Thermal magnetic fluctuations in plasmas near the electron cyclotron frequency

Abstract

Summary form only given. In a large dense laboratory plasma the thermal fluctuation spectrum between the lower hybrid frequency and multiples of the electron cyclotron frequency has been measured with shielded magnetic loop antennas. A 1/f-like noise spectrum is observed in the whistler wave regime of a Maxwellian afterglow plasma. Cross-spectral measurements show that the noise consists of propagating whistler waves filling the resonance cone with equal probability in all allowed propagation directions. The randomly moving electrons induce shot noise in the magnetic antenna rather than exciting eigenmodes by Cherenkov radiation as in the whistler wave regime. During the active discharge the plasma is non-Maxwellian due to the presence of a shell-distributed tail of energetic electrons. The fluctuation spectrum now exhibits lines at the electron cyclotron frequency and its harmonics. Up to 15 cyclotron harmonics are observed for high pitch angle electrons generated by an electron beam injected into a Maxwellian afterglow plasma. The fluctuations are found to consist of ballistic modes of spiralling electrons with thermal motions. A line spectrum emerges from the broadband thermal spectrum.