The fundamentals of free electron laser theory are now well-established and can provide a sophisticated description of experiments over a wide range of parameters. While new technology is being developed for systems working from 1mm to 10nm wavelengths, the theory remains the same.
A review of free-electron laser research at UCSB is presented here. Among the topics included are: 1) the development of high-quality electron beam sources based on electrostatic accelerating fields, 2) the analysis of present FEL operating characteristics, 3) the development of advanced FEL concepts, and 5) the utilization of the UCSB FEL in scientific research.
The storage ring based free electron laser is capable of producing a high repetition rate UV laser beam tunable in the range 500 to 50 nm, and ultimately to 10 nm. The predicted output characteristics and the technological constraints on its operation are described.
The geometry of a free-electron laser (FEL) resonator differs from a conventional high energy laser (HEW. In most high powered lasers, the gain is provided by atoms or molecules that are excited into a higher energy level. It is usually possible to produce this type of excited state over a relatively large volume. This provides the cavity designer of more conventional HELs with both opportunities and problems. On the plus side, it is easy for a designer to use the large cross section of the active (lasing) material to spread the power of the laser beam over a larger surface of the cavity mirrors. On the negative side, the larger extent of this relatively dense active media produces variations in both the optical phase and intensity of the beam, which need to be controlled if the beam is to be focused effectively on a target. In contrast, the active medium of a free-electron laser consists of a beam of high speed electrons passing through a series of small magnets. The magnets make the electrons wiggle as they move, and this wiggling makes the electrons give off light. From an optics point of view, the main difference is that it is not practical, at this time, to produce very thick beams of high energy, high quality electrons so that the active area in an FEL has an overall shape of a long, very thin rod. In order for the laser to work effectively, the beam of light must also take on this shape so that the laser light and the electron beam overlap. This presents the cavity designer with the problem of dealing with a beam of light of very high power, concentrated into a very small cross-sectional area. This light can be too concentrated for a normal mirror, which makes it necessary to use some unusual cavity designs.
The free-electron laser (FEL) has achieved impressively high power levels at high efficiency in the millimeter wavelength regime. Recent advances in this regime include the incorporation of efficiency enhancement techniques, the successful operation of high-gain amplifiers and oscillators, and the design of more compact, more practical devices. In this review, the development of these relatively long wavelength FELs will be traced through the past decade. The major differences between these long wavelength devices and the infrared to ultraviolet FELs will be described. Finally, 'some of the technology issues particularly relevant to millimeter wave operation will be discussed.
Radio-frequency linear accelerators have been, and will continue to be, the most important source of electrons for free-electron lasers. This article describes the principles of operation of these devices along with some of their technological problems and expectations for the future. Some results of rf-linac free-electron laser experiments are discussed, with emphasis on those features of free-electron laser operation which are peculiar to devices powered by rf linacs.
Recent analyses of the self-consistent nonlinear interaction of the gain medium and the optical field in free-electron lasers (FELs) are reviewed. A unique longitudinal feature of the FEL stems from the closed synchrotron orbits of electrons trapped in the ponderomotive potential well. Such oscillations result in the generation of optical sidebands. Sideband buildup enhances extraction efficiency in untapered systems but leads to electron detrapping in FEL oscillators with long, highly tapered undulators. Fortunately, full extraction may be recovered with use of intracavity wavelength selectivity. Analysis of the transverse mode structure in FEL oscillators shows that typical geometries produce near diffraction-limited beam quality. However, under certain conditions, namely high gain and with near-concentric resonators, effects due to the radially nonuniform gain medium may be noticed. In particular, the one-way gain medium may result in a near diffraction-limited mode which is somewhat mismatched to the cavity. Preliminary three-dimensional analysis of the sideband instability indicates that beam quality is unaffected when the instability is active.
The electron beam in a free-electron laser (FEL) can act as an optical fiber, guiding or bending the optical beam. The refractive and gain effects of the bunched electron beam can compensate for diffraction, making possible wigglers that are many Rayleigh ranges (i.e., characteristic diffraction lengths) long. The origin of optical guiding can be understood by examining gain and refractive guiding in a fiber with a complex index of refraction, providing a mathematical description applicable also to the FEL, with some extensions. In the exponential gain regime of the FEL, the electron equations of motion must be included, but a self-consistent description of exponential gain with diffraction fully included becomes possible. The origin of the effective index of refraction of an FEL is illustrated with a simple example of bunched, radiating dipoles. Some of the properties of the index of refraction are described. The limited experimental evidence for optical beam bending is summarized. The evidence does not yet provide conclusive proof of the existence of optical guiding, but supports the idea. Finally, the importance of refractive guiding for the performance of a high-gain tapered-wiggler FEL amplifier is illustrated with numerical simulations.
The free-electron laser (FEL) directly converts the energy of an electron beam to electromagnetic radiation. The properties of the radiation are intimately related to the characteristics of the accelerator that produces the electron beam. A radio-frequency (rf) accelerator produces a high-frequency (1-100 MHz) burst of short electron pulses (10-30 ps) at relatively low peak current. The burst typically lasts tens of microseconds. The low peak current (0.1-1 kA) implies that an FEL driven by an rf accelerator must operate with low single-pass optical gain and produce radiation pulses of relatively low peak power. By contrast, an induction linac (IL) accelerator produces an electron beam consisting of longer (50 ns), higher current (1-10 kA) pulses. These pulses can be produced in a variety of pulse formats, including a series of high repetition rate bursts or a cw pulse train. For equal average powers, the induction linac operates at a lower duty factor than the RF accelerator.
Free-Electron Laser (FEL) experiments place extreme demands upon the accelerator that provides the electron beam that drives the wiggler. A wide range of diagnostic devices is needed to determine the properties of the accelerated beam, to assist in adjusting the accelerator and the associated beamline, and to monitor the changes to the beam that occur after it interacts with the laser beam in the wiggler. We explain the FEL physics issues that set such severe requirements and describe a representative selection of diagnostic devices that are currently in use to provide the measurements.
Encouraged by experimental operation of free-electron lasers in the visible to far-infrared, several research centers are now designing and/or building devices to extend into the vacuum-ultraviolet and soft x-ray regions below 100 nm, collectively referred to as the extreme ultraviolet. Here, the peak- and average-power output of FEL oscillators and amplifiers should surpass the capabilities of any existing, continuously tunable photon sources by many orders of magnitude. These devices are certain to greatly enhance the future research capabilities of a number of scientific disciplines and industrial applications beyond those possible with synchrotron light sources. We review the features and output parameters of several of these active or proposed programs and the technological challenges that must be met to guarantee their success.