MHD instabilities near theβlimit in the doublet III-D tokamak

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Abstract
Saturation or slow collapse of β is observed in DIII-D divertor discharges at high-values of normalized β [(β/(I/aB)≥2.4]. The sequential appearance of MHD instabilities with poloidal/ toroidal mode numbers m/n=5/4, 4/3, 3/2, and 2/1 at progressively larger minor radii leads to the saturation or collapse of β. Only n=2 and n=1 modes affect global energy confinement. A model is suggested in which the instabilities originate as high-n ballooning modes near the center of the discharge, progressing to lower-n modes by modification of pressure and current profiles.