FEL Operation Research Articles

Recent progress of the compact AFEL at Los Alamos

The Advanced Free Electron Laser (AFEL) is a compact, infrared, rf-linac FEL that uses a high-brightness photoinjector and a short-period permanent-magnet wiggler. Lasing at saturation with and without sidebands was achieved over the 4–6 μm region with a nominally 15-MeV electron beam energy and a 1-cm-period, 24-period wiggler. Sideband-free FEL operation was optimized with respect to outcoupling by tuning off the reflectivity curve of the multilayer dielectric mirrors. Sideband operation was achieved using copper mirrors with a 1.1% outcoupling hole. The measured macropulse energy with ∼ 1300 micropulses was approximately 50 mJ per 1 nC of beam, corresponding to an output efficiency of 0.25% and a maximum extraction efficiency of 1.4%.

Upper hybrid wave pumped free electron laser

A large amplitude upper hybrid wave has potential to be employed as a wiggler for the generation of high frequency coherent radiation via free electron laser (EEL) instability. At a density fluctuation level of a few percent, due to the upper hybrid wave, the growth time of FEL instability, with electron beam current of a few kilo ampere, turns out to be of the order of a nanosecond. The growth rate of FEL instability depends sensitively on /spl omega//sub 1/ and this dependence comes through /spl nu//sub osc/ the beam oscillatory velocity. /spl nu//sub osc/ acquires large values at k/sub 0z//spl sime/[/spl omega//sub 0/-(/spl omega//sub e///spl gamma//sub 0//sup 0/)]/spl nu//sub b//spl sime//spl omega//sub c/[1-(1//spl gamma//sub 0//sup 0/)]/spl nu//sub b/. At this value of k/sub 0z/,/spl omega//spl sime/2/spl gamma//sub 0//sup 02//spl omega//sub c/[2-(1//spl gamma//sub 0//sup 0/)]. However, this scheme of FEL operation suffers from a severe limitation due to the parametric instability of the upper hybrid wave. The process of parametric decay into lower hybrid and upper hybrid waves may have growth rate comparable to that of the FEL instability.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electron beam characterization for a compact far-infrared free-electron laser

A compact, far-infrared free-electron laser (FIR-FEL) is operating at 85 /spl mu/m at Stanford University, where the electron beam is obtained from a 1 1/2 cell, thermionic RF (2.856 GHz) cavity gun. This gun was not designed for FEL operation, and under the condition at which it was intended to operate, with a peak acceleration gradient below 85 MV/m, it would not be suitable. We have explored new parameter ranges, and have found that at high peak gradients, from 100-140 MV/m, the gun will function satisfactorily as an FEL accelerator. For example, thirty-nine percent of the total gun current was transmitted through a 1% energy window at a beam energy of /spl gamma/=9.72. At /spl gamma/=9.56, a 304 mA macropulse current with an estimated rms micropulse length of 3.4 ps was obtained, and the normalized rms emittance was measured to be 11.6 /spl pi/-mm-mrad for the 1% energy-spread electrons, corresponding to a beam brightness of 2.3/spl times/10/sup 11/ A/m/sup 2/. For these parameters, the calculated small small-signal gain for our 0.5 m-long wiggler is 110% at a wavelength of 85 /spl mu/m. In this paper we characterize the electron beam from a 1 1/2 cell, thermionic cathode, RF cavity gun in a parameter range where it can be used as the accelerator for a far infrared FEL. >

A study of linewidth, noise and fluctuations in a FEL operating in SASE

We study the evolution of the FEL radiation intensity and spectrum starting from noise in the electron beam longitudinal distribution. Due to the slippage both the time and frequency structure of the emitted radiation pulse show a very different behavior when the bunch length is much longer than or of the order of the cooperation length l c. The occurrence of superradiant spikes is discussed. We present analytical and numerical results.

The SLAC soft X-ray high power FEL

We discuss the design and performance of a 2 to 4 nm FEL operating in Self-Amplified Spontaneous Emission (SASE), using a photoinjector to produce the electron beam, and the SLAC linac to accelerate it to an energy of about 7 GeV. Longitudinal bunch compression is used to increase the peak current to 2.5 kA, while reducing the bunch length to about 40 μm. The FEL field gain length is about 6 m, and the saturation length is about 60 m. The saturated output power is about 10 GW, corresponding to about 10 14 photons in a single pulse in a bandwidth of about 0.1%, with a pulse duration of 0.16 ps. Length compression, emittance control, phase stability, FEL design criteria, and parameter tolerances are discussed.

