The Free Electron Laser: Properties and Prospects 1

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1 The Free Electron Laser: Properties and Prospects 1 Gunnar Ingelman and Kai Siegbahn Uppsala University Abstract: Novel electron accelerator techniques can be used to create free electron lasers in a wide wave length interval extending to the X-ray region, i.e. providing an X-ray laser. Due to the coherent emission, very large intensities can be obtained, with peak brilliance ten orders of magnitude higher than in conventional synchrotron radiation. This opens completely new prospects for sciences that study material and molecular structures as well as their dynamics using the high time resolution. One should consider three-dimensional imaging, e.g. using holography, with very high space-time resolution envisage the potential of femto-second snap-shots of atomic structures! We describe briefly the basic physics of free electron lasers, the present development and future plans, and discuss the potential applications. 1 Introduction The technology for electron accelerators has made substantial progress in recent years. A main driving force for this development is the desire of particle physicists to build a linear electron-positron collider in the energy region of 500 GeV to 1 T ev. This would extend the present studies of fundamental forces between point-like particles to a new energy region where new physics questions can be addressed and new phenomena could be expected. Therefore, the Deutsches Elektronen-Synchrotron laboratory (DESY) in Hamburg has a large development program in accelerator technology in collaboration with institutes from all over the world. This has led to a recent break-through by the achievement of high accelerating gradients (25 M V/m) using superconducting niobium cavities of extremely high quality. Together with other developments to reduce the costs, this would make such an e + e linear collider feasible. A linear electron accelerator with such low-frequency superconducting cavities is an ideal driver for a Free Electron Laser (FEL). The successful operation of very precise undulator magnets for synchrotron radiation is here also of importance. With further developments such undulators can be used to obtain Self-Amplified Spontaneous Emission (SASE) which is the underlying principle for the coherent laser effect. DESY is presently building a test facility [1] to demonstrate that a free electron laser with short wavelength can be obtained in this way. It will also be used for experiments with laser light of wavelengths down to 6 nm or a photon energy of 207 ev (E [ev ] = 1240/λ[nm]). This can be compared to usual X-ray sources, e.g. K α emission from Cu and Al with λ = 0.15 nm (E = 8 kev ) and λ = 0.9 nm (E = 1.4 kev ), respectively. For the future, one is planning for a linear e + e collider where higher energy electron beams can also be used to drive free electron lasers that extend the accessible wavelengths down into the < 0.1 nm region, i.e. suitable for X-ray diffraction work at high spatial resolution. 1 Published in the Swedish Physical Society journal Fysik-Aktuellt no. 1, p. 3,

2 These developments are therefore of interest to several different research areas. Particle physics would reach new high energy frontiers and nuclear physics exploit high quality electron beams. The integrated FEL producing coherent radiation with orders of magnitude increased intensity would open a whole new field for synchrotron radiation users in physics, chemistry and biology. An international meeting was held 1996 in Uppsala to review and discuss these prospects in an interdisciplinary research community. A summary of the meeting can be found in [2] and the full proceedings in [3] [13]. Here, we concentrate on the more nearby future based on electron accelerators in the 1 GeV region, which can provide soft X-ray lasers of large potential interest in many areas. Our objective is to give a short account for the basic principles, ongoing developments and propsects for future applications. For more details we refer to the recent reviews, research papers and technical design reports we quote at the end of our report. 2 The physics of free electron lasers To get a simple basic understanding of the phenomenon of a free electron laser, consider the electromagnetic radiation from an accelerated charge. From basic principles, the radiation is proportional to the charge squared e 2. If N charges radiate incoherently the effect will be P incoherent N e 2, which is the normal case and applies, e.g., in synchrotron radiation. However, suppose that the N charges would radiate coherently, i.e. effectively as a single charge, then the effect would be P coherent (N e) 2. This gives an enormous increase provided that N is large. The requirement that the charges effectively act as a single point charge means that the spatial extent of the charge distribution must be smaller than the wavelength of the emitted radiation. Thus, the basic requirement is to have a very small bunch containing many electrons to get a sizeable effect. However, this causes Coulomb repulsion that tends to enlarge the bunch. This problem can be minimized by rapidly bringing the electron beam to relativistic velocities where the transverse electric space charge force is approximately cancelled by the self-induced magnetic field of the beam, while the longitudinal repulsion is less effective due to the increased relativistic mass. The way to accomplish this is by using a very short (few picoseconds) laser pulse on a photocathode in a high gradient radio frequency (rf) accelerating field. This should result in an intense electron burst containing about one nanocoulomb, i.e. an instantaneous current of a few hundred amperes. Both the electron source and the accelerator must therefore meet very stringent demands. A synchrotron is not a priori suitable since radiation in the bent orbit results in electron bunches that are too spread-out to radiate coherently. Also the necessary instantaneous currents cannot be reached. Instead one must consider linear accelerators with properties such as to keep the small dimension of the bunches and even compress them in the longitudinal direction. The laser emission should arise when the electron bunch traverses a long undulator magnet as illustrated in Fig. 1. The alternating magnetic fields force the electron on a sinusoidal trajectory, i.e. a transverse acceleration causing spontaneous emission of electromagnetic radiation (like in normal synchrotron radiation). This radiation field interacts with the electron bunch and can stimulate further emission such that a laser effect is obtained with a large increase in the radiated power (Fig. 1c). 2

