Los Alamos Free-electron Laser Research Articles

Measuring emittance of nonthermalized electron beams from photoinjectors

The Los Alamos free-electron laser (FEL) facility APEX is the first high-charge (greater than 1 nC), low-emittance (normalized rms emittance less than 5π mm mrad) photoinjector-driven accelerator. The rapid beam acceleration, which is essential for maintaining high brightness, also prevents the beam from thermalizing its distribution. As a result, the phase-space the electron beam occupies appears as a contorted structure with a large surface area but little volume. The transverse beam shape is mostly non-Gaussian, often with a large halo at beam waists. This shape often results in unusual effects, some of which are outlined in another paper for this conference. In this paper, we will examine how this physical property influences emittance measurement techniques, such as quadrupole scans and beam filtering thorugh tubes and slits. We will show that common emittance measurement techniques typically underestimate the rms emittance by factors of two or more. We will also discuss the beam quality relevant to FEL interaction.

Time-resolved electron-beam characterizations with optical transition radiation

Time-resolved characterizations of electron beams using optical transition radiation (OTR) as a prompt conversion mechanism have recently been extended on the Los Alamos free-electron Laser (FEL) facility 40-MeV linac. Two key timescales for rf-linac driven FELs are the mupulse (10 ps) and the macropulse (5 μs to 1 ms). In the past we have used gated, intensified cameras to select a single or few mupulses (25–400 ns gate width) out of the pulse train to evaluate submacropulse effects. Recently, we have obtained some of the first measurements of mupulse bunch length (7–10 ps) and submacropulse spatial position and profile using OTR and a Hamamatsu streak camera. Additionally, mupulse elongation effects and head-to-tail transverse kicks are reported as a function of charge.

The Los Alamos FEL photoinjector drive laser

Abstract The drive-laser system of the photoelectric injector for the Los Alamos free-electron laser (FEL) is described. The bright electron beams required for high-performance FELs demand stringent specifications on the drive-laser output levels, beam quality, and stability. The laser system generates 7–15 ps (FWHM) Gaussian pulses of 527 nm light at a repetition rate of 21.67 MHz. These “micropulses” are contained within a “macropulse” envelope less than 200 μs long, which is emitted from the drive laser at a rate of 1 Hz. Micropulse energies as large as 12 μJ are obtained. The design specification, laser architecture, and operating performance following the most recent round of upgrades are presented.

Initial electron-beam characterizations for the Los Alamos APEX facility

The ongoing upgrade of the Los Alamos Free-Electron Laser (FEL) Facility involves the addition of a photoelectric injector (PEI) and acceleration capability to about 40 MeV. The electron-beam and high-speed diagnostics provide key measurements of charge, beam position and profile, divergence emittance, energy (centroid, spread, slew, and extraction efficiency), micropulse duration, and phase stability. Preliminary results on the facility include optical transition radiation interferometer measurements of divergence (1 to 2 mrad), FEL extraction efficiency (0.6 {plus_minus} 0.2%), and drive laser phase stability (< 2 ps [rms]). 10 refs.

First dual-sweep streak camera measurements of a photoelectric injector drive laser

Abstract We have performed the first dual-sweep streak camera measurements of a photoelectric injector (PEI) drive laser in regard to micropulse duration, phase jitter, and phase slew. As the drive laser features are directly coupled to the eventual electron-beam properties, the characterization of these features is critical to optimizing free-electron laser (FEL) performance. The features have been investigated on both the intermacropulse and intramacropulse regimes at the Los Alamos FEL facility. In this initial phase we measured the doubled frequency component from the Nd:YLF-based laser system using a Hamamatsu C1587 universal streak camera with a synchroscan plug-in and the dual-sweep upgrade unit. We demonstrated that the macropulse-to-macropulse jitter was about 8 ps (rms) without laser phase stabilization and less than or equal to 2 ps (rms) with laser optimization and phase stabilization. Additionally, for the 100 μs long macropulse the phase slew within the macropulse was reduced with phase feedback from about 5 ps to near the resolution limit ( ps ).