Observations of frequency chirping and phase of a free electron laser amplifier

We report observations of frequency chirping and phase of a free electron laser amplifier operating in the Raman regime. The FEL is driven by a mildly relativistic electron beam (750 kV, 25 ns) subjected to a combined axial magnetic field and a helical wiggler field. The input into the FEL amplifier is provided by a high power magnetron tuned to a frequency of 33.39 GHz. Phase and frequency shifts are measured both as a function of time and interaction length. It is found that in the Group I regime of FEL operation the output frequency is upshifted by ∼ 100 MHz, but smaller upshifts are observed in the Group II or the Reversed Field configurations.

An accelerator/wiggler for high efficiency FEL operation

A possible approach to high efficiency FEL operation is to combine a microwave linear accelerator and magnetic wiggler into a single structure. As the electrons lose energy to the radiation at the FEL oscillation wavelength (e.g. 10 μm), energy is replaced by the microwave linac. The electron beam acts as a catalyst for the conversion of microwave power to infrared power. Several advantages to the accelerator/wiggler are: it is possible to obtain high conversion efficiency in a short length; small-signal gain reduction can be avoided; power extraction may be increased by increasing length; there is little detrapping; and electron beam energy out of the wiggler is relatively monochromatic, permitting efficient energy recovery. A six period, full scale model of the accelerator/wiggler has been fabricated to check the computer simulations for both microwave and magnetic properties. Microwave measurements on this model indicate that the predicted Q is accurate, that there is an approximate 1.5% error in the predicted resonant frequency and that critical coupling to the structure in the presence of the electron beam can be achieved with a direct, rectangular waveguide feed.

Closure relations in macroscopic FEL equations

Based upon a macro-electron approach, several closure relations are presented to truncate the hierarchy of moments in macroscopic equations for the electron motion in a Compton FEL. In the non-linear and saturation regime of FEL operation, the optical power predicted by the models is compared with results from a multi-electron simulation code and with results from the literature. An idealized macro-electron model shows that the oscillatory non-linear behaviour is completely determined by the sloshing of energy between the electrons and the wave. A more realistic model which includes the slippage between the pulses, describes the behaviour of the optical power in the linear regime as well as the saturation level and the oscillations in the non-linear regime.

Design elements of a high-power VUV free electron laser

We explore the feasibility of a high-power FEL operating in the VUV and configured as a coupled oscillator-tripler. The dynamical behaviour of the system and the dependence of the output power on the characteristics of the e-beam are analysed along with the parameters of the oscillator. Finally, the possibility of enhancing the output power by means of appropriate tapering of the second undulator is discussed.

Investigation of 3D effects on FEL operation

An investigation is made of 3D effects on FEL operation by comparing the 3D simulation code TDA with a 1 1 2 D model. In the latter model, the full spatial dependence of the radiation field is taken into account, whereas the electrons are treated as moving in a 1D, density-averaged ponderomotive potential. Three dimensional effects like a radial beam profile, emittance and betatron oscillations are investigated. For FELIX parameters, the 1 1 2 D model is in good agreement with TDA.

First operation of a compact FEL in the millimeter wave region

We report the first operation of a compact free-electron laser at wavelengths in the range between 2 and 2.6 mm with an output power up to 1 kW in 4 μs pulses. The innovative characteristic of this device, driven by a 2.3 MeV microtron, is its considerable small size, obtained by using a short undulator with a 2.5 cm period and a length of 22.5 cm.

Far infrared Smith Purcell experiment: measurement of radiated power

Infrared radiation produced by a fully relativistic, 3.6 MeV, electron beam passing close to a metal grating has been observed and the radiated power estimated by direct comparison with a high pressure mercury vapour lamp. The dependence of wavelength on angle of emission, over angles from 56° to 150° and wavelengths from 350 μm to 1860 μm, agrees remarkably well with the Smith Purcell dispersion relation. The observed radiated power of about 10 μW/cm 2 sr A for a 100 μm band width at a wavelength of 1 mm is comparable with or greater than that typical for infrared beam lines at synchrotrons, and also suggests that the Smith Purcell mechanism deserves greater attention as a possible basis for FEL operation in the mm and sub-mm wavelength bands.

DELTA, a status report of the storage ring FEL facility at the University of Dortmund

The storage ring FEL facility DELTA, under construction at the University of Dortmund since the beginning of 1990, has reached the stage of assembling the accelerator components, now starting the setup of the injection linac. DELTA is a low emittance 1.5 GeV storage ring optimized for FEL operation. Two straight sections are available for insertion devices up to 14 m length. To achieve the design goal of extremely high brilliance of the electron beam a triplet lattice is chosen, giving low emittance values comparable to FODO designs. This lattice has the advantage of more space between the elements, allowing further instrumentation. Furthermore, the vacuum chamber was optimized for low impedance. Even in the kicker magnets there is no change of the cross section. This yields optimized operation with respect to bunch lengthening and other wakefield effects. Due to the integrated pumping using NEG and ion getter pumps in a continuous antechamber, only negligible lifetime degradations due to vacuum pressure have to be expected. Thus, DELTA represents an optinum accelerator to operate FELs in the extremely short wavelength regime down to about 20 nm.

