A low-frequency magnetic mode is observed to grow in a line-tied axisymmetric mirror with increasing plasma β up to βc as predicted by ideal-MHD ballooning-mode theory. The instability is excited like a standing wave in a region of unfavourable curvature and steep radial pressure gradient, travelling outward from the mirror well with a velocity near the Alfvén one for β ⪅ βc, but close to the sound velocity for β < βc. It propagates azimuthally around the ion-diamagnetic drift frequency with m = 2 for β ⪅ βc, while appearing to propagate in the electron diamagnetic direction for β < βc. Results are consistent with transitional structrues from resistive to ideal ballooning modes.
Confinement and ballooning stability at the beta limit