Electromagnetic emission from a neutral-beam-injected plasma

Results obtained from experiments in PDX with neutral beam injection show the emission of several harmonics of a frequency centred in the ion cyclotron frequency range. The structure of the magnetic signals registered is suggestive of an electromagnetic ion cyclotron type instability. A theory is developed for explaining the generation of neutral-beam-driven ion cyclotron instability (ion Bernstein mode) and the results are compared with the experimental data. An increase in beam density towards the outside of the plasma is possible if the beam ion population in that region increases owing to either counter-injection orbits or 'fishbone' effects. A calculation is made of the threshold density necessary to drive such an instability in a plasma with anisotropic beam ion distribution, and the effects due to the curvature drift of hot ions are included. The quasi-linear evolution of the beam is studied in order to understand possible beam isotropization and spreading in velocity space. (author)