Simulation of an optical klystron waveguide FEL

It is suggested that an optical klystron FEL operating in a waveguide could provide a compact source of mm wave and far infrared radiation with both a low voltage and a low electron beam requirement. As a first step in determining the feasibility of this suggestion, a method of calculating the small signal gain of a waveguide optical klystron is outlined and some results are presented.

Characteristics and beam tracking of a compact storage ring for FEL operation

The CSA (Compact Storage and Acceleration) ring for FEL was projected in 1989 [T. Hattori et al., Proc. 14th Linear Accelerator Meeting, Japan (1989) 212]. The lattice of the ring is a double bend double achromatic type. The circumference is 15 m. In spite of the short circumference, the ring has two long straight sections for undultors. The bending magnets were completed and their magnetic field strength was measured [Y. Takahashi et al., Proc. 8th. Symp. on Accelerator Science and Technology 8 (1991) 419]. This paper will present the design and the characteristics of the ring. Furthermore, results of beam tracking are reported.

Study on the storage ring NIJI-IV with an optical klystron

The experimental setup of the FEL storage ring NIJI-IV was completed in July 1992. This paper will describe the preparation of the electron bunch to 1 or 2 bunch operation at an energy range of 240–300 MeV in order to reach FEL operation at about 600 nm.

Design considerations for simultaneous FEL and nuclear physics operation at CEBAF

As conceived in a recent design study, electron beams of quite distinct character would be provided for nuclear physics experiments and FEL wigglers at CEBAF. When full nuclear physics operation begins, coordination between these two programs becomes critical. FEL operation requires electron bunches carrying charge of 120 pC at repetition rates of 2.5 and 7.5 MHz, whereas the nuclear physics users need a relatively small charge per bunch, ~ 0.13 pC, but at a repetition rate of 1.5 GHz. To allow maximal operation of the FEL facility without interfering with CEBAF's primary mission of conducting nuclear physics research, the principal mode of operation should accelerate and deliver the two disparate beams simultaneously with negligible degradation of beam quality. Various RF power, RF control, wakefield, and beam transport questions that are encountered in designing for concurrent operation are discussed.

Photoemission using femtosecond laser pulses

Successful operation of short wavelength FEL requires an electron bunch of current >100 A and normalized emittance < 1 mm-mrad. Recent experiments show that RF guns with photocathodes as the electron source may be the ideal candidate for achieving these parameters. To reduce the emittance growth due to space charge and RF dynamics effects, the gun may have to operate at high field gradient (hence at high RF frequency) and a spot size small compared to the aperture. This may necessitate the laser pulse duration to be in the subpicosecond regime to reduce the energy spread. We will present the behavior of metal photocathodes upon irradiation with femtosecond laser beams, comparison of linear and nonlinear photoemission, and scalability to high currents. Theoretical estimate of the intrinsic emittance at the photocathode in the presence of the anomalous heating of the electrons, and the tolerance on the surface roughness of the cathode material will be discussed.

FELS, a 3-dimensional FEL simulation code including storage-ring operation of high-gain FELS

Abstract The free-electron-laser code FELS is a program to model a high-gain free-electron laser in the short-wavelength long pulse regime. The code can be run to describe the single-pass amplifier operation, and the oscillator configuration. Two different types of optical cavities are implemented, the nearly concentric and the ring resonator. Furthermore, the modelling not only of operation with linacs but also with storage ring devices is included. Two versions provide calculation with and without the harmonics of the spontaneous undulator spectrum included [1,2]. This paper will review the models of the FEL, the optical resonator, and the storage ring, and the implementation of the code, shortly. Furthermore, simulation results which point out the differences between ID, 2D and 3D calculations for the example of FELICITA II, the second FEL experiment at DELTA, are included [3,4].

The Los Alamos POP Project: FEL oscillator experiments in the ultraviolet and beyond

The Los Alamos POP Project will include a series of proof-of-principle FEL oscillator experiments in 1992 designed to extend FEL operation into the ultraviolet (UV) and vacuum ultraviolet (VUV). With beam energy extended to 50 MeV, enhanced beam brightness with a photoinjector, and appropriate UV and VUV resonator optics, the Los Alamos FEL oscillator should have sufficient single-pass gain (20--30%) to reach below 200 nm. The first goal will be lasing at 250 nm utilizing a permanent-magnet undulator with 5-mm period or alternatively on the third harmonic with a 1-cm period. To operate at VUV wavelengths of 200 and 150 nm (third harmonic), pulsed electromagnetic microwigglers with periods of 5.7- and 4.3 mm will be employed. 23 refs.