Search Results (9 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.

In this paper we report on the numerical calculations and experimental testing of a nonadiabatically electrostatic pumped beam. These initial investigations are intended to lay the groundwork for generating a high-quality electron beam, which will be used for cyclotron autoresonance maser driving. The present gun geometry produces a beam at a voltage of 500 kV in a 30-ns pulse, with total power between 150 and 300 MW The experimental data characterizing the beam properties are compared with numerical simulation results, indicating that the gun has operated essentially as predicted by theory. © 1996 The American Physical Society.

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.

We report the observation of coherent emission from a Cherenkov free-electron laser driven by a 5-MeV radio-frequency microtron. Power up to 50 W in pulses of 4-μs duration has been generated at the wavelengths of 1.6 and 0.9 mm using two different dielectric-loaded waveguides.

The double-slab waveguide configuration for a Čerenkov free-electron laser is discussed. It is shown that it may allow higher emission brightness with respect to the single-slab configuration.

The off-axis symmetry of the F_A center in KCl:Li⁺ has been investigated by optical experiments. We have studied static and dynamic reorientational properties of the F_A center by measurements of absorption and emission. The off-axis model of the center has been confirmed. A theoretical treatment, which takes into account the small deviation of the center with respect to the crystal axes, is in good agreement with the experimental results. Best fittings of the theory to the experimental data yield a value of the off-axis angle theta=8°.

The low-temperature optical conversion of F centers in NaBr and NaI into defects with absorptions at equal or higher energies was studied under magnetic fields up to 80 kG. A strong reduction of the optical conversion was observed under fields giving clear evidence that the reaction product of the conversion is an F^′ center with two electrons in a singlet state, the formation of which becomes forbidden in spin-polarized F-center systems. This magnetic field quenching of the F^′ conversion ("Porret-Luty effect"), however, is incomplete even under full spin polarization indicating the existence of some loss mechanisms. To clarify the latter, we measured the spin-memory loss during the optical cycle by monitoring the magnetic circular dichroism (MCD) under known pumping conditions, obtaining spinmixing parameters of ε=0.03 and 0.12 for NaBr and NaI, respectively. These ε values can explain only in part the incompleteness of the magnetic field effect, hinting towards the existence of a bound triplet F^′ state. From the MCD measurement, the spin-lattice relaxation time of the F ground state as a function of magnetic field and the spin-orbit—splitting parameter Δ of the F center were determined too.