Experimental Investigation of m=1 Diocotron Mode Growth at Low Electron Densities
Authors: Stephen F. Paul, Kyle Morrison, and Ronald C. Davidson
Date of PPPL Report: August 2003
Presented at: the 2003 Workshop on Non-neutral Plasmas, 7-11 July 2003, Santa Fe, New Mexico.
Previous experiments on the Electron Diffusion Gauge showed that the diocotron mode damping increases with higher neutral gas filling pressure. Yet the energy dissipated from a rotating plasma by collisions with neutrals is predicted to excite the mode. To resolve this, experiments have been conducted to examine the coupling between expansion and the m=1 diocotron mode. Results from recent experiments have shown interesting phenomena: (1) The degree and sensitivity of mode growth is observed to be strongly dependent on filament conditions. Mode growth rates of nearly 20 sec-1 have been observed even with negligible resistive drive. Specifically, at low filament bias voltages (and correspondingly low electron densities ~1-2 x 107 electrons/cm), the mode growth is very sensitive to the heating voltage across the filament, even though changes in filament heating voltage barely affect the plasma expansion, the plasma density profile, the filament emission, or the resulting electron density. (2) At low neutral gas pressure (<10-9 Torr), the diocotron mode growth rate increases with neutral pressure. However, the growth rate is several orders of magnitude larger than theoretical predictions.