Observation of ballooning modes in high-temperature tokamak plasmas

The beta-degradation phase of a high-${\mathrm{\beta }}_{\mathit{p}}$ plasma in the TFTR tokamak is analyzed with x-ray and electron cyclotron emission imaging techniques. Medium-n (toroidal mode number) instabilities with ballooning characteristics are observed near and within the q=1.5 surface during a slow degradation in the plasma β and precede a sudden partial collapse in the central plasma pressure. This is the first reported observation of a ballooning instability in the interior of a large, collisionless tokamak plasma.