependicular to/ and qualitatively consistent with the formation of an anisotropic ion velocity distribution and with toroidal rotation. Quantitatively, the difference between ..beta../sub I..delta../ and ..beta../sub I perpendicular to/ is more than can be accounted for on the basis of the usual classical fast ion calculations and spectroscopic rotation measurements.