Excitation of parametric instabilities by electron cyclotron waves in a plasma column

Experimental results are presented illustrating parametric instabilities with thresholds as low as a fraction of a watt. Two types of instabilities are observed under different plasma conditions. The type I instability has a broad sideband below the pump and a corresponding low frequency acoustic component. The type II instability occurs at higher neutral pressure, where a 30% dip in density on the column axis is created and has an upper sideband with harmonic structure and no observable low frequency components. Wavelength measurements of each wave components are made to compare with parametric theory. The time domain evolution of the sidebands in the form of an rf burst is observed and discussed. (author)