Performance of the Los Alamos HIBAF accelerator at 17 MeV

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.

Compact photoinjector accelerators for free electron lasers

A compact photoinjector accelerator has been built for the Los Alamos free electron laser facility. This accelerator produces a very high quality beam at 6 MeV with a peak current of 300 A and an emittance < 50 π mm mrad. Two more compact photoinjector accelerators now being designed and built will incorporate features to eliminate certain problems that affected the first photoinjector accelerator. One problem, multipacting in the coupling cells, has been solved in the 6-MeV accelerator. Another problem, observed in this accelerator, is a small quadrupole component in the accelerating field, which is caused by the coupling slots. The new design will reduce this effect. This phenomenon was detectable only because the beam quality was excellent.

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. >

The Los Alamos FEL photoinjector drive laser

The drive laser system of the photoelectric injector for the Los Alamos free-electron laser (FEL) is described. This system consists of a 1.053 mu m mode-locked Nd:YLF oscillator, a grating-rhomb pulse compressor, a fast electrooptic shutter to reduce pulse repetition frequency, a pulsed Nd:YLF amplifier chain, and a KTP frequency doubler. The bright electron beams required for high performance FELs demand stringent specifications on drive laser output levels, beam quality, and stability. The design specifications, laser architecture, and operating performance following the most recent round of upgrades are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Macroclump model of the nonlinear evolution of the sideband instability in a helical wiggler free-electron laser

The nonlinear evolution of a helical wiggler, free-electron laser is investigated within the framework of a macroclump model for the trapped electrons. The model describes the nonlinear evolution of a right-circularly polarized electromagnetic wave with frequency ωs and wave number ks, and slowly varying amplitude âs(z,t) and phase δs(z,t) (eikonal approximation). The model further assumes that the trapped electrons can be treated as tightly bunched macroclumps that interact coherently with the radiation field. The analysis is carried out in the ponderomotive frame, which leads to a substantial simplification in both the analytical and numerical studies. As a first application, the nonlinear evolution of the primary signal is examined when ∂/∂l′=0 (no spatial variation of the wave amplitude and phase). The evolution equations are reduced to quadrature, and the maximum excursion of the wave amplitude âs,max is calculated analytically. Subsequently, the nonlinear evolution of the sideband instability is investigated, making use of the equations describing the self-consistent evolution of the wave amplitude âs and phase δs, which vary slowly with both space and time, together with the macroclump orbit equation. In the present analysis, the sideband signals are treated as perturbations (not necessarily small) about a constant-amplitude (â0s =const) primary electromagnetic wave with slowly varying phase δ0s(z′). The coupled orbit and field equations are investigated analytically and numerically over a wide range of system parameters to determine detailed scaling properties of the sideband instability. The results of the present analysis suggest that free-electron lasers operating with system parameters corresponding to the strong-pump regime [(Ω′B/Γ0)6/4≫1] are least vulnerable to the sideband instability. Moreover, the nonlinear evolution of the sideband instability is investigated numerically for system parameters corresponding to the Los Alamos free-electron laser experiment [Warren et al., IEEE J. Quantum Electron. QE-21, 882 (1985)]. In several aspects, the numerical results are found to be in good qualitative agreement with the experimental results.

Electron-beam and high-speed optical diagnostics for the Los Alamos HIBAF facility

Characterization of the electron beam's properties will be a major task after the upgrade of the Los Alamos Free Electron Laser (FEL) Facility with a photoelectric injector (PEI) and increased acceleration capability to 40 MeV. Adjustments to the previous diagnostics package that address the lower beam emittance, higher energy and wake-field source reduction issues are discussed.

Potential biophysical application of the Los Alamos infrared free electron laser: DNA spectroscopy

Recently, the Los Alamos Free Electron Laser (FEL) has demonstrated optical output at wavelengths from 9 to 45 μm (1100 to 220 cm−1). Potential applications of such a laser are proposed for the study of vibrational modes predicted in different conformations of DNA and in DNA complexed with drugs and/or proteins that regulate replication and/or transcription.

