Search Results (7 total)

Coherent Smith-Purcell (SP) radiation originating from three different gratings has been measured at End Station A, SLAC, and has been used to reconstruct the time profile of the electron bunches. The beam energy during these experiments was 28.5 GeV (γ≅55 773) and the number of electrons in the bunch was 0.9–1.4×10¹⁰. The spectral distribution of the radiated energy was measured by means of an array of 11 pyroelectric detectors. Typical values of the FWHM of the bunch length are about 2.5 ps, but sharper peaks with FWHM less than 2.0 ps have also been observed. The longitudinal profile also varies with accelerator conditions and can best be approximated by a superposition of 3–4 Gaussian curves. Some typical profiles are presented, together with a discussion of the limitations and strengths of coherent SP radiation as a diagnostic tool. It is concluded that SP radiation offers excellent prospects in this respect, not only in the picosecond range, but potentially in the femtosecond range as well.

We have used coherent Smith-Purcell radiation in order to investigate the longitudinal (temporal) profile of the electron bunch at the FELIX facility. Detection of the far-infrared radiation was achieved by a simple and compact experimental arrangement, consisting of an array of 11 room-temperature pyroelectric detectors. Accurate determination of the background radiation, use of high quality optical filters, and an efficient light collection system are essential for this type of experiment. The radiated power is in good agreement with the predictions of the surface current description of this process. It is concluded that 90% of the bunch particles are contained within 5.5 ps, with a temporal profile that could be approximately triangular in shape.

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 diffraction grating mounted in the electron beam focal region of a scanning electron microscope has been used to produce superradiant emission over the 300–900 μm wavelength range. Feedback is provided by the grating itself and the electron beam is focused and positioned over the grating by the microscope's electron optical system. Extensions of this technique promise a new tunable, coherent, cw source for the difficult to access far infrared (30–1000 μm) range of the spectrum.

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.

A beam of 3.6-MeV electrons has been used to study the generation of radiation in the far infrared (FIR) by the Smith-Purcell mechanism. The dependence of wavelength on angle of emission, over angles from 56° to 150° and wavelengths from 350 to 1860 μm, is in excellent agreement with the Smith-Purcell dispersion relation. Comparison of the yield with that from a 5000-K source suggests that the spontaneous Smith-Purcell effect offers an easily tunable alternative to the synchrotron as a coherent FIR source, and that it could also form the basis of an inexpensive, compact free-electron laser.

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.