Dependence of Ion Cyclotron Instabilities on Particle Distribution Functions in the Alice Experiment

Plasma formed in an open-ended system by energetic particle injection is subject to instability because of the departure of the trapped -ion distribution function from the Maxwellian form. The objective of the Alice neutral injection experiment is to study the stabilizing effects of changes in the ion and electron distribution functions. A series of perturbations and alterations of the distribution functions is in progress. The results of the first of these perturbations are reported here. The plasma is contained in a quadrapole magnetic well generated by a coil in the shape of a baseball seam. Protons are trapped by Lorentz -force ionization of excited atoms in the injected beam, forming a steady-state plasma. At densities above 108/cm3, intermittent oscillations have been observed both at the ion gyrofrequency, ωci, and at higher frequencies. At these densities observed oscillations are accompanied by anomalous loss of particles; bursts of electrons and ions have been detected emerging through the minor regions. At a density of 5 x 108/cm3, ω2pi/ω2ci is of order 0.5, where ωpi is the ion plasma frequency. At this density the mean anomalous loss and charge-exchange loss times (∼0.5 s) are comparable. Energy modulation of the injected beam by 20% results in a broadening of the energy spectrum of the trapped protons. As a consequence, an increase of about 25% in the threshold density for onset of the plasma oscillations has been observed in this case. A more pronounced effect on the high-frequency activity has been observed by alteration of the electron distribution. Application of a few watts of microwave radiation at a frequency corresponding to the electron gyrofrequency in the magnetic field near the region of peak ion density results in a decrease in the activity at wci and an increase in trapped-proton density. However, when the microwave frequency is adjusted to correspond to a different region of the plasma, enhanced activity at the ion gyrofrequency is observed together with increased proton losses. (author)