3 Figure 1: Principle of a single-pass free electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. (a) Electron motion through the Undulator with alternating magnetic fields forcing the electrons into a sinusoidal trajectory leading to electromagnetic radiation which recouples to the electron bunch causing laser action through SASE. (b) Increase in radiated power along the electron beam path, showing the exponential increase due to SASE. With high enough electron current and long enough undulator, the power is saturated and energy oscillates between the electron and photon beam. (c) Energy exchange between electron and photon beam with resonance condition leading to microbunching with coherent emission. 3

4 The undulator gives a resonance condition between the electron bunch and the electromagnetic wave as illustrated in Fig. 1c. Over one undulator period λ u, the time difference between the electron bunch and the wave must correspond to the wavelength λ phot of the spontaneously emitted light, i.e. the longitudinal slippage of the electrons relative to the light must equal λ phot. Due to the coherence of the radiation and the specific properties of the electron beam (extremely small emittance and high charge density), the electron bunch can interact sufficiently strong with the electromagnetic field to form a microbunch structure with periodicity λ phot, as illustrated in Fig. 2. Each microbunch is then effectively a point charge with respect to radiation of wavelength λ phot. With N electrons in the microbunch, the radiated intensity is (Ne) 2 or N times the intensity of the sponaneously emitted radiation. Moreover, many (n) microbunches are lined up in phase and therefore stimulate each other giving an additional amplification through coherence in their radiation, P coherent = n bunches N P incoherent. Thus, the emission from one electron is very much enhanced due to the presence of the field of all the other electrons, i.e. stimulated emission. Figure 2: Illustration of microbunch structure formed in an electron bunch through interaction with the radiation field created in the undulator. The number of microbunches is in reality much larger as is clear from the indicated smallness of microbunch separation relative to the overall bunch length. The start of the SASE process is through a perturbation of the electron bunch. This could either be from some external source or from intrinsic fluctuations, i.e. from noise. In this latter case, microbunching starts with different phases at different positions inside the electron bunch, leading to sections of coherent structures within a cooperation length that is smaller than the bunch length. These coherent sections in the bunch gives a fine-structure with spikes in each laser pulse. The overall pulse length is of order 100 femtoseconds (f s) with individual spikes of the order 1 f s. This spiky time structure, which varies from pulse to pulse, may cause problems in some types of experiments. It may, however, be avoided if the SASE process is initiated by a seed which can, e.g., be obtained from a preceeding undulater. For a more detailed account, see [7]. It should be noted that the FEL does not require any mirrors or resonating laser cavity structure. This is a great advantage at short wave lengths where, e.g., mirrors and optics for X-rays are difficult technological problems. 4

5 3 Ongoing developments and future plans The SASE principle has not yet been experimentally demonstrated at short wavelengths. Some evidence for SASE has only been obtained in the µm range. Therefore, the first goal of the R&D program at DESY is a proof of principle down to nm wavelength. For this purpose the TESLA Test Facility (TTF) is presently under construction at DESY, see Fig. 3. Figure 3: Layout of the free electron laser in the TESLA test facility at DESY (the LINAC is < 100 m long). The electron energies are given together with the bunch length σ s before and after compression. The electron gun uses a laser with very short pulses that hit a photocathode located in a synchronized high gradient rf accelerating field. This produces the necessary short and dense bunches of electrons mentioned above. The longitudinal bunch length is initially 2 mm, but is compressed in three stages along the accelerator to become 50 µm, which corresponds to 150 fs bunch length. The linear accelerator (LINAC) is based on the superconducting niobium cavities mentioned above. Their frequency is 1.3 GHz and they operate at liquid helium temperature of 2 K. Eight one-meter long cavities are mounted in a 12 m long unit that includes quadrupole focusing magnets etc. In phase I of the project, three such units will be installed resulting in electron energies up to 380 MeV. In phase II an additional five units will be installed, raising the energy to 1 GeV. It is, of course, the high accelerating gradients ( 25 MeV/m) in these cavities that facilitate the small length of the LINAC. The following undulator is long (30 m) and of very high precision in order for the SASE effect to saturate. It is based on permanent magnets arranged with alternating field directions of periodicity 2.7 cm. Very high mechanical precision is needed as well as integrated quadrupole elements for focussing. After the undulator, the electron beam is bent away by magnets and the FEL photon beam can be used in experiments. The time schedule is as follows. The electron gun and the first accerelating unit became operational in summer Phase I should be operational and give the proof of the principle of the SASE FEL by the end of Installing phase II should lead to the first experiments using the 1 GeV electron beam in This should not only be for accelerator R&D, but also for users of the FEL radiation and for developing detector techniques for various user communities. 5

