Recent results of the Los Alamos free-electron laser oscillator experiment

Since the last Lasers '83 Conference, significant progress has been made on the Los Alamos free-electron laser (FEL). Although the previously reported results were in agreement with theory, the data were plagued by gain fluctuations, and no spectral measurements had been obtained. Since then, the source of the fluctuations has been analyzed and substantially reduced. Also, the optical power and spectra dependence on beam emittance, alignment, cavity length, energy spread, and current were systematically studied. This paper reports on the results obtained with a uniform-period wiggler. 3 refs., 14 figs., 2 tabs

Proposed extended tuning range for the Los Alamos mid-infrared adjustable, coherent light experiment (MIRACLE) Facility

The Los Alamos Free-Electron Laser (FEL) Facility has been in operation as an oscillator in the 10..mu..m wavelength regime since 1983. Operations from 10 to 45..mu..m have been recently demonstrated which would provide a new applications capability: A Mid-Infrared Adjustable Coherent Light Experiment (MIRACLE) Facility. We propose to extend this tunability from 3 to 160..mu..m by upgrading the injector, accelerator, and resonator cavity. Potential applications in material science (high-temperature superconductors) and biophysics (DNA spectroscopy) for this wavelength regime are briefly addressed. 7 refs., 2 figs., 2 tabs

Multiple-shot laser damage thresholds of ultraviolet reflectors at 248 and 308 nanometers

Multiple-shot damage thresholds of dielectric reflectors have been measured at 248 and 308 nm. Standard irradiation conditions were a 10-ns pulsewidth, 0.6-mm spot diameter and 35-Hz pulse repetition frequency. The reflectors, from various sources, were composed of oxide and fluoride films. Although damage was generally initiated at visible film defects, there was no correlation between damage susceptibility and the appearance of these defects. At levels near threshold, damage was most often observed as an increase in white-light scatter of a site with no growth upon continued irradiation; at higher levels, the damage site grew with successive shots. Test sites were subjected to at least 10/sup 3/ shots and some sites received as many as 2.5 x 10/sup 4/ shots; however, with only one exception damage was found to occur within the first few shots or not at all. Reflectors at 248 nm typically had damage thresholds in the 1.0 to 1.8 J/cm/sup 2/ range with two samples exhibiting unexpectedly high thresholds of 2.8 and 3.0 J/cm/sup 2/. In some cases, a subthreshold pre-irradiation treatment resulted in a 20 to 25% enhancement in damage resistance

Multiple-shot ultraviolet laser damage resistance of nonquarterwave reflector designs for 248 NM

The damage resistance of multilayer dielectric reflectors designed for 248 nm has been substantially increased by use of nonquarterwave (QW) thicknesses for the top few layers. These designs minimize the peak standing-wave electric field in the high-index layers, which have proven to be weaker than the low-index components. Previous damage tests of infrared- and visible-wavelength reflectors based on these designs have produced variable results. However, at the ultraviolet wavelength of 248 nm, 99% reflectors of Sc/sub 2/O/sub 3/, MgF/sub 2/, and SiO/sub 2/ strongly demonstrated the merit of non-QW designs. Four sets of reflectors of each of four designs (all QW thickness; one modified-pair substitution; two modified-pair substitution; one modified pair plus an extra half-wave layer of Sc/sub 2/O/sub 3/) were tested for damage resistance with a KrF laser operating at 35 pps with a pulsewidth of 8 ns and spot-size diameter of 0.6 mm. Each of 50 sites were irradiated for 1000 shots or until damage occurred. On the average, the reflectors with one-modified-thickness pair had a 50% higher threshold (10 to 10 sites survived) than the all-quarterwave design. Addition of a second modified-layer pair resulted in no further increase in threshold but the saturation fluence (10 of 10 sites damage) was 110% higher. Reflectors with an additional half-wave of Sc/sub 2/O/sub 3/ had lower thresholds of the order of 10% as expected. The thresholds correlated best with peak-field models, whereas the best model correlating the saturation fluences involved the sum of the upper two scandia layer thicknesses

Angular dependence of multilayer-reflector damage thresholds

The damage resistance of HfO/sub 2//SiO/sub 2/ multilayer dielectric reflectors was measured as a function of angle of incidence with 351-nm XeF-laser irradiation. The laser produced nominal 10-ns pulses at a repetition rate of 35 pps. A series of reflectors designed for 0/sup 0/, 30/sup 0/, 45/sup 0/, 60/sup 0/, 75/sup 0/, and 85/sup 0/ was tested with an S-plane polarized beam. To account for variations in the separate coating depositions, some of the coating designs were tested at two angles of incidence. At large angles of incidence, we did not observe the anticipated large increases in damage thresholds predicted theoretically on the basis of spatial dilution (1/costheta) of the intensity at the reflector surface and standing-wave electric fields. For example, the threshold for a reflector designed and tested at 85/sup 0/ was only a factor of 2.5 larger than that of normal-incidence reflectors tested at 0/sup 0/. Several possible mechanisms to explain this discrepancy were considered. 11 refs

LASL free electron laser experiment

The accomplishment of a high-power, free-electron laser (FEL) is dependent on efficient transfer of electron energy to the laser electromagnetic field and efficient recovery of the remaining electron energy. Electron energy transfer to the laser field occurs in a wiggler. In the past, wigglers with a uniform period have been used. Recently, however, it has been proposed that a tapered wiggler would enhance the efficiency of energy transfer. In the Los Alamos Scientific Laboratory (LASL) experiments, the efficiency of energy transfer in tapered wigglers will be studied and recovery of a portion of the electron energy will be investigated. First, an amplifier experiment will be performed to demonstrate efficient energy extraction and laser gain of a tapered wiggler. In the second experiment, the FEL amplifier will be converted to an oscillator by increasing the duration of the electron beam pulse and by adding laser mirrors. In the third experiment, a racetrack beam-transport system will be added to the FEL oscillator to determine the recovery efficiency of the remaining electron energy

XUV/VUV free-electron laser oscillator

It is shown, from computations based on a detailed theoretical model, that modest improvements in electron beam and optical mirror technologies will enable a free-electron laser, driven by an rf linear accelerator, to operate in the 50 to 200-nm range of optical wavelengths. 10 references