Fast Ion Effects on Magnetic Instabilities in the Pdx Tokamak

A study of the modification and excitation of nondisruptive magnetic instabilities due to near perpendicular neutral beam injection on the PDX tokamak has been made to determine the importance of these instabilities at low q. The instabilities consisted of resistive MHD modes, beam driven ideal MHD modes, and beam driven ion cyclotron modes. Evidence of enhanced transport (beyond the anamolous transport associated with auxiliary heating) is presented for several of these instabilities as well as comparison of the experimental results with theory. The main results can be summarized as follows: (1) The loss of electron thermal energy due to sawteeth oscillations, although small, increases with increasing auxiliary power and decreasing q, but it was suppressed with high power perpendicular injection due to a loss of the beam ions. (2) m = 2 resistive modes have a disastrous effect on the achieved density, but at present power levels, they can be controlled. (3) Evidence from Mirnov coils, soft x-ray emission, neutron emission, and fast charge-exchange flux indicates that a degradation of the global energy confinement above (beta)(,T)q (DBLTURN) 0.045 is due to a rapid loss of beam ions from the bulk plasma ("fishbone instability"). Scalings of the data indicate that this loss occurs due to a destabilizing resonance between an internal kink mode and the precessing beam ions. The flux of escaping ions was studied using a charged particle detector calibrated for the fast deuterons injected by the beam and an array of high frequency electromagnetic coils. Bursts in the beam ion flux and the rf emission were found to be correlated with drops in the neutron emission and bursts of 20 kHz magnetic oscillations (fishbones). (4) A quiescent level of rf emission was observed to be correlated with increases in the density of trapped beam ions at the outer edge of the plasma. The harmonic structure of this instability was the same as that during fishbones (harmonics of (OMEGA)(,ci) at the magnetic field at the plasma edge). Possible consequences for reactor type tokamaks and high power auxiliary heating systems are discussed.