EXCITATION OF HIGH-FREQUENCY PRESSURE-DRIVEN MODES IN NON-AXISYMMETRICAL EQUILIBRIUM AT HIGH BETA(POL) IN PBX-M

High frequency pressure driven modes have been observed in high poloid al beta discharges in the Princeton Beta Experiment Modification (PBX- M). These modes are excited in a non-axisymmetric equilibrium characte rized by a large, low frequency m(l) = 1/n(l) = 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 m(h) = n(h) or (le ss probably) m(h)/n(h) = m(h)/(m(h) - 1), with m(h) varying between 3 and 10. Occasionally these modes are simultaneously localized in the v icinity of the m(l) = 2/n(l) = 1 island. The high frequency modes near the q = 1 surface also exhibit a ballooning character, being signific antly stronger on the large major radius side of the plasma. When a la rge m(l) = 1/n(l) = 1 island is present, the mode is poloidally locali zed in the immediate vicinity of the X point of the island. The modes occur exclusively in high beta beam heated discharges and are likely t o be dri\en by the beam ions. They can thus be a manifestation of eith er a toroidicity induced shear Alfven eigenmode (TAE) at q = (2m(h) 1)/2n(h), a kinetic ballooning mode, or some other type of pressure dr iven (high beta) mode. Most of the data are consistent with the theore tical predictions for the TAE gap mode. Since the high frequency modes in PBX-M, however, are found exclusively on or in the immediate neigh bourhood of magnetic surfaces with low rational numbers (q = 1, 2,...) , other possibilities are not excluded.