Observations of odd-half cyclotron harmonic emissions in a shell-Maxwellian laboratory plasma

During the last 14 years, the subject of odd-half electron cyclotron harmonic emission from plasma has been given considerable attention. It was apparently first reported to occur in a beam plasma machine. The existence of the emission in space has been well documented through observations made with satellites. Because wavelengths are difficult to observe in space, no wave number spectrum has ever been obtained for the half-odd harmonic emission. Such a spectrum together with a frequency spectrum might provide the basis for a successful modeling of the instability. The present investigation is concerned with the design of a laboratory experiment in which a plasma with an anisotropic velocity distribution is produced and measured. In addition, the dispersion relation is directly measured, and a noise analysis is conducted. The obtained plasma, a mixture of shell and Maxwellian distributions, is found to emit waves within the Bernstein wave branches. By correlation measurements, a mode is found which is essentially an absolute instability in a narrow frequency band.