Excitation of high frequency pressure driven modes by non-axisymmetric equilibrium at high βpol in PBX-M

High-frequency pressure-driven modes have been observed in high-poloidal-β discharges in the Princeton Beta Experiment-Modification (PBX-M). These modes are excited in a non-axisymmetric equilibrium characterized by a large, low frequency m1=1/n1=1 island, and they are capable of expelling fast ions. The modes reside on or very close to the q=1 surface, and have mode numbers with either mh=nh or (less probably) mh/nh=mh/(mh-1), with mh varying between 3 and 10. Occasionally, these modes are, simultaneously localized in the vicinity of the m1=2/n1=1 island. The high frequency modes near the q=1 surface also exhibit a ballooning character, being significantly stronger on the large major radius side of the plasma. When a large m1=1/n1=1 island is present the mode is poloidally localized in the immediate vicinity of the x-point of the island. The modes, which occur exclusively in high-β discharges, appear to be driven by the plasma pressure or pressure gradient. They can thus be a manifestation of either a toroidicity-induced shear Alfven eigenmode (TAE) at q=(2mh+ 1)/2nh, a kinetic ballooning mode (KBM), or some other type of pressure-driven mode. Theory predicts that the TAE mode is a gap mode, but the high frequency modes in PBX-M are found exclusively on or in the immediate neighborhood of magnetic surfaces with low rational numbers