Lasing on the third harmonic

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.

Developments in on-line, electron-beam emittance measurements using optical-transition radiation techniques

We have developed image analysis software to facilitate the analysis of optical-transition radiation (OTR) patterns generated by the electron beam from the Los Alamos free-electron laser facility. The software can be used for beam alignment, beam profile and angular divergence measurements, and the programs run on an IBM AT microcomputer. The programs and their use are described and some results shown.

The Los Alamos High-Brightness Accelerator FEL (HIBAF) facility

The 10 μm Los Alamos free-electron laser (FEL) facility is being upgraded. The conventional electron gun and bunchers have been replaced with a much more compact, 6 MeV photoinjector accelerator. By adding existing parts from previous experiments, the primary beam energy will be doubled to 40 MeV. With the existing 1 m wiggler (λw = 2.7 cm) and resonator, the facility can produce photons with wavelengths from 3 to 100 μm when lasing on the fundamental mode and produce photons in the visible spectrum with short-period wigglers or harmonic operation. After installation of a 150° bend, a second wiggler will be added as an amplifier. The installation of laser transport tubes between the accelerator vault and an upstairs laboratory will provide experimenters with a radiation-free environment for experiments. Although the initial experimental program of the upgraded facility will be to test the single-accelerator master-oscillator/power amplifier configuration, some portion of the operational time of the facility can be dedicated to user experiments.

Comparative analysis of optical-transition-radiation-based electron-beam emittance measurements for the Los Alamos free-electron laser

A series of measurements have been performed to determine the emittance of the electron beam of the LANL FEL. These measurements were made both before and after installation of a photoelectric injector on the LANL linac. An optical-transition-radiation (OTR) interferometer was used to determine the beam divergence while simultaneously using the beam spot imaged in OTR to determine the spatial profile at a beam waist. We have developed two methods to analyze the OTR interference patterns in order to determine the beam divergence from the observed interference-fringe visibility. The first method relies on a simple analytical model in which a Gaussian beam divergence is convolved with the OTR interference pattern for a single particle. The second method uses a numerical convolution of the phase-space distribution in beam angle and energy with the single-particle OTR interference pattern. The six-dimensional phase space used to simulate the experimental OTR patterns was produced by the particle code PARMELA which was run for the beam parameters appropriate for the two sets of experiments. We present a comparison of these two methods with each other and with the experimental observations. The effects of both the beam divergence and the beam energy spread on the emittance measurement is illustrated.

Identification and reduction of wake-field effects in the Wiggler region of the Los Alamos free-electron laser

Improving the performance of the Los Alamos free-electron laser (FEL) in 1988 involved the identification and reduction of wake-field effects generated in the wiggler region. The wake-field effect was dramatically reduced by inserting a smooth metal tube into the wiggler gap, and the FEL performance was concomitantly improved.

High extraction efficiency experiments with the Los Alamos free electron laser

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.

Optical-transition radiation measurements for the Los Alamos and boeing free-electron laser experiments

Optical-transition radiation (OTR) measurements of the electron-beam emittance have been performed at a location just before the wiggler in the Los Alamos free electron laser (FEL) experiment. Beam profiles and beam divergence patterns from a single macropulse were recorded simultaneously using two intensified charge-injection device (CID) television cameras and an optical beam splitter. Both single-foil OTR and two-foil OTR interference experiments were performed. Preliminary results are compared to a reference variable quadrupole, single-screen technique. New aspects of using OTR properties for pointing the e-beam on the FEL oscillator axis, as well as measuring e-beam emittance are addressed.

Sideband suppression in the Los Alamos free electron laser using a littrow grating

Results are reported on successful experiments using intracavity diffraction gratings to suppress synchrotron sidebands in the Los Alamos free electron laser. The effect of the gratings on wavelength, optical mode orientation and the shape of the cavity-length detuning curves are discussed, along with problems and limitations of the technique.