Electrostatic Ion Waves Modified by Parallel Ion-Velocity Shear

The objective of this work is the experimental study of the effect of inhomogeneous magnetic-field-aligned (parallel) ion drift on the destabilization and propagation of electrostatic ion waves. Such inhomogeneous ion velocity is created in the cylindrical barium plasma column produced in a Q-machine. Noninvasive measurements of the ion velocity distribution are performed using the laser-induced fluorescence technique. Multi-harmonic ion cyclotron waves are identified, and experimental evidence is presented that ion cyclotron damping can become inverted to result in net wave growth. For other experimental conditions, low frequency ion acoustic waves are identified, characterized and compared favorably with theory. These results provide experimental support for the inhomogeneous-parallel-ion-flow-model interpretation of electrostatic ion-cyclotron and ion-acoustic waves observed in the auroral region. The results also broaden fundamental plasma physics concepts such as ion-cyclotron damping to include the possibility of inverse ion-cyclotron damping.