Edge density profile variation during a magnetic axis shift in the CHS Heliotron/Torsatron

The edge density profile was measured during natural and forced magnetic axis shift in a low-aspect-ratio heliotron/torsatron CHS, using an 8 keV fast neutral lithium beam probe. The Shafranov, shift of the magnetic axis (and hence the dislocation of the LCFS) that was negligible for a low density ECH discharge became substantial (∼ 4.5 cm) for a high density NBI discharge (< ne > ∼ 2.5 x 1013 cm-3) and the corresponding radius of the LCFS increased about 1.2 cm in the major radius direction compared to the vacuum case. For NBI discharges with different settings of the vacuum magnetic axis Rax (fixed during the discharge), the measured edge density profiles indicated reasonable agreements between the theoretically and experimentally obtained LCFS radii for 90 cm < Rax < 101.6 cm while for Rax < 90 cm the measured radius was 10% larger than expected. When a change of Rax from 94.7 cm to 89.9 cm during a discharge was imposed externally, a well behaved plasma boundary moved inward smoothly by about 7.5 cm, while the steepness of the edge density profile changed for different values of Rax. the steepest profile was attained for Rax = 92.1 cm when highest energy content and average density were achieved. (author). 15 refs, 8 figs