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When an intense relativistic electron beam is injected into an unmagnetized plasma, the plasma develops into a strong Langmuir turbulence state, and high-power broadband microwave radiation is emitted. The emission source has been confirmed to be the modulated beam interacting with the caviton fields. Total power of this radiation depends on the beam-to-plasma density ratio almost in the same way as the level of the beam modulation. The present paper indicates that by performing two and a half-dimensional particle-in-cell simulations, this dependence comes from the dependence of activity of the beam electron-plasma electron two-stream instability on the beam-to-plasma density ratio. Index Terms--Beam modulation, intense relativistic electron beam, microwave radiation, particle-in-cell (PIC) method, strong Langmuir turbulence.

High-power broad-band millimeter-wave radiation is emitted from a plasma in a strong Langmuir turbulence state driven by an intense relativistic electron beam. We measured directivity and spectrum of this radiation with a filter-bank spectrometer, a heterodyne spectrometer, and a filter-waveguide-combination spectrometer covering 18-140 GHz. The directivity measurement indicated that the radiation was relativistically beamed. The observed spectra were nearly flat up to about 40 GHz and declined steeply above 40 GHz. Discussion is given on the experimental results in connection with the collective Compton boosting model proposed by Benford and Weatherall. Index Terms - Beam bunching, broad-band millimeter-wave radiation, heterodyne spectrometer, IREB, strong Langmuir turbulence.

It is observed that the length of a coaxial cavity strongly affects the induced gap voltage and the level of automodulation. For high-frequency automodulation for a beam with slow rise-time, a series of cavities with decreasing lengths is proposed. Index Terms--Automodulation, coaxial cavity, current modulation, relativistic electron beam, subnanosecond.

It has been shown experimentally that in the two-stage autoacceleration scheme the acceleration voltage at the second-stage can be increased by using two cavities with the same length as the second-stage instead of using a single-cavity. Index Terms--Autoacceleration, coaxial cavity, intense relativistic electron beam, pulse compression.

Experiments on four-stage autoacceleration were carried out to generate a subnanosecond, intense relativistic electron beam (IREB). An annular electron beam with energy of 500 keV, current of 5 kA, and pulse length of 12 ns was injected into a series of four coaxial cavities with decreasing lengths. The energy and pulse length of the most accelerated part of the beam electrons were 1.1 MeV and 0.8 ns, respectively. The transmission line theory that was used to explain single-stage autoacceleration process was found to be applicable to the multistage autoacceleration process. Index Terms--Autoacceleration, coaxial cavity, intense relativistic electron beam, superradiance.

Strong Langmuir turbulence fields driven by an intense relativistic beam were probed using a weak electron beam after the intense beam passed through a plasma, Persistence of the strong Langmuir turbulence state was proved by observing broadening of the profile of the probe beam after it traversed the plasma column. Index Terms--Caviton, electron beam probing, intense relativistic electron beam (IREB), strong Langmuir turbulence, wake fields.

Two-stage autoacceleration was experimentally realized by passing an annular electron beam through a two-stage passive cavity structure. The kinetic energies of the last quarter part in the beam pulse were increased. Possibility Of compression of an intense beam using multistage autoacceleration is discussed. Index Terms: Autoacceleration, coaxial cavity, relativistic electron beam.