Interaction of a density modulated electron beam with a magnetized plasma: Emission of whistler waves

Abstract

Whistler wave excitation in a magnetized laboratory plasma by a density modulated electron beam is studied for frequency modulation below, but in the range of, the electron gyrofrequency. When the beam velocity differs from the phase velocity of the whistler at the frequency modulation of the beam, a whistler wave, and, by Čerenkov emission, a beam wave at the same frequency are excited as single poles. When both velocities are equal, a resonance occurs leading to a maximum in the emission of whistler waves. The experiment performed in a magnetized plasma (np≊1011 cm−3, Te≊0.2 eV, B≊100 G) with a modulated electron beam (nb≊109 cm−3, Eb≊300 eV) is in agreement with a simple model which predicts the characteristics of the excited waves.

Effect of beam premodulation on excitation of surface plasma waves in a magnetized plasma

“…The normalized growth rate of the unstable wave has the value of 13.8 and the growth rate is 4.07ϫ 10 10 rad/ s when k Z ϳ k OZ and ϳ o , i.e., when the phase velocity of the wave is comparable to the premodulated beam velocity. Krafft et al 18 have shown in their experimental observations that the maximum emission of the whistler waves occurred when the phase velocity was equal to the beam velocity. The normalized growth rate ␥ / pb is also plotted as a function of modulation index ⌬ in Fig.…”

“…Krafft et al 17 have done the study of the energy transfer between the modulated spiraling electron beam and the whistler wave. Krafft et al 18 have studied emission of whistler waves by a density modulated electron beam in a laboratory plasma and results have been compared to the excitation by loop antenna. In this case the fraction of the power of the modulated electron beam radiated by the whistler wave has been measured and is of the order of 10 −5 .…”

“…There has been a great deal of interest in studying electrostatic 14 and electromagnetic [15][16][17][18][19] waves by modulated electron beams. Sharma et al 14 have examined the effect of beam density modulation on excitation of lower hybrid waves in a magnetized plasma cylinder.…”

Whistler waves produced by a modulated spiraling electron beam: Linear approach

“…Moreover, we suppose that the beam, injected at a finite pitch angle keeps its thickness unchanged, much less than the Larmor radius, forming a spiral structure. Such a structure was considered in many theoretical works [13][14][15][16] and observed in laboratory experiments, 9,18 as well as in space. It is clear that during the interaction of beam electrons with induced electromagnetic fields, this spiral structure will be spread.…”

“…The interest to the problem of whistler wave excitation in space is connected with some recent experiments in space and laboratory. [2][3][4][5][6][7][8][9][10] In the present paper, we consider the oblique injection case; thus we have to take into account both Č erenkov and Doppler resonances.…”