6 The development of FEL s is considered also at other places in the world, in particular at the large electron accelerator laboratories. The Stanford Linear Accelerator Center (SLAC) persues the Linac Coherent Light Source (LCLS) project [14] using electrons in the energy range 5 15 GeV from the existing linear accelerator. This would provide photon wave lengths in the range nm and a peak power of 10 GW, i.e. a high power X-ray laser. Substantial progress has been made in developing the necessary high performance electron gun, as well as in designing a 100 meter long undulator [14]. The new 3rd generation synchrotron radiation facilities APS at Argonne (USA) and SPring-8 in Japan may use their LINACs for operating SASE FEL in the VUV wavelength range. New ideas and projects may also arise in this fast developing field having specialized international conferences, e.g. [15]. The expected intensities of free electron lasers are shown in Figs. 4 and 5 in comparison with various synchrotron radiation sources. As can be seen, the brilliance of FEL s are many order of magnitude higher than other sources. Although the main cause for this enormous increase is the coherent laser emission effect, a substantial increase is due to the conventional spontaneous emission in the special undulators used. The difference between FELs and synchrotrons is enhanced by using the quantity brilliance, which is the number of photons per unit time, bandwidth, divergence and size of the source (i.e. the dimension used in Figs. 4 and 5). The FELs, with their short pulse radiated from a very small source, have a very high brilliance although the time-averaged intensity is less prominent. It depends on the experimental situation whether one can exploit the particular properties of the FEL radiation. For example, if the coherence of the radiation is not used and if the detection effectively makes an averaging over a time interval much longer than the pulse length, then the gain by a FEL is reduced. However, in many cases the coherence and the short pulse time are essential to obtain the desired information and then a FEL is the obvious choice. 4 Potential applications Free electron lasers promise to give radiation with unique properties. Of special advantage is the high brilliance, the extremely short pulse length, the coherence, the high degree of polarization and the fast tunability of the photon energy. This should open new and exciting areas of basic and applied research in several disciplines in physics, chemistry and biology. The scientific applications are discussed in some detail in [1, 3, 4, 7, 8, 9, 10, 11]; only a short résumé is given here. Generally speaking, the short wave length provides a high spatial resolution and the high intensity allows for short exposure times such that the time evolution of processes can be followed or unstable targets be investigated. The high peak power can be used to induce and study non-linear phenomena. Areas of research that are expected to profit greatly from a FEL in the VUV to soft X-ray region include [1]: atomic and molecular spectroscopy, spectroscopy of ion beams, ionized gases and plasmas, spectroscopy of atomic and molecular clusters, photoelectron spectroscopy, solid state spectroscopy, physics and chemistry of surfaces and thin films, photochemical reactions, biological structures and dynamics. In material science, the FEL will be a valuable tool in the study of materials and surface processing, multilayer 6

7 Figure 4: Peak brilliance of free electron lasers compared to synchrotron radiation sources (BESSY, ESRF, PETRA etc). TTF-FEL is presently under construction at DESY, whereas TESLA-FEL and SBLC-FEL are in conjunction with a future linear collider; the SASE radiation can be compared with the spontaneous undulator radiation. LCLS is the FEL considered at SLAC. Also shown is the state-of-the-art plasma laser source. 7

8 Figure 5: Average brilliance of free electron lasers compared to synchrotron radiation sources; cf. Fig. 4. magnetic films, electronic structure of semiconductors, heavy fermion materials and high temperature superconductors, as well as the dynamics of catalytic reactions. One may also speculate over the possibility to use this kind of soft X-ray laser in lithography for the manufacturing of integrated semiconductor circuits. This remains to be investigated. The short pulse length can be used to do time-resolved studies of various phenomena, e.g. the electronic structure of molecules during chemical reactions or reaction dynamics 8

