W.E. Stein

Results of the Los Alamos free-electron laser experiment

A free-electron laser (FEL) amplifier experiment to test the performance of a tapered wiggler at high optical power has been successfully completed. A well-separated two-component electron energy distribution has been obtained that is characteristic of a tapered wiggler. Energy distribution spectra and extraction efficiencies have been measured as a function of initial electron energy, energy spread, emittance, optical power, and spatial and temporal misalignments of the laser and electron beams. A maximum efficiency of ∼ 4 percent was measured, and good agreement of efficiency with a one-dimensional theory was obtained.

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.

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.

Lasing on the third harmonic

Abstract The Los Alamos free-electron laser has recently lased near 4 μm on the third harmonic of the fundamental frequency of about 12 μm. By a choice of intercavity apertures and cavity length, lasing can be forced to occur on both frequencies simultaneously or on either one alone.

Experimental results from the Los Alamos FEL photoinjector

The authors report some initial measurements of electron beam properties from the new photoinjector installed as the front end on the Los Alamos free-electron laser (FEL). The FEL is being rebuilt with the photoinjector, added acceleration to 40 MeV, new diagnostics, and a beam line designed to minimize emittance growth. The authors measured the spatial and temporal properties of the beam at energies of about 15 MeV as a function of several parameters and the results have been compared to simulations. The operational characteristics of the important elements of the system and the theoretical comparisons are described. >

Initial results from the Los Alamos photoinjector-driven free-electron laser☆

Abstract We report initial results on the APEX (APLE prototype experiment) photoinjector-driven infrared free-electron laser (FEL). The APEX FEL is operating in support of a Boeing Aerospace and Electronics/Los Alamos National Laboratory collaboration to build the average power laser experiment (APLE). Our system uses a high quantum efficiency (3–7%) multi-alkali photocathode, illuminated with a frequency-doubled Nd:YLF mode locked laser at 21.7 MHz. The photocathode is located in this first cell of a six-cell 1.3 GHz, 6 MeV photoinjector that feeds a linac with a final energy up to 40 MeV. Because the illuminating laser pulse on our photocathode is short (10 ps), no pulse compression is required in the linac. Emittance measurements made after the second linac tank at 15 MeV have shown that a normalized emittance (for 90% of the particles) of less than 50π mm mrad can be achieved at a peak micropulse current of 300 A. Our initial lasing has been at a wavelength of 3.6 μm over a 30 μs macropulse with an electron beam energy of 35 MeV and a 2.7 cm period permanent magnet wiggler. We are continuing to characterize and optimize our system, with particular emphasis on understanding and minimizing electron beam emittance-growth mechanisms, and subsequently improving the quality of the beam delivered to the wiggler.

The accelerator for the Los Alamos free-electron laser - IV

The design parameters and performance of the accelerator used with the Los Alamos free-electron laser are described. Special emphasis is placed on those features of the accelerator that affect emittance, energy spread, and micropulse temporal shape.

Energy recovery in the Los Alamos free electron laser

Abstract Experiments to demonstrate recovery in conjunction with the Los Alamos free electron laser are reported in this paper. Deceleration of the electron beam greater than 70% has been observed. Beam transport through the system down to 3.5 MeV has been obtained and power flow measurements have been made that demonstrate the conversion of beam energy back into rf power. The resonant bridge couplers appear to function as designed. Predicted instabilities in the beam transport system have been observed.

Performance of the Los Alamos HIBAF accelerator at 17 MeV

Abstract The Los Alamos free electron laser (FEL) is being rebuilt with a photoelectric injector and 40 MeV beam energy for a lower emittance, brighter beam. Tests of the Los Alamos high-brightness accelerator FEL (HIBAF) system have been conducted including the photoinjector and first 17 MeV of acceleration. The photoinjector is designed to operate with a micropulse charge of 5 nC, peak current of 300 A. energy spread of 0.3%, and emittance of 50π mm mrad. Measurements of temporal and spatial beam characteristics have been made up to 10 nC per micropulse for both single micropulses and macropulses. The operational characteristics of the system components, e.g., drive laser and photoinjector are described. The results of beam and rf measurements are briefly presented and compared to simulation. The effects of nonaxially symmetric rf fields in the on axis coupled accelerator structure were observed and are discussed.

High extraction efficiency experiments with the Los Alamos free electron laser

Abstract 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 in amplifier experiments and in 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.