The mechanism of nonlinear harmonic generation in the exponential gain regime, which is driven by bunching at the fundamental wavelength, may provide a path toward both enhancing and extending the usefulness of an x-ray free- electron laser (FEL) facility. Related exotic generation schemes, which exploit properties of harmonic production in various undulator topologies, have been discussed both in the past and more recently. Using three different numerical simulation codes, we explore the possible utility of such schemes (e.g., harmonic afterburners and biharmonic undulators) at future light source facilities.
Over the last few years, there has been a growing interest in self-amplified spontaneous emission (SASE) free-electron lasers (FELs) as a means for achieving a fourth-generation light source. In order to correctly and easily simulate the many configurations that have been suggested, such as multi- segmented wigglers and the method of high-gain harmonic generation, we have developed a robust three-dimensional code. The specifics of the code, the comparison to the linear theory as well as future plans will be presented.
Construction of a single-pass free-electron laser (FEL) based on the self-amplified spontaneous emission (SASE) mode of operation is nearing completion at the Advanced Photon Source (APS) with initial experiments imminent. The APS SASE FEL is a proof-of-principle fourth-generation light source. As of January 1999 the undulator hall, end-station building, necessary transfer lines, electron and optical diagnostics, injectors, and initial undulators have been constructed and, with the exception of the undulators, installed. All preliminary code development and simulations have also been completed. The undulator hall is now ready to accept first beam for characterization of the output radiation. It is the project goal to push towards full FEL saturation, initially in the visible, but ultimately to UV and VUV, wavelengths.
It is commonly assumed that a matched electron beam optimizes free-electron laser (FEL) performance; however, this assumption has not been proven for FELs operating in the high gain regime. We test this hypothesis using a 3D multimode analysis which is capable of modeling matched and unmatched electron beams and for design parameters suitable for a SASE experiment at Duke University using the PALADIN wiggler. The gain length predicted for a matched beam is in good agreement with analytic theory. Further, the simulation indicates that while the gain length is optimized for a matched beam the saturated power is not necessarily optimized.
Two distinct types of free-electron laser simulations are in general use. In 'wiggler-averaged' simulation codes such as FRED and TDA, the Lorentz force equations are averaged over a wiggler period. A reduced orbit analysis is obtained requiring integration only of equations for the energy and ponderomotive phase for each electron. In contrast, 'non-wiggler-averaged' codes such as MEDUSA use the complete three-dimensional Lorentz force equations. A direct comparison of the two techniques is discussed in this paper using the MEDUSA and TDA codes.
The nonlinear simulation of x ray free-electron lasers is dealt with using the non-wiggler-averaged MEDUSA simulation code and realistic wiggler model built up from the contributions of permanent magnets in a Halbach configuration in conjunction with a model for a FODO lattice for enhanced focusing. The specific parameters of interest are relevant to the linac coherent light source design at SLAC and deals with a 15 GeV/5 kA electron beam, and a wiggler with a 3.0 cm period and an on-axis amplitude of approximately 12.7 kG which results in x ray emission at wavelengths in the vicinity of 1.3 angstrom.
We consider linac-driven SASE (stimulated amplification of spontaneous emission) using the Duke linac and the PALADIN wiggler. SASE design studies using this equipment are described based on the non-wiggler-averaged FEL simulation code MEDUSA. We have identified a new collective Compton regime in which ac space-charge is unimportant but the dc self-fields of the beam are important.
The Naval Research Laboratory is investigating innovative magnetic wigglers to reduce beam energy requirements for millimeter wave FELs and to enhance the gain and efficiency. Recent work has focused on coaxial designs. The advantages of this are twofold. First, annular configurations are advantageous for propagating high current beams. The annular geometry permits use of the central structure to enhance the wiggler field, hence, allowing shorter wiggler periods. One such wiggler is referred to as the Coaxial Hybrid Iron (CHI) wiggler, and employs a solenoid enclosing periodic arrays of ferromagnetic and nonferromagnetic material arranged as an outer ring and an inner rod. A second wiggler uses both outer and inner bifilar helical current windings. Both wiggler designs result in substantial enhancements in the wiggler field experienced by the electron beam as compared with the fields in the absence of the central structure. A prototype CHI wiggler is discussed along with a 35 GHz amplifier experiment which is under construction. Preliminary performance calculations for a two helix wiggler system are discussed. This will include both orbit theory and a fully 3D nonlinear simulation of the interaction.
A self-consistent analysis of weight field errors in free-electron lasers is described using the 3D simulation code WIGGLIN. The 3D wiggler field model chosen is able to treat gradients in the wiggler amplitude since both the divergence of the field as well as the axial component of the curl vanish identically while the transverse components of the curl are small. Hence, the field model is well-suited to the treatment of small imperfections in the wiggler amplitude. In order to describe the wiggler imperfections, a random variation is chosen to determine the pole-to-pole variations in the wiggler amplitude and a continuous map is used between the pole faces. The specific parameters chosen for study correspond to the high-power 35-GHz free- electron laser experiment conducted at Lawrence Livermore National Laboratory; however, the fundamental physics issues are relevant to the entire range of free-electron laser experiments.
A magnetic wiggler design has been developed for applications in free-electron lasers which is scalable to small periods with high field amplitude, high beam current acceptance, and excellent transverse focusing and beam propagation properties. The Coaxial Hybrid Iron (CHI) wiggler design consists of a coaxial arrangement of alternating ferromagnetic and non- ferromagnetic rings with the central portion of the coax shifted by one half period. The entire arrangement is immersed in a solenoidal field which results in a cylindrically symmetric periodic field. A key advantage of this wiggler configuration is its capacity to handle very high beam currents with excellent focusing and transport properties. FEL configuration using the CHI wiggler design have the potential for high power, high frequency coherent generation in relatively compact systems. Analytic and simulated characteristics of the CHI wiggler are presented.
In conventional microwave tubes electron beams propagate through structures which effectively reduce the phase velocity of electromagnetic waves below the speed of light in vacuo. In such cases, an electron beam propagating parallel to the waves with a velocity close to the wave phase velocity can engage in a resonant interaction with a high intrinsic efficiency.