1 Lasers, Free-Electron Claudio Pellegrini and Sven Reiche • Q1 University of California, Los Angeles, CA 90095-1547, USA e-mail: [email protected]; [email protected] Abstract Free-electron lasers are radiation sources, based on the coherent emission of synchrotron radiation of relativistic electrons within an undulator or wiggler. The resonant radiation wavelength depends on the electron beam energy and can be tuned over the entire spectrum from micrometer to X-ray radiation. The emission level of free-electron lasers is several orders of magnitude larger than the emission level of spontaneous synchrotron radiation, because the interaction between the electron beam and the radiation field modulates the beam current with the periodicity of the resonant radiation wavelength. The high brightness and the spectral range of this kind of radiation source allows studying physical and chemical processes on a femtosecond scale with angstrom resolution. Keywords free-electron laser; undulator; microbunching; SASE FEL; FEL oscillator; FEL amplifier; FEL parameter; gain length. 1 Introduction 2 2 Physical and Technical Principles 5 2.1 Undulator Spontaneous Radiation 6 2.2 The FEL Amplification 7 2.3 The Small-signal Gain Regime 10 2.4 High-gain Regime and Electron Beam Requirement 11 2.5 Three-dimensional Effects 13 OE038 2 Lasers, Free-Electron 2.6 Longitudinal Effects, Starting from Noise 14 2.7 Storage Ring–based FEL Oscillators 16 3 Present Status 17 3.1 Single-pass Free-electron Lasers 17 3.2 Free-electron Laser Oscillators 18 4 Future Development 20 Glossary 21 References 23 Further Reading 24 1 accelerator and can be used to accelerate Introduction the electron beam to higher energies. The energy transfer can take place only A free-electron laser (FEL) [1] transforms if a condition of synchronism between the the kinetic energy of a relativistic electron wave and the beam oscillations is satisfied beam produced by a particle accelerator (Sect. 2.2). This condition gives a relation- like a microtron, storage ring, or radio ship between the radiation wavelength λ, • Q2 frequency (RF) linear accelerator into the electron beam velocity βz along the un- electromagnetic (EM) radiation. The trans- dulator axis (measured in units of the light formation occurs when the beam goes velocity c), and the undulator period λ0: through an alternating magnetic field, pro- λ (1 − β ) duced by a magnet called an undulator [2], λ = 0 z (1) that forces the electrons to move in an βz oscillatory trajectory about the axis of the For relativistic electrons, this condition can system, as shown in Fig. 1. An electromag- alsobewritteninanapproximate,but netic wave propagates together with the more convenient, form using the beam electron beam along the undulator axis, energy γ (measured in the rest energy and interacts with the electrons. units E = mc2γ ): The undulator magnet is a periodic structure in which the field alternates λ λ = 0 (1 + a2 ). (2) between positive and negative values and 2γ 2 w has zero average value. It produces an electron trajectory having a transverse The dimensionless quantity velocity component perpendicular to the eB0λ0 axis and parallel to the electric field of the aw = (3) 2πmc wave, thus allowing an energy exchange between the two to take place. One can is the undulator vector potential normal- either transfer energy from the beam to the ized to the electron rest mass mc2; e is the wave, in which case the device is an FEL, or electron charge and B0 is the undulator from the wave to the beam, in which case it peak magnetic field. (Here, and in the rest is an inverse FEL (IFEL) [3]. In the second ofthepaper,weuseMKSunits.)Thequan- case, the system is acting like a particle tity aw is called the undulator parameter, OE038 Lasers, Free-Electron 3 Undulator Electron Outcoupled beam field Optical cavity mirror Radiation field Partial mirror Fig. 1 Schematic representation of an FEL oscillator, showing its main component: the electron beam, undulator magnet, and optical cavity. The undulator shown is a permanent-magnet planar undulator. The arrows indicate the magnetic field direction. In an FEL amplifier of SASE FEL setup the optical cavity is omitted. The FEL amplifier is seeded by an external radiation field which is the normalized vector potential of fact suggests that high- to average-power the undulator field. The undulator can be FELs can be designed without the problem, of two types: helical and planar (Sect. 2). common in atomic and molecular lasers, Equation 2 is valid for a helical undulator. of heating the lasing medium. In the case of a planar undulator, Eq. 2 The time structure of the laser beam is still valid if we replace the undulator√ mirrors that of the electron