9 at semiconductor surfaces. Diffraction experiments will profit from the high brilliance, since a sufficient number of photons may be obtained in a single pulse. This may become very valuable in, e.g., protein crystallography. With laser radiation it is possible to consider new techniques for imaging. For example, one may produce three-dimensional holographic pictures with high spatial resolution due to the short wave length. Developments on X-ray holography is already under way [7, 9]. Imagine what could be studied using three-dimensional imaging having pico- or femtosecond time resolution at the atomic level. There are also potential medical applications of these kinds of facilities [11]. For example, monochromatic X-rays may be used for imaging and treatment of cancer with stereotactic surgery. Of course, there are many uncertainties and problems that have to be solved. Many targets will not stand the high FEL intensity, but be distorted or even disintegrated. The question is whether the time scale for these processes is shorter or longer than the time needed to collect enough scattered radiation in a detector. The first series of test experiments can use the beam as emitted by the FEL. Later, experiments will require beam modifications, e.g. monochromatization, focussing or defocussing, expanded transverse beam size. This requires challenging developments of optical elements and detectors that are capable of withstanding the high power of the FEL radiation. Detectors must also be developed to cover a large enough area and give fast enough response and read-out in order to exploit the short pulses. This will require cooperation between experts from different fields, e.g. biologists collaborating with physicists. The Uppsala meeting [2, 3] ended with a discussion on such collaborations and plans are in progress along this line. 5 Conclusions and outlook It is obvious that the special properties of free electron lasers provide very exciting prospects in many fields of research. It is likely to be the next important step following the fantastic development in the recent two decades of research based on synchrotron radiation. The TTF-FEL presently under construction at DESY will provide a full scale test of the SASE FEL concept and, hopefully, demonstrate its feasibility for giving short wave length laser radiation. The results should be available in late A few years later, around 2001, experience with different types of user experiments will become available. Developments are underway for the new detector technologies necessary to exploit the spectacular properties of this radiation. For example, X-ray holography methods have already shown very interesting progress. The development of the new technologies for accelerators, undulators etc. involve large costs and substantial efforts by highly specialized experts. Therefore, this can only be made at large laboratories like DESY. However, once the problems are solved and the technology developed, one may very well consider to build FEL facilities at somewhat smaller laboratories of national character. The know-how will be available and many 9

10 components may be built by high-technology industries. This will reduce the cost substantially compared to the prototype development stage. The cost will still be large, but certainly within reach for a national laboratory. A crude estimate gives a few hundred MSEK, but a reliable cost estimate will only be available after the commissioning of the DESY TTF-FEL facility. The cost may be reduced through technological developments or by reducing the electron beam energy such that a smaller LINAC is needed. Of course, the starting point should be the scientific case which sets the demands on the energy and other properties of such a facility. As usual with large projects, one should follow the forefront of scientific and technological developments from the early stage if one wants to become an active participant in the future. Acknowledgments. We are grateful to several colleagues at DESY for providing us the exciting information on the free electron laser and demonstrating the TESLA Test Facility, in particular B. Wiik, J. Schneider, J. Roßbach and H. Weise. References [1] A VUV free electron laser at the TESLA test facility at DESY, DESY print June 1995, TESLA-FEL [2] G. Ingelman, L. Jönsson, An integrated electron facility, Europhysics News 27 (1996) 182; G. Ingelman, L. Jönsson, Summary of the meeting, in [3] p. ix [3] Proceedings Future Electron Accelerators and Free Electron Lasers Prospects and Opportunities in Natural Sciences, Uppsala 1996, Eds. G. Ingelman, L. Jönsson, Nuclear Intruments and Methods 398 (1997) [4] B.H. Wiik, The TESLA project: an accelerator facility for basic science, in [3] p. 1 [5] P.M. Zerwas, Physics with e + e linear colliders, in [3] p. 19 [6] A. Wagner, Particle physics experiments at a linear collider, in [3] p. 31 [7] J.R. Schneider, Properties and scientific perspectives of a single-pass X-ray freeelectron laser, in [3] p. 41 [8] S. Svanberg et al., Atomic physics using short-wavelength coherent radiation, in [3] [9] I. Lindau, Condensed matter physics using a coherent X-ray source, in [3] p. 65 [10] M. Wulff et al., Time-resolved structures of macromolecules at ESRF, in [3] p. 69 [11] B. Larsson et al., Future electron accelerators and free electron lasers: Potentials in clinical medicine, in [3] p. 85 [12] P. Hoyer, Physics opportunities at ELFE, in [3] p. 91 [13] B. Frois, ELFE at DESY: Experimental perspectives, in [3] p. 99 [14] SLAC Annual report 1995, p. 13; Stanford Synchrotron Radiation Laboratory, 1996 Activity Report, p. 15 and Users Newsletter, October 1997, p. 13 [15] Proc. 18th International Free Electron Laser Conference, Rome 1996, Eds. G. Dattoli, A. Renieri, Nuclear Intruments and Methods 393 (1997)