Cyclotron harmonic lines in the thermal magnetic fluctuation spectrum of spiraling electrons in plasmas

Abstract

Radio frequency (rf) magnetic fluctuations B have been measured with loop antennas in a large pulsed discharge plasma column (ne≲1012 cm−3, kTe≲3 eV, B0≂20 G, Ar, 2×10−4 Torr, 1 m diam×2.5 m length). A 1/f‐like noise spectrum is observed in the whistler wave regime (ωce1/2ωci1/2<ω<ωce) both in the Maxwellian afterglow plasma and in the active discharge which contains energetic (45 eV) electrons. Discrete emission lines at the electron cyclotron frequency and its harmonics are found only in the presence of spiraling energetic electrons. These are naturally present in the active discharge but have also been injected as a controlled oblique electron beam into the Maxwellian afterglow plasma. In the latter case up to 15 cyclotron harmonic lines with weak amplitude decay Bz(ω) are generated in the beam flux tube. From two‐point correlation measurements it is shown that the line spectrum is due to ballistic beam modes rather than plasma eigenmodes driven unstable by the beam. The lines evolve from broadband t...