beam. De- parameter with its rms value aw/ 2. pending on the accelerator used, one can Because of the dependence of the radi- design systems that are continuous-wave ation wavelength on the undulator period, (cw) or with pulses as short as picoseconds magnetic field, and electron-beam en- or subpicoseconds. ergy – quantities that can be easily and Tunability, high efficiency, and time continuously changed – the FEL is a tun- structure make the FEL a very attractive able device that can be operated over a source of coherent EM power. In some very large frequency range. At present, the wavelength regions, like the X-ray, the range extends from the microwave to the FEL is unique. Its applications range from UV [4]. A new FEL is now under construc- purely scientific research in physics, chem- tion in the United States [5] to reach the istry, and biology to military, medical, and X-ray region, about 0.1 nm. A similar pro- industrial applications. gram is being developed in Germany [6], FELs originate in the work carried out and other FEL to cover the intermediate in the 1950s and 1960s on the generation region between 0.1 nm and the UV [7] are of coherent EM radiation from electron also being considered by several countries. beams in the microwave region [2, 8]. The efficiency of the energy transfer As scientists tried to push power sources from the beam kinetic energy to the EM to shorter and shorter wavelengths, it wave is between 0.1 to a few percent for became apparent that the efficiency of most FELs, but it can be quite large, up to the microwave tubes, and the power about 40%, for specially designed systems. they produced, dropped rapidly in the The beam energy not transferred to the millimeter region. It was then realized that EM wave remains in the beam and can this problem could be overcome by using be easily taken out of the system, to be an undulator magnet to modify the beam disposed of, or recovered elsewhere. This trajectory [1], making it possible for the OE038 4 Lasers, Free-Electron beam to interact with a wave, away from 1. portions of the EM spectrum, like the any metallic boundary. Two pioneering far infrared (FIR), or the soft and experiments at the Stanford University [9, hard X-ray region, where atomic or 10] proved that the FEL is a useful source molecular lasers are not available or of coherent radiation. are limited in power and tunability; The current disadvantage of FELs is the 2. large-average power, high-efficiency greater complexity and cost associated with system. the use of a particle accelerator. For this reason, the use and development of FELs An order-of-magnitude comparison of are mainly oriented to the following: the peak brightness obtained or expected 1035 TESLA SASE FELs 1033 DESY TTF-FEL LCLS (seeded) 1031 Spontaneous spectrum SASE FEL 1 0.1% bandw.)] 29 DESY 2 10 TTF-FEL 30 GeV mm 15 GeV 2 Spontaneous spectrum TESLA SASE FEL 4 spontaneous undulator 1027 SPring8 undulator 25 10 (30m in vacuum) ESRF-undulator TTF-FEL (ID23) spontan 1023 Peak brilliance [Phot./(sec mrad Peak BESSY-II APS U-49 undulator BESSY-II (Typ A) 21 U-125 10 PETRA undulator ALS U5.0 1019 101 102 103 104 105 106 Energy (eV) Fig. 2 Peak brightness of free-electron lasers in the VUV and X-ray regime, compared to 3rd generation light sources [11] OE038 Lasers, Free-Electron 5 for the FEL, compared to other sources of The accelerators used to provide the elec- EM radiation, is given in Fig. 2, pointing tron beam are of many types: electrostatic, out again the interest for FELs in the soft- induction line, radio-frequency (rf) linear to-hard X-ray regions. accelerator, pulsed diode, or storage rings. In Sect. 2 of this article, we discuss Some of their basic characteristics, their the general physical principles of FELs energy ranges, and the FEL wavelengths and the properties and characteristics of for which they are more commonly used their main components, that is, the un- are given in Table 1. dulator magnets, electron beams, and Undulator magnets are of two main optical systems. We also describe the types: helical or planar. In the first case, main properties of the spontaneous ra- the magnetic field vector rotates around diation from an undulator, the process the axis as a function of axial distance; leading to the amplification of radia- in the second case, its direction is fixed, tion, the high-gain and small-signal-gain and its amplitude oscillates along the axis. regimes, diffraction effects, saturation These magnets can be, and have been, effects, the limits on the system effi- built using a wide variety of technologies: ciency, the dependence of gain on beam pulsed or DC electromagnets, permanent parameters. magnets, and superconducting magnets. In Sect.