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We have theoretically and experimentally investigated the superradiance from a bunch of electrons rotating in a homogeneous magnetic field. A RADAN-303B modulator equipped with a subnanosecond pulse slicer has been used to generate high current subnanosecond electron bunches (250 kV, 0.1-1 kA, 0.3-0.5 ns). Transverse momentum was imparted to the electrons by a kicker. It is shown that for the experimental observation of cyclotron superradiance from high current electron bunches the optimum conditions are the conditions of group synchronism, when the translational velocity of the bunch coincides with the group velocity of the radiation propagating in the waveguide. In the 35 GHz range microwave pulses with record short duration, down to 0.4 ns, with a peak power level up to 200 kW, have been obtained. Index Terms - Cyclotron emission, cyclotron maser, free electron devices, high power microwaves, microwave pulses, super-radiance.

The influence of the Q-factor of a two-mirror Bragg resonator on the efficiency and spectrum of radiation was studied both theoretically and experimentally for the JINR-IAP millimeter-wave free-electron laser (FEL)-oscillator driven by a linear induction accelerator (1 MeV/200 A/200 ns). In accordance with theoretical predictions, single-frequency operation in a low (optimal) Q-factor resonator and multifrequency operation in a high Q-factor resonator were observed. For an optimal resonator configuration at the frequency 31 GHz, single-mode generation with power of approximately 35 MW and efficiency of 26% was obtained. A significant drop in FEL efficiency was observed for multifrequency generation. Index Terms - Bragg resonators, free-electron laser, microwave radiation, reversed guide magnetic field, single-mode and multimode generation.

Results of theoretical and experimental studies of free-electron laser (FEL)-oscillators with guide magnetic field of both conventional and reversed orientation are presented. It is shown that in regimes far from the cyclotron resonance (including reversed guide field regimes) the quality of the electron beam formed in slowly up-tapered wiggler entrance is better than in regimes close to the cyclotron resonance. As a result, FEL operating in the reversed guide field regime possesses a rather small sensitivity to the initial spread of beam parameters. A capability of high-efficiency FEL operation in reversed guide field regime is corroborated experimentally, and a record for millimeter-wavelength FEL-oscillators efficiency of 26% is obtained. In accordance with the simulations in conventional guide field regimes, under a magnetic field smaller than the resonance value, the process of RF-generation was accompanied with beam current losses caused by electron fall-out on the wall of the transporting channel. Index Terms - Free-electron laser, helical wiggler, microwave radiation, reversed and conventional guide magnetic field.

All-optical ultrafast time-resolved plasma diagnostics of plasma-based accelerators (PBA's) are described, with emphasis on the laser Wakefield accelerator (LWFA). Specifically, the diagnostic techniques involve replacing the trailing particle bunch in the LWFA with a trailing photon bunch: a weak ultrashort laser pulse. Since this photon pulse is derived directly from the intense pump pulse, practical difficulties such as synchronization and dephasing are eliminated. The interaction of the photon bunch with the plasma wake is essentially a simple time-domain shift in optical phase, which can produce both 'dc' phase shifts and frequency blue/red-shifting of the probe pulse spectrum. These phase/frequency shifts are recorded in frequency domain interferograms, which are formally equivalent to time-domain holograms. Experimental results of longitudinal plasma density profiling are presented in which plasma density oscillations (Langmuir waves) in the wake of an intense ([I.sub.peak] ~ 3 x [10.sup.17] W/[cm.sup.2]) laser pulse (~100 fs) were measured with ultrafast time resolution. Phase shifts consistent with large amplitude (~80%) density oscillations at the electron plasma frequency were observed in a fully tunnel-ionized He plasma, corresponding to longitudinal electric fields of ~10 GV/m. Strong radial ponderomotive forces enhance the density oscillations. Finally, proposed single-shot schemes for simultaneous transverse and longitudinal profiling are discussed.

Experimental measurements of coherent stimulated radiation from intense, subnanosecond electron bunches moving through a periodic waveguide and interacting with a backward propagating [TM.sub.01] wave are presented. The ultra-short microwave pulses in Ka, W, and G band were generated with repetition frequencies of up to 25 Hz. Observation of RF breakdown of ambient air, as well as direct measurements by hot-carrier germanium detectors, gives an estimate of the peak power up to 140 MW for the 300-400 ps pulses at 38 GHz. The initial observation of 75 GHz 10-15 MW radiation pulses with duration less than 150 ps, and of 150 GHz microwave spikes with a risetime of 75 ps are also reported. Index Terms--Backward-wave oscillator, microwave pulse, subnanosecond pulse, superradiance.