Search Results (5 total)

We report the first observation of enhanced coherent emission of terahertz radiation in a compact free electron laser. A radio-frequency (rf) modulated electron beam is passed through a magnetic undulator emitting coherent radiation at harmonics of the rf with a phase which depends on the electron drift velocity. A proper correlation between the energy and phase distributions of the electrons in the bunch has been exploited to lock in phase the radiated field, resulting in over 1 order of magnitude enhancement of the coherent emission.

Coherent enhancement of the Smith-Purcell radiation produced from the interaction of a 1.8 MeV electron beam with a grating has been observed. The emitted radiation has been measured at angles in the 40° to 120° range, which correspond to wavelengths from 0.65 to 4 mm, approximately. The radiated power was 320 mW at 90°. Its angular distribution agrees well with the description of the process in terms of induced surface currents and has been used to infer the longitudinal profile of the electron bunch. It is concluded that the bunch has an approximately triangular profile, with 85% of the bunch particles contained within 14 ps. The possibilities of the technique as a bunch-shape diagnostic tool are also discussed.

A radio-frequency (rf) modulated electron beam passing through a magnetic undulator emits coherent radiation at harmonics of the rf with a phase which depends on the electron drift velocity. Treating the bunches as ensembles of electrons, each with its energy and phase, the radiated field is calculated as a sum over the particle distribution in the phase space. At long wavelengths a proper correlation between the energy and phase distributions of the electrons in the bunch can be exploited to lock the radiated field in phase, resulting in a significant enhancement of the coherent emission.

The waveguide operation of a compact free-electron laser driven by a 2.3-MeV S-band microtron is reported. Power up to 1 kW in 4-μs pulses has been generated at wavelengths in the range between 2.1 and 2.6 mm. Novel tuning features and temporal characteristics of the emitted radiation, related to the bunched nature of the electron beam and to the dispersive effect of the waveguide, have been observed.

The photostimulated reorientation of F_A centers in RbCl:Li⁺ has been studied by means of luminescence and absorption measurements. Experimental data of both stationary and transient emission intensities have been interpreted on the basis of a theoretical model of the center reorientation. Such a treatment, already applied to similar F_A centers in KCl:Li⁺, takes into account the influence on the reorientation process of two different phenomena: the overlap between the F_A1 and F_A2 absorption bands, where the luminescence is excited, and the off-axis configuration of the F_A dipoles. The results allow for the first time a quantitative determination of the off-axis angle of F_A centers in RbCl:Li⁺: θ≃7°.