Los Alamos Scientific Laborato­ Ry

For 10-µm work, the optical damage threshold of some substrates is sufficiently predictable to make them useful for beam diagnostics. 1 However, as at other wavelengths, the absolute threshold for physical exposure is often dependent upon the incident spot size as well as the intensity of the incident illumination. In this Letter we present data describing the spot size dependence of the exposure threshold for two commonly used 10-µm detection materials: Kalvar 2 and type 107 Polaroid 3 films. Data are presented over the range of spot sizes from 100 µm to 600 µm.

Over the years the Los Alamos FEL team has reduced or eliminated many of the experimental problems that resulted to non-ideal lasing . The mayor problems were accelerator instabilities that cause noise and fluctuations m current, energy, and timing ; wakefield effects in the wiggler and beamline that introduce fluctuations in the beam's energy, and nurror nonlmearittes caused by free carriers produced in the mirror by the high light levels, which caused extra light losses and interfered with the diagnostics. Lasing is now thought to be ideal m that it lacks major disturbing effects and is linuted only by emittance, energy spread, and peak current. In this paper we describe the features of lasing that we have observed over a range of optical power of 1000, from the onset of lasing, to the threshold of the stdeband instability, to the organization of regular optical spikes, to the region of chaotic spikes . Cavity-length detuning is presented as an ideal technique, in most circumstances, to completely suppress stdebands. With detunmg one can easily switch operating modes from that giving the highest efficiency (chaotic spiking) to that giving the narrowest spectral line (no stdebands) . Alternative techniques for stdeband suppression normally use some kind of wavelength selective device (e.g., a grating) inserted in the cavity . With detuning, there is no need for such a device, and, therefore, no conflict between the wavelength control exerted by thus extra optical component and that exerted by the energy of the electron beam Lasmg, therefore, starts easily, a shift m wavelength, 1 e ., chirp, is easily accomplished, and the consequences of inadequate control of the electron beam energy are not severe.

The injector, radio-frequency power system, beam transport, and cavity optics of the Los Alamos free electron laser system have been significantly improved . We report here on experiments to determine the effects of these improvements on extraction efficiency and to demonstrate performance comparable to that found to amplifier experiments and m reasonable agreement with simulations. The experiments used wigglers with 12% and 30% wavelength taper. Measurements were made with and without a prebuncher and with sideband suppression accomplished by cavity-length detuning . The free electron laser produced extraction efficiencies up to 4 .4% and showed well-defined buckets of decelerated electrons.