TUOAS1 Proceedings of 2011 Particle Accelerator Conference, New York, NY, USA A TUTORIAL ON ACCELERATOR-BASED LIGHT SOURCES∗ M. Borland† , ANL, Argonne, IL 60439, USA Abstract light compared to protons, electrons are far easier to ac- celerate to relativistic energies, and hence are preferred Accelerator-based light sources are some of the largest for radiation generation. The first accelerator-generated and most successful scientific user facilities in existence, x-rays were created by the rapid deflection and decel- serving tens of thousands of users each year. These im- eration electrons experience when hitting a metal target portant facilities enable research in diverse fields, includ- (bremsstrahlung radiation). A more controlled technique ing biology, pharmaceuticals, energy conservation and pro- uses a magnetic field to deflect the particle trajectory in a duction, data storage, and archaeology. In this tutorial, circular arc, which produces acceleration at right angles to we briefly review the history of accelerator-based light the direction of motion. sources. We present an overview of the different types of accelerator-based light sources, including a description of For circulating electrons with β 1, radiation is emit- their various operating principles, as well as a discussion of ted in a broad angular pattern at the revolution frequency. measures of performance. Technical challenges of current Radiation is emitted most strongly in the forward and back- and future light sources are also reviewed. ward directions, for which a distant observer sees the great- est acceleration. However, when β ≈ 1 the emitting elec- tron follows closely behind the forward-directed radiation, INTRODUCTION which has β =1. Thus, the forward-directed radiation pulse seen by a distant observer is dramatically shortened, By the early 20th century, it was known that electri- by the factor γ3, and therefore has much higher frequency cal charge was carried by two types of particles, the elec- by the same factor [3]. Also, the radiation intensity in- tron and proton, that are constituents of ordinary matter. creases by a factor γ4 because the apparent time taken to Interactions between electrically charged objects and be- “turn the corner” is much shorter, indicating a larger appar- tween currents in wires and magnetic fields were recog- ent acceleration [4]. Because the apparent acceleration is so nized as macroscopic manifestations of microscopic inter- much greater for that part of the arc where the observer sees actions between charged particles, mediated by the elec- a change of direction, the radiation is narrowly collimated tric and magnetic fields they produce. Visible light itself with an rms angle 1/γ, like a moving searchlight. These was recognized as consisting of mutually-supporting, time- effects explain the usefulness of SR from high-energy elec- varying electric and magnetic fields, originating in the mo- tron accelerators. tions of charged particles. With a growing ability to create and control electric fields E and magnetic fields B ,and armed with the Lorentz force equation F = qE + qv × B , researchers acquired increasingly precise control of the motion of charged particles. By the 1940’s, circular accel- erators (“betatrons” and “synchrotrons”) could accelerate electrons to nearly the speed of light (v/c = β ≈ 1). Lienard predicted in 1898 [1] that a charged particle moving on a circular trajectory would radiate, but it was not until 1947 that 70 MeV electrons circulating in a syn- chrotron were observed to emit visible light [2], leading to the name “synchrotron radiation” (SR). Rigorous anal- ysis [3], shows that only acceleration of charged particles Figure 1: Radiation flux from a 100 mA beam of different produces radiation, which is characterized by magnetic and energies on a circular trajectory in a 1T magnetic field. electric fields transverse to the propagation direction that fall off with the first power of distance. Analysis also shows that the radiation output is pro- Fig. 1 shows flux curves for a 100 mA beam of vari- portional to a high power of the Lorentz factor, γ = ous energies circulating in a 1T magnetic field. The crit- 2 1/ 1 − β2. (Note that beam energy E is E(MeV) ≈ ical photon energy ec(eV) = 665E(GeV) B(T),which 0.511γ.) Hence, to produce radiation effectively, we need divides the power spectrum into two equal parts, is also relativistic particles, i.e., β ≈ 1. Because they are very marked. This illustrates the increasing ease with which en- ergetic radiation is produced as the beam energy increