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Recent technological developments have opened the possibility to construct a device which we call a linac coherent light source (LCLS) (C. Pellegrini et al., Nucl. Instr. and Meth. A 331 (1993) 223; H. Winick et al., Proc. IEEE 1993 Particle Accelerator Conf., Washington, DC, May 1993; C. Pellegrini, Nucl. Instr. and Meth. A 341 (1994) 326; J. Seeman, SPIE Meet. on Electron Beam Sources of High Brightness Radiation, San Diego, CA, July 1993 [1–4]); it would be a fourth-generation light source, with brightness, coherence, and peak power far exceeding other sources. Operating on the principle of the free electron laser (FEL), the LCLS would extend the range of FEL operation to much shorter wavelength than the 240 nm that has so far been reached. We report the results of studies of the use of the SLAC linac to drive an LCLS at wavelengths from about 3 to 100 nm initially and possibly even shorter wavelengths in the future. Lasing would be achieved in a single pass of a low emittance, high peak current, high-energy electron beam through a long undulator. Most present FELs use an optical cavity to build up the intensity of the light to achieve lasing action in a low-gain oscillator configuration. By eliminating the optical cavity, which is difficult to make at short wavelengths, laser action can be extended to shorter wavelengths by self-amplified-spontaneous-emission (SASE), or by harmonic generation from a longer wavelength seed laser. Short wavelength, single pass lasers have been extensively studied at several laboratories and at recent workshops (M. Cornacchia and H. Winick (eds.), SLAC Report 92/02; I. Ben-Zvi and H. Winick (eds.), BNL report 49651 [5,6]). The required low-emittance electron beam can be achieved with recently-developed rf photocathode electron guns (B.E. Carlsten, Nucl. Instr. and Meth. A 285 (1989) 313; J. Rosenzweig and L. Serafini, Proc. IEEE 1993 Particle Accelerator Conf., Washington, DC, 1993 [7,8]). The peak current is increased by about an order of magnitude by compressing the bunch to a lenght of about 0.2 ps (rms). Techniques for beam transport, acceleration, and compression without emittance dilution have been developed at SLAC as part of the linear-collider project (J. Seeman, Advances of Accelerator Physics and Technologies, ed. H. Schopper (World Scientific, Singapore, 1993 [9]). The undulator length required to saturate the laser varies from about 15 m for a 100 nm FEL to about 60 m at 3 nm. Initial experiments, at wavelengths down to about 50 nm are planned using the 25-m long Paladin undulator now located at LLNL. In a proposed future LCLS R&D facility the short wavelength light pulses are distributed to multiple end stations using grazing-incidence mirrors. About 10 14 photons per pulse can be produced at a 120 Hz rate, corresponding to average brightness levels of about 10 21 photons/s/mm 2 /mrad 2 within 0.1% BW and peak brightness levels of about 10 31 photons/s/mm 2 /mrad 2 within 0.1% BW. Peak power levels are several hundred megawatts to several gigawatts. Electron energies required range from about 500 MeV for the 100 nm FEL to about 7 GeV for 3 nm.

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 1014 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.

Bi-phase shift keying (BPSK) is a modulation scheme used in communications and radar in which the phase of a transmitted rf signal is switched in a coded pattern between discrete values differing by π radians. The transmitted information rate (in communications) or resolution (in imaging radar) depends on the rate at which the transmitted signal can be modulated. Modulation rates of greater than 1 GHz are generally desired. Although the instantaneous gain bandwidth of a mm-wave FEL amplifier can be much greater than 10 GHz, slippage may limit the BPSK modulation rate that can be amplified. Qualitative slippage arguments would limit the modulation rate to relatively low values; nevertheless, simulations with a time-dependent FEL code (GINGER) indicate that rates of 2 GHz or more are amplified without much loss in modulation integrity. In this paper we describe the effects of slippage in the simulations and discuss the limits of simple slippage arguments.

The CERN linear collider (CLIC) [W. Schnell, Proc. Workshop on Physics of Linear Colliders, Capri, 1988, eds. l. Palumbo, S. Tazzari and V.G. Vaccaro (1989) p. 345] is a two-beam-accelerator (TBA) scheme in which the driving beam consists of an intense 3 to 5 GeV electron beam bunched at 30 GHz. One possible way to produce this drive beam is to start with a low-energy (or order 10 MeV), high-current (about 5 kA) beam from an induction linac. In passing through a wiggler, this beam is bunched at 30 GHz into micropulses, each with about 10 12 electrons. To construct the format required for the CLIC drive beam, the bunched beam is subsequently chopped at 350 MHz. It is then accelerated to 3 to 5 GeV in an rf linac driven by conventional, low-frequency klystrons. Rf power is extracted at the bunch frequency of 30 GHz and fed into high-gradient structures to accelerate electron or positron beams to TeV energies. The drive beam is repeatedly reaccelerated in 350 MHz superconducting cavities. This study examines the design trade-offs of the proposed “front end” of the CLIC TBA, the linear induction accelerator and FEL. We examine the relevant figure of merit, the efficiency of bunching, as a function of beam energy, current, and emittance, and we consider the effects of wiggler errors, energy spread and slew, and beam misalignment.

This paper describes designs for 280-GHz and 560-GHz microwave sources based on free electron lasers (FELs). These 10-MW units are based on technology developed over the last 5 years. A first demonstration of high-average-power microwave production with an FEL system is expected in the Microwave Tokamak Experiment (MTX) facility. This paper gives details on the design and construction of that 250-GHz, 2-MW system and discusses specific applications for the Compact Ignition Tokamak (CIT).

We describe modeling and simulation of long-range terrestrial laser communications links between static and mobile platforms. Atmospheric turbulence modeling, along with pointing, tracking and acquisition models are combined to provide an overall capability to estimate communications link performance.

During nearly a decade of remote sensing programs under the auspices of the U.S. Department of Energy (DOE), LLNL has developed a set of performance modeling codes -- called APRS -- for both Active and Passive Remote Sensing systems. These codes emphasize chemical detection sensitivity in the form of minimum detectable quantities with and without background spectral clutter and in the possible presence of other interfering chemicals. The codes have been benchmarked against data acquired in both active and passive remote sensing programs at LLNL and Los Alamos National Laboratory (LANL). The codes include, as an integral part of the performance modeling, many of the data analysis techniques developed in the DOE's active and passive remote sensing programs (e.g., "band normalization" for an active system, principal component analysis for a passive system).

The Microwave Tokamak Experiment (MTX) is under construction at Lawrence Livermore National Laboratory to investigate the feasibility of intense, pulsed microwave radiation for plasma heating on future ignition tokamaks. A high-average-power free-electron laser (FEL) will use the Experimental Test Accelerator (ETA-II), a linear induction accelerator, in combination with an advanced high-field wiggler, to produce 1-2 MW of power at 1-2-mm wavelengths for periods of up to 0.5 s. The design of the FEL, termed the Intense Microwave Prototype (IMP), is described, along with the status and major issues associated with the experiment. >

We describe the use of the SLAC linac to drive a unique, powerful, short wavelength Linac Coherent Light Source (LCLS). Operating as an FEL, lasing would be achieved in a single pass of a high peak current electron beam through a long undulator by self-amplified spontaneous emission. The main components are a high-brightness rf photocathode electron gun; pulse compressors; about 1/5 of the SLAC linac; and a long undulator with a FODO quadrupole focusing system. Using electrons below 8 GeV, the system would operate at wavelengths down to about 3 nm, producing /spl ges/10 GW of peak power in sub-ps pulses. At a 120 Hz rate the average power is /spl ap/1 W. >

Recently the SSRL/SLAC and its collaborators elsewhere have considered the merits of a 2 to 4 nm high power FEL utilizing the SLAC linac electron beam. The FEL would be a single pass amplifier excited by spontaneous emission rather than an oscillator, in order to eliminate the need for a soft X-ray resonant cavity. We have used GINGER, a multifrequency 2D FEL simulation code, to study the expected linewidth and coherence properties of the FEL, in both the exponential and saturated gain regimes. We present results concerning the effective shot noise input power and mode shape, the expected sub-percent output linewidths, photon flux, and the field temporal and spatial correlation functions. We also discuss the effects of tapering the wiggler upon the output power and linewidth. >

We have used the improved ETA-II linear induction accelerator (ETA-III) and the IMP steady-state wiggler to generate high power (1-2 GW) microwaves at 140 GHz. The FEL was used in an amplifier configuration with a gyrotron driver. Improved control of energy sweep and computerized magnetic alignment in ETA-III resulted in small beam corkscrew motion ( >

The ETA-II linear induction accelerator utilizes four pulse power conditioning chains. Magnetic pulse compression modulators (MAG1-Ds) form the last state of each chain. The upgrading and characterizing of the power conditioning chain on a high-average-power test stand (HAPTS) is reported. On the HAPTS, the pulse-to-pulse amplitude stability has been improved to less than 0.7% (one sigma) and the random jitter to 3-5 about a systematic timing variation. A description is presented of the work to achieve the desired performance level of the MAG1-D to allow high average power operation of ETA-II. >

SLAC/SSRL and collaborators elsewhere are studying the physics of a single-pass, FEL amplifier operating in the 1-2 /spl Aring/ wavelength region based on electron beams from the SLAC linac at /spl sim/15 GeV energy. Hoping to reduce the total wiggler length needed to reach saturation when starting from shot noise, we have examined the benefits of making the first part of the wiggler resonant at a subharmonic wavelength (e.g. 4.5 /spl Aring/) at which the gain length can be significantly shorter. This leads to bunching of the electron beam at both the subharmonic and fundamental wavelengths, thus providing a strong coherent "seed" for exponential growth of radiation at the fundamental in the second part of the wiggler. Using both multi-harmonic and multi-frequency 2D FEL simulation codes, we have examined the predicted performance of such devices and the sensitivity to electron beam parameters such as current, emittance, and instantaneous energy spread.

Application of a novel transform operator, the Sticklet transform, to the quantitative estimation of trace chemicals in industrial effluent plumes is reported. The sticklet transform si a superset of the well-known derivative operator and the Haar wavelet, and is characterized by independently adjustable lobe width and separation. Computer simulations demonstrate that we can make accurate and robust concentration estimates of multiple chemical species in industrial effluent plumes in the presence of strong clutter background, interferent chemicals and random noise. In this paper we address the application of the sticklet transform in estimating chemical concentrations in the effluent plumes in the presence of atmospheric transmission effects. We show that this transform retains the ability to yield accurate estimates using on-plume / off-plume measurements that represenst atmospheric differentials up to 10% of the full atmospheric attenuation.

LLNL has utilized optical parametric oscillator technology to develop and field a rapidly-tunable mid-wave IR DIAL system. The system can be tuned at up to 1 KHz over the 3.3- 3.8 micron spectral region, where hydrogen-bond stretching modes provide spectroscopic signatures for a wide variety of chemicals. We have fielded the DIAL system on the LLNL site on range, turbulence, and receiver aperture size. In this paper we describe the interplay of turbulence and speckle to produce the observed nose fluctuations at short range.

Differential absorption LIDAR (DIAL) with multiple wavelengths provides capabilities for separately identifying and quantifying chemicals in mixtures that are impossible for conventional two-line DIAL. It also permits several choices about how to convert individual laser pulse returns into estimates of gas concentrations. These choices concern both averaging techniques and procedures for fitting averaged data to library spectra of possible gases. The purpose of this paper is to compare several analysis options using real data taken with 8 or 10 mid-wave IR wavelengths in field test. The options fall naturally into two groups, as implied above. The first group comprises ways to combine data averaging with ratioing. The second group comprises various maximum-likelihood estimators and least-squares fits of the averaged data. Several options arise in the second group because data is taken at multiple wavelengths; for two-line DIAL there would be only a single option in this second group. This paper compares the result of field data analysis for these two groups of options. The properties of the data acquired by the multi-line DIAL system are first described. Then the three averaging/ratioing techniques are discussed. The various options for extracting concentration estimates from averaged data are compared. Finally, the implications for remote sensing data analysis are discussed.

This paper reports on recent progress made in developing rapidly tunable MWIR lidar system for the detection and identification of multiple trace atmospheric molecules. The lidar systems and multiline DIAL approach are described in detail. The unique advantages of the lidar system and multiline DIAL measurement technique are demonstrated with results from field measurements.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Electron bunch requirements for single‐pass saturation of a Free‐Electron Laser (FEL) operating at full transverse coherence in the Self‐Amplified Spontaneous Emission (SASE) mode include: 1) a high peak current, 2) a sufficiently low relative energy spread, and 3) a transverse emittance e[r‐m] satisfying the condition e≤λ/4π, where λ[m] is the output wavelength of the FEL. In the insertion device that induces the coherent amplification, the prepared electron bunch must be kept on a trajector sufficiently collinear with the amplified photons without significant dilution of its transverse density. In this paper we discuss a Linac Coherent Light Source (LCLS) based on a high energy accelerator such as, e.g., the 3 km S‐band structure at the Stanford Linear Accelerator Center (SLAC), followed by a long high‐precision undulator with superimposed quadrupole (FODO) focusing, to fulfill the given requirements for SASE operation in the 1000 A−1 A range. The electron source for the linac, an RF gun with a laser‐excited photocathode featuring a normalized emittance in the 1–3 mm‐mrad range, a longitudinal bunch duration of the order of 3 ps, and approximately 10−9 C/bunch, is a primary determinant of the required low transverse and longitudinal emittances. Acceleration of the injected bunch to energies in the 5–25 GeV range is used to reduce the relative longitudinal energy spread in the bunch, as well as to reduce the transverse emittance to values consistent with the cited wavelength regime. Two longitudinal compression stages are employed to increase the peak bunch current to the 2–5 kA levels required for sufficiently rapid saturation. The output radiation is delivered, via a grazing‐incidence mirror bank, to optical instrumentation and a multi‐user beam line system. Technological requirements for LCLS operation at 40 A, 4.5 A, and 1.5 A are examined.

We describe the possible use of the SLAC linac to drive a unique, powerful, short wavelength Linac Coherent Light Source. Using the FEL principle, lasing is achieved in a single pass of a high peak current electron beam through a long undulator by self-amplified-spontaneous- emission (SASE). The main components are a high-brightness electron RF gun with a photocathode, two electron bunch length compressors, the existing SLAC linac, beam diagnostics, and a long undulator combined with a FODO quadrupole focusing system. The RF gun, to be installed about 1 km from the end of the SLAC linac, would produce a single bunch of 6 X 109 electrons with an invariant emittance of about 3 mm-mrad and a bunch length of about 500 micrometers . That bunch is then accelerated to 100 MeV and compressed to a length of about 200 micrometers . The main SLAC linac accelerates the bunch to 2 GeV where a second bunch compressor reduces the length to 30 - 40 micrometers and produces a peak current of 2 - 3 kA. The bunch is then accelerated to 7 - 8 GeV and transported to a 50 - 70 m long undulator. Using electrons below 8 GeV, the undulator could operate at wavelengths down to 2 nm, producing about 10 GW peak power in sub-ps light pulses.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Motivated by the LLNL/LBL FEL experiment (ELF) we have developed a theory of small signal FEL performance including high gain, space charge, multiple waveguide modes, and beam emittance. A comparison is made between the theoretically predicted and experimentally observed exponential growth rates. These two rates are seen to agree well in the amplifier configurations for which detailed experimental measurements have been performed. It is observed, through numerical simulation, that the nonlinear shift in wave number produced by the FEL can slow the rate of particle detrapping when two modes overlap. Also, it is found that the operation in a higher order mode is degraded by the presence of a lower order mode.

We have used the free electron laser particle simulation code FRED to examine the design of an FEL for amplifying radiation in the 1–2 mm wavelength range for use in electron heating and current drive in a tokamak device such as ALCATOR-C. We have used as a desired design goal a peak output power of 8 GW, with a minimum input power in the 1–100 W range. We have examined the effects of electron beam current, energy and brightness, laser frequency and input power as well as wiggler wavelength and overall wiggler length on performance of the FEL. We find that to obtain the desired power output, we require a 3 kA electron beam with a brightness of about 1 × 105 A/cm2. For a 1 mm wavelength system, the beam energy must be at least 10 MeV, the laser input power must be at least 100 W, and the wiggler wavelength is 8 cm, with an overall length of at least 6 m. For a 2 mm wavelength system, the output power goals can be met with a 7 MeV electron beam, with 10 W of input laser power, and a wiggler with an 8 cm wavelength and total length of at least 6 m. The beam and laser requirements to meet the design criteria are less stringent for the 2 mm wavelength system. The magnet requirements to meet the design power level goal is more difficult to meet with a wiggler wavelength of 8 cm compared to a 9.8 cm wiggler wavelength. With the 10 MeV beam energy of the 1 mm system and the 7 MeV beam energy of the 2 mm system, the peak magnetic field changes from about 0.4 to 0.6 T when the wiggler wavelength changes from 9.8 to 8 cm.

The coupling between betatron and synchrotron oscillations in a tapered wiggler free-electron laser (FEL) is investigated analytically and numerically. The resonance between the oscillations, first investigated by Rosenbluth, is shown to be important only for a very limited range of parameters. In general, the resonance is not a serious detrapping mechanism for high-power FEL amplifiers.

We have substantially increased the output power and extraction efficiency of a free-electron laser operating at 34.6 GHz by tapering the wiggler magnetic field. In the exponential-gain regime, the laser exhibited a measured gain of 34 dB/m. With a 50-kW input signal, the amplifier saturated in 1.3 m with a 180-MW output signal. By using a taper that brought the magnetic field at the end of the wiggler down to 45% of its initial (peak) value, we increased the output signal to 1.0 GW. This corresponds to an extraction efficiency of 34%.

A theoretical analysis is given of an FEL operating in the millimeter range and, hence, operating in a waveguide. For study of the situation in which the average longitudinal velocity of electrons v/sub parallel/ is close to the electromagnetic pulse group velocity v/sub g/, it is shown that the usual paraxial approximation is invalid. The complete analysis is then specialized to a single particle model which, for operation in a steady state, yields a dispersion relation. The resulting dispersion relation is analyzed analytically and numerically. It is shown that when v/sub parallel/ greater than or equal to v/sub g/ there is no sideband instability, and an analytic expression is given for the stability boundary. 10 refs., 7 figs.

As a part of the program in induction-linac free-electron laser research, the authors summarize the Laboratory's work in a variety of activities addressing the requirements imposed on wiggler systems. The development of improved designs is reported for DC iron-core electromagnetic wigglers to attain higher peak fields, greater tunability, and lower random-error levels. Specialized control systems are noted (such as magnetic-field and beam-position controllers) that can relax requirements on the wiggler itself. Basic studies to establish the effect of radiation on permanent magnets are also discussed briefly. >

A high-gain, high extraction efficiency, free electron laser (FEL) amplifier operating at the Experimental Test Accelerator (ETA) at 34.6 GHz has demonstrated a small signal gain of 13.4 dB/m. With a 30 kW input signal, the amplifier has produced a saturated output of 80 MW and a 5% extraction efficiency. Comparison of these results with a linear model at small signal levels indicates that the amplifier can deliver saturated output starting from noise, if the brightness of the electron beam is sufficiently high. The brightness of the ETA is far below that possible with optimized choice of practical design characteristics such as peak voltage, cathode type, gun electrode geometry, and focusing field topology. In particular, the measured brightness of the ETA injector is limited by plasma effects from the present cold, plasma cathode. As part of a coordinated theoretical and experimental effort to improve injector performance, we are using the EBQ gun design code to explore the current limits of gridless, relativistic, Pierce columns with moderate current density (>50 A/cm/sup 2/) at the cathode. The chief component in our experimental effort is a readily modified electron gun that will allow us to test many candidate cathode materials, types, and electrodemore » geometries at field stresses up to 1 MV/cm. 8 references, 5 figures.« less

The standard synchronous tapering method used to design the wiggler magnetic field for free electron laser (FEL) amplifiers operating in the Compton regime will not work for amplifier systems where space-charge effects are important. The space-charge effects lower the overall gain in the amplifier system and, even more importantly, shift the peak in the gain curve to magnetic field values that are significantly less than the synchronous magnetic field value. As a result, the overall predicted gain using the synchronous tapering method is too low. Moreover, the synchronous magnetic field corresponds to the peak in the gain curve for a frequency below the fundamental frequency. Consequently, shot noise at frequencies below the fundamental frequency can grow to levels that may prevent amplification of the fundamental. We have developed a new tapering strategy that improves the predicted amplifier gain and circumvents the shot-noise growth for systems with moderate space-charge effects. For this new strategy, we hold the wiggler magnetic field constant at a value below the synchronous value but near the peak of the gain curve for the fundamental frequency, for some optimized length at the front end of the wiggler. Beyond this constant wiggler section, the field is tapered using the standard synchronous tapering algorithm. This new tapering scheme results in significant improvement in predicted amplifier gains and limits the growth of shot noise to insignificant levels. We demonstrate the effectiveness of this new tapering algorithm using the tapered wiggler design for the proposed microwave heating experiment (MTX) at the Lawrence Livermore National Laboratory (LLNL).

Early free electron laser (FEL) development was guided by simple performance criteria based on the number of undulator periods, electron beam quality, and current. The beam quality (emittance and energy spread) was used to characterize the initial distribution of axial electron velocities along the undulator axis. While the emittance and energy spread determine the overall width of the distribution, its detailed shape is also important. As new accelerators are designed specifically for FEL applications, it becomes important to obtain distribution shape information from simulations that include the electron gun, accelerator, and beam transport in addition to the usual electron/optical interaction in the undulator. The distribution at the entrance to the undulator can be calculated from numerical simulations of the cathode emission, acceleration, and transport of an electron beam. We have modeled the beam generation, from cathode emission up to the energy of the accelerator injector, using an axisymmetric, cylindrical geometry particle simulation (DPC). This code solves the relativistic force equation with fields obtained from Maxwell's equations in the Darwin model. The DPC calculation is run repeatedly varying parameters such as accelerating stress, electrode configuration, and axial magnetic field profile until a good match is obtained for the accelerator. The beam exiting from the injector can be accelerated and transported using the transfer matrix technique with a simple model for accelerating gaps and magnets. Alternatively, acceleration and transport can be simulated with a particle code that solves for the axisymmetric evolution of a slice of an electron beam including possible emittance growth. The phase space obtained from the accelerator can be evaluated for performance using either the simple FEL integral equation method or the more complete FRED simulation code.

High-power free electron lasers (FEL's) can be realized using induction linear accelerators as the source of the electron beam. These accelerators are currently capable of producing intense currents (102-104A) at moderately high energy (1-50 MeV). Experiments using a 500 A, 3.3 MeV beam have produced 80 MW of radiation at 34.6 GHz and are in good agreement with theoretical analysis. Future experiments include a high-gain, high-efficiency FEL operating at 10.6 μm using a 50 MeV beam.

Random errors that affect the transport of electron beams through long wigglers can degrade the performance of free electron laser amplifiers. We describe the origin and magnitude of the degradation, and test (with simulations) possible ways to improve amplifier performance in the presence of errors.

Tapering a free electron laser (FEL) amplifier to improve extraction after gain saturation can be accomplished by varying both the wiggler period and the wiggler field. This paper considers specific FEL designs and demonstrates the improvement in extraction efficiency that can be achieved by utilizing both variations. Fabrication considerations make a continuously varying wiggler period impractical for long wigglers, and so this technique entails discontinuous steps in the wiggler period. Such steps in wiggler period must be introduced in a manner such that they do not induce steering in the electron beam for reasonable ranges of transverse velocities and energies nor induce a large spread in phase shifts. We discuss such techniques.

Experiments at the Electron Laser Facility have generated peak microwave power of 180 MW at 35 GHz. The facility is operated as a single pass amplifier. Gain in excess of 30 dB/m has been observed up to saturation of the amplifier. For the 3.6 MeV, 850 A electron beam, the radiation corresponds to 6% energy extraction from the electron beam. Beyond saturation, the electron beam output power exhibits oscillations corresponding to the synchrotron motion of the trapped electrons in the ponderomotive well. In addition, the TE 21 and TM 21 modes have been studied and have power levels comparable to the fundamental. Third harmonic (105 GHz) radiation has been measured at power levels on the order of a few percent of the peak fundamental power.

We estimate the growth rate of the filamentation instability in a relativistic electron beam travelling parallel to an intense laser beam. We further estimate how the altered index of refraction in a high power FEL will affect the growth rate. We find that due to the finite emittance, FELs will be stable against filamentation.

A report is presented of the design, fabrication, and testing of a 25.6-m-long wiggler for a free-electron-laser (FEL) experiment. It is a DC iron-core electromagnetic wiggler that incorporates a number of important and unique features: permanent magnets are used to suppress saturation in the iron and extend the linear operating range; steering-free excitation allows real-time adjustment of the field taper without causing beam steering; and wiggle-plane focusing is produced by curved pole tips. The magnitude of random pole-to-pole field errors is minimized by a mechanical design concept that reduces tolerance stackup in critical locations. Three five-meter sections of this wiggler design were tested individually, and when connected together the measurements show exceptionally low levels of random errors. Detailed analysis of the error distribution has shown that the errors are not truly random, and the net effect on steering is much less than would be expected from random errors of the same magnitude. Analysis is still in progress to identify the cause of this beneficial effect. >

The phase change of an electromagnetic wave propagating through a free-electron laser (FEL) amplifier has been measured. The FEL operated at 34.6 GHz, with both a uniform and a tapered wiggler. The results of the experiment show a nearly constant phase derivative with increasing wiggler length through the exponential gain region and are in good agreement with analytical theory. For the tapered-wiggler amplifier, the phase derivative decreases at a point approximately one third of a synchrotron period past saturation; the decrease is in good agreement with numerical simulation. For the untapered-wiggler amplifier, however, numerical simulations predict an increase in phase derivative past saturation, whereas in the experiment the phase derivative abruptly vanishes. Several explanations for the discrepancy are discussed.

The effect on optical guiding of the undulations of an electron beam in an FEL is investigated, and a model for a fully saturated FEL amplifier with no remaining gain is developed. The density of the electrom beam in the model fully includes the effects of both transverse and longitudinal undulation of the beam. The longitudinal density modulation is expressed in terms of the Bessel functions of ζ, where ζ = a w 2 2(1 + a w 2 ) is the shift of the electron phase in the electron bucket caused by its longitudinal undulation. The transverse density modulation is evaluated to second order in the ratio of undulation amplitude δr to beam radial scale length rb. The radiation field from the modeled beam is calculated in terms of spatial modes proportional to exp[i(k + δk + lkw)z − iωt], where l is an arbitrary integer. Here, δk is the change of the wavenumber of the radiation caused by the electron bunches. Radially radiating modes with intensity on the order of ( δk k w )ζ 2 are found. Optical guiding is found to be modified by the transverse undulations of the beam at second order in δr r b , and by the longitudinal undulations to first order in δk k w . For the usual FEL parameters, the correction is quite small.

PALADIN is a single pass, free electron laser amplifier located at the Lawrence Livermore National Laboratory. This FEL is designed to run at 10.6 um. The 1 -kA, 45 -MeV electron beam is provided by the Advanced Test Accelerator. The wiggler is 25 m long with an 8 cm period. The input optical signal to theamplifier is provided by a conventional CO2 laser, which can produce a peak input power of either 18 kW or 3.6 MW. We have demonstrated 31 dB of gain with the 18 -kW input and 12.9 dB of gain for the 3.6 -MW input,producing over 70 MW of optical power. Using the 18 -kW input, the gain saturated at about 12 m into thewiggler; with the 3.6 -MW input, the gain saturated at about 8 m. Modeling results are shown.INTRODUCTIONIn 1986, the Electron Laser Facility (ELF) at LLNL demonstrated high gain and high extraction efficiency from a tapered -wiggler FEL amplifier.) The ELF experiments were done using microwaves in a waveguide anddid not address some issues pertinent to an optical- wavelength FEL. The PALADIN experiment was designed toexamine the physics pertaining to an FEL operating in the optical regime (10.6 um). The PALADIN wigglerwas designed to be assembled in 5 -m long sections, with a final length of 25 m. Preliminary experimentshave been performed using wiggler lengths of 5 and 15 m (Ref. 2). These earlier experiments have shown thatwe can successfully transport the electron beam through long wigglers without losing beam current into thewalls of the wiggler. The design of the wiggler magnet pole pieces made transport of the electron beam

The frequency offset of the sideband instability in a free-electron laser (FEL) should depend on (1-v/sub X//v/sub g/)/sup -1/, where v/sub X/ is the average longitudinal velocity of the electrons and v/sub g/ is the group velocity of the electromagnetic waves. We have tested the v/sub X//v/sub g/ dependence of the sideband shift in a 2-mm, Raman-regime FEL oscillator. A change of v/sub X//v/sub g/ from 0.93 to 0.98, accomplished by an increase in the undulator period, resulted in the measured sideband shift increasing from 6% to 40%, in approximate agreement with theory.

A high-gain, high--extraction-efficiency, linearly polarized free-electron laser amplifier has been operated at 34.6 GHz. At low signal levels, exponential gain of 13.4 dB/m has been measured. With a 3-=kW input signal, saturation was observed with an 80-MW output and a 5% extraction efficiency. The results are in good agreement with linear models at small signal levels and nonlinear models at large signal levels.

In recent years significant studies have been initiated on the feasibility of utilizing a portion of the 3 km S-band accelerator at SLAC to drive a short wavelength (4.5−1.5 A) Linac Coherent Light Source (LCLS), a Free-Electron Laser (FEL) operating in the Self-Amplified Spontaneous Emission (SASE) regime. Electron beam requirements for single-pass saturation in a minimal time include: 1) a peak current in the 7 kA range, 2) a relative energy spread of e = λ 4π , where λ[m] is the output wavelength. Requirements on the insertion device include field error levels of 0.02% for keeping the electron bunch centered on and in phase with the amplified photons, and a focusing beta of 8 m/rad for inhibiting the dilution of its transverse density. Although much progress has been made in developing individual components and beam-processing techniques necessary for LCLS operation down to ∼20 A, a substantial amount of research and development is still required in a number of theoretical and experimental areas leading to the construction and operation of a 4.5−1.5 A LCLS. In this paper we report on a research and development program underway and in planning at SLAC for addressing critical questions in these areas. These include the construction and operation of a linac test stand for developing laser-driven photocathode rf guns with normalized emittances approaching 1 mm-mrad; development of advanced beam compression, stability, and emittance control techniques at multi-GeV energies; the construction and operation of a FEL Amplifier Test Experiment (FATE) for theoretical and experimental studies of SASE at IR wavelengths; an undulator development program to investigate superconducting, hybrid/permanent magnet (hybrid/PM), and pulsed-Cu technologies; theoretical and computational studies of high-gain FEL physics and LCLS component designs; development of X-ray optics and instrumentation for extracting, modulating, and delivering photons to experimental users; and the study and development of scientific experiments made possible by the source properties of the LCLS.