High Power, Very Broadband Microwave Radiation from the Interaction of a Relativistic Electron Beam with Plasmas in the Low Magnetic Field Regime, with Application to the Type III Solar Burst Phenomenon.

We observe prodigious quantities of microwave energy uniformly across a very wide frequency band when a relativistic electron beam penetrates a plasma. Typically we measure 20 MW total in the calibrated range and estimate as high as 350 MW total in our full observational range for (DELTA)f (TURNEQ) 100 GHz. We fire an intense, annular, pulsed REB (I (TURNEQ) 128 kA; r (TURNEQ) 3 cm; (DELTA)r (TURNEQ) 1 cm; 50 ns FWHM; (gamma) (TURNEQ) 3) through an unmagnetized or weakly magnetized plasma column (n(,plasma) (TURN) 10('13) cm('-3)). We use 0.01 (LESSTHEQ) n(,beam)/n(,plasma) (LESSTHEQ) 2, the higher values of this range being an unconsidered region for most previous theoretical and experimental efforts. For these high n(,b)/n(,p) values, the observed emission with (omega) >> (omega)(,p) and weak harmonic structure is wholly unanticipated from Langmuir scattering or soliton collapse models. A model of Compton-like boosting by the beam electrons of ambient plasma waves, with the collateral emission of high frequency photons, qualitatively explains our observed spectra. Power emerges largely in an angle (TURN) l/(gamma), as required by Compton mechanisms. As n(,b)/n(,p) falls, we observe (omega)(,p)-2(omega)(,p) structure and harmonic power ratios consistent with soliton collapse theories. With further reduction of n(,b)/n(,p) only the (omega)(,p) 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. We also observe the presence of radiation for (omega)(, )<(, )(omega)(,p) after the beam is off, and interpret this "afterglow" radiation as due to the scattering of short wavelength Langmuir waves which are the parametric decay products of beam-excited, long-wavelength Langmuir waves. For frequencies slightly below the broadband region, an (omega)(,p) line is observed with high power ((TURN) 1 MW); the line disappears in an external B(,z) (TURN) 400 G. Changing (gamma)(,b) over a range of 2.2-3.7 has little effect on the spectra.