Design and Electronic Compensation of a Diamagnetic Loop and Its Application in the ASDEX Tokamak

摘要

Owing to the perpendicular motion of electrons and ions the plasma excludes a small amount of diamagnetic flux (created by the plasma itself) of the total toroidal flux. Measuring the diamagnetic flux allows determination of the diamagnetic beta poloidal, defined as the ratio of transverse plasma pressure to magnetic pressure at the plasma surface. The perpendicular component of beta poloidal is measured by a separate loop mounted outside of the stainless-steel vacuum vessel. The design and mounting of the loop are described. As in tokamaks one has a ratio of diamagnetic flux to total toroidal flux of about 10 sup -4 , even very slight magnetic stray fields, coil displacements and eddy currents in the structure during the discharge pulse can influence the diamagnetic signal. We show the separate interference of vertical, ohmic-heating, divertor, radial and toroidal fields, and finally the effect of misalignment with respect to the plasma current. We also describe the method and the electric system for compensating these spurious effects. The final section presents experimental results which demonstrate the operation of the diamagnetic loop during the ohmic-heating and neutral-injection phases. (ERA citation 11:042340)