MHD induced neutral beam ion loss from NSTX plasmas

Bursts of ∼60 kHz activity on Mirnov coils, identified as energetic particle modes, occur frequently in NSTX plasmas and these are accompanied by bursts of neutral beam ion loss over a range in pitch angles. These losses have been measured with a scintillator type loss probe imaged with a high speed (> 10 000 frames s^-1) video camera, giving the evolution of the energy and pitch angle distributions of the lost neutral beam ions over the course of the events. The instability occurs below the TAE frequency in NSTX (∼100 kHz) in high beta plasmas and may also be identified as a beta-induced Alfvén acoustic (BAAE) mode. The data from one burst that causes a 13% reduction in the neutron rate have been studied extensively and have several interesting features. First, the burst begins with the mode having a purely n = 1 character, sweeping downwards in frequency. However, there is no change to the underlying loss signal during this time interval. This indicates there is no phase space transport of fast (80 keV D) ions into the loss cone seen by the probe from the frequency sweeping. As the burst evolves further in time, a concurrent n = 2 mode arises, followed quickly by a concurrent n = 3 mode. During this period when multiple modes are present, loss over a wide range of pitch angles is seen simultaneously, suggesting stochastization of the beam ion phase space. There is no evidence of any sweeping in pitch angle of the loss in either phase of the burst, at least not on the 100 νs time scale.