Detailed spectra of high power broadband microwave radiation from interactions of relativistic electron beams with weakly magnetized plasmas

摘要

We observe prodigious quantities of microwave energy uniformly across a wide frequency band when a relativistic electron beam penetrates a plasma. Typically we measure 20 MW total for Δυ = 40 GHz with preliminary observations of bandwidths as large as 100 GHz. We fire an intense annular pulsed REB (I ≃128kA; r ≃ 3cm; Δr ≃ 1 cm; 50ns FWHM; Y ≃ 3) through an unmagnetized or weakly magnetized plasma column (np ∼ 1013cm−3). We use 0.01 < nbp < 2, the higher values of this range being an unconsidered region for most previous theoretical and experimental efforts. For these high nbp values, the observed emission with ω >> ωp and weak harmonic structure is wholly anticipated from Langmuir scattering or soliton collapse models. A model of Compton-like boosting of ambient plasma waves by the beam electrons, with collateral emission of high frequency photons, qualitatively explains our spectra. Frequencies up to ∼ Y2θ2ωp should be emitted with substantial power, where θ is the angle between beam electrons and plasma waves. For our experiment, ψ > 0.5 radians. Power emerges largely in an angle ∼ l/y, as required by Compton mechanisms. As nbp falls, we observe ωp − 2ωp structure and harmonic power ratios consistent with soliton collapse theories. With further reduction of ωp only the ωp line persists. Thus we have observed a transition in spectral behavior from the weak to strong turbulence theories advocated for Type III solar burst radiation, and further into a regime we characterize as super-strong REB-plasma interactions.