Modification of tokamak edge instability character through control of ballooning mode second stability regime accessibility

The frequency and amplitude of edge localized instabilities in DIII-D tokam ak H mode discharges are shown to depend on the discharge shape squareness. An abrupt increase in instability frequency and decreases in instability a mplitude and edge pressure gradient are observed when the squareness is cha nged to a sufficiently large or a sufficiently small value. The observed ch anges in the instability frequency and amplitude correlate with a predicted loss, at high and low; values of discharge shape squareness, of access in the H mode edge region to thr ideal infinite toroidal mode number balloonin g mode second regime of stability. When no edge region second stability reg ime access is predicted, the measured pressure gradient is in agreement wit h the predicted ballooning mode first stability regime threshold. When the second stability regime is predicted to be locally accessible, the edge reg ion pressure gradient increases to values well above the first stability re gime limit.