Controlled axial density gradient formation for studies of modified, driven electromagnetic fluctuations in ALEXIS*

The Auburn Linear Experiment for Instability Studies (ALEXIS) is a 2m long, 10 cm diameter, inductively coupled, linear plasma device that is designed to support a variety of plasma instabilities and waves. Recent studies in ALEXIS have focused on the production and detection of electrostatic and electromagnetic waves in the ion cyclotron to lower hybrid frequency regimes and then using a biased cylindrical electrode to change local plasma conditions to modify wave propagation. Experiments have been performed in which applying a bias voltage to the cylindrical electrode has been demonstrated to systematically vary the local plasma parameters. It has been observed that certain portions of the plasma column become significantly brighter or dimmer when the electrode is biased, corresponding to a modification of the axial density profile in the plasma. Probes were used to make plasma measurements and it was found that at low RF input power (10W) and large negative bias voltages (<-60V) the electron density decreased by an order of magnitude in the dark regions of the plasma column. Spectroscopic measurements were also made along the entire plasma column to fully characterize the effect of biasing the can electrode. The results of these measurements are presented.