John C. Goldstein

Optical performance of the Los Alamos free-electron laser

During a year of oscillator experiments, the Los Alamos free-electron laser has demonstrated high-power and diffraction-limited output capabilities with a factor-of-4 wavelength tunability in the infrared. A conventional, L -band RF linear accelerator produced a 100 μs long, 2000 pulse train of 35 ps wide electron-beam pulses with peak currents to 50 A and nominal energy of 20 MeV. Small-signal gain in excess of 40 percent was generated in a 1 m, plane-polarized, uniform-period undulator for wavelengths between 9 and 11 μm. Best performance included an electron-energy extraction efficiency of 1 percent, 10 MW peak output power, and a corresponding average power of 6 kW over a 90 μs pulse train. A Strehl ratio of 0.9 characterized the output spatial beam quality. By reducing the electron energy by a factor of 2, the wavelength was tuned continuously from 9 to 35 μm.

Raman spectra and the Los Alamos free-electron laser

Spectra have been obtained of the light generated near 10 μm by the Los Alamos free-electron laser. The spectra contain a main line and copious sidebands, mostly to the long-wavelength side of the main line. The spectra vary in overall width as the detuning parameter is altered or as the sidebands are preferentially allowed to escape the optical cavity. A comparison is made between these measurements and the results of numerical simulations.

Near-ideal lasing with a uniform wiggler

Abstract Over the years the Los Alamos FEL team has reduced or eliminated many of the experimental problems that resulted in non-ideal lasing. The major problems were accelerator instabilities that cause noise and fluctuations in current, energy, and timing; wakefield effects in the wiggler and beamline that introduce fluctuations in the beams energy; and mirror nonlinearities 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 in that it lacks major disturbing effects and is limited 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 sideband 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 sidebands. With detuning one can easily switch operating modes from that giving the highest efficiency (chaotic spiking) to that giving the narrowest spectral line (no sidebands). Alternative techniques for sideband 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 this extra optical component and that exerted by the energy of the electron beam. Lasing, therefore, starts easily, a shift in wavelength, i.e., chirp, is easily accomplished, and the consequences of inadequate control of the electron beam energy are not severe.

Spiking mode operation for a uniform-period wiggler

The onset of saturation in a uniform-period wiggler has been examined experimentally and through numerical simulations. Models have been constructed that explain the observations in simple and consistent ways. The models are based upon the development of strong frequency and amplitude modulation of the optical wave as a way to increase extraction efficiency and optical power.

The Los Alamos free electron laser oscillator: Optical performance ☆

Abstract During nearly a year of oscillator experiments, the Los Alamos free electron laser has demonstrated high-power and diffraction-limited output capabilities with continuous wavelength tunability in the infrared. A conventional L-hand rf linear accelerator produced a 100-μs-long, 2000-pulse train of 35-ps-wide electron-beam pulses with peak currents to 50 A and nominal energy of 20 MeV. Small-signal gain in excess of 40% was generated in a 1-m, plane-polarized, uniform-period undulator for wavelengths between 9 and 11 μm. Best performance included an electron-energy extraction efficiency of ∼1%, 10-MW peak output power, and a corresponding average power of 6 kW over a 90-μs pulse train. A Strehl ratio of 0.9 characterized the output spatial beam quality.

Demonstration of ultraviolet lasing with a low energy electron beam

Abstract We report on the design details of the first ultraviolet (UV) free-electron laser (FEL) oscillator driven by low-energy electrons from a radio-frequency linear accelerator. In our experiment we used a high-current, high brightness electron beam in combination with a wiggler of novel design to produce an FEL that lased at wavelengths from 369 to 380 nm using 45.9–45.2 MeV electrons. In addition we performed a proof-of-principle experiment that demonstrated the first ever photolithography on a photoresist-coated silicon wafer using an FEL light source.

First lasing of the regenerative amplifier FEL

The Regenerative Amplifier Free-Electron Laser (RAFEL) is a high-gain RF-linac FEL capable of producing high optical power from a compact design. The combination of a high-gain and small optical feedback enables the FEL to reach saturation and produce a high optical power and high extraction efficiency without risk of optical damage to the mirrors. This paper summarizes the first lasing of the Regenerative Amplifier FEL and describes recent experimental results. The highest optical energy achieved thus far at 16.3 {micro}m is 1.7 J over an 9-{micro}s macropulse, corresponding to an average power during the macropulse of 190 kW. They deduce an energy of 1.7 mJ in each 16 ps micropulse, corresponding to a peak power of 110 MW.

Recent results from the Los Alamos free-electron laser

In this paper, we review the most recent experimental results of the Los Alamos free-electron laser program. Three major efforts will be described: lasing at improved efficiency over that previously attained, electron beam improvement, and energy recovery. An extraction efficiency of 2 percent was achieved with a wiggler having a 12 percent wavelength taper. The beam has been improved so that limits to its quality are now caused, not by injector performance, but by wake fields related to the high peak currents achieved. Limits to optical power are set by mirror damage. Experiments are described that demonstrate the successful operation of an energy-recovery system.

Initial performance of Los Alamos Advanced Free Electron Laser

Abstract The Los Alamos compact Advanced Free Electron Laser (AFEL) has lased at 4.7 and 5.2 μm with a 1-cm period wiggler and a high-brightness electron beam at 16.8 and 15.8 MeV, respectively. The measured electron beam normalized emittance is 1.3 π mm mrad at a peak current of 100 A, corresponding to a beam brightness greater than 2 × 10 12 A/m 2 rad 2 . Initial results indicate that the AFEL small signal gain is ∼ 8% at 0.3 nC (30 A peak). The maximum output energy is 7 mJ over a 2-μs macropulse. The AFEL performance can be significantly enhanced by improvements in the rf and drive laser stability.

Conceptual designs of a 50 nm FEL oscillator and a 20–40 nm SASE amplifier☆

Abstract This paper consists of two parts; (1) the conceptual design, and optical performance characteristics, of a grazing angle of incidence ring resonator utilizing multifaceted metal mirrors for use with a 50 nm rf linac driven XUV FEL oscillator, and (2) electron beam and wiggler requirements for a self-amplified spontaneous emission (SASE) amplifier to produce high power in the 20–40 nm wavelength range. The basis for these studies is the 3- d FEL simulation code FELEX which, in part (1), is used to derive tolerances on mirror figure and thermal distortion, alignment sensitivity, and alternative output coupling methods. In part (2), the sensitivity of the output characteristics of an XUV FEL SASE amplifier to wiggler field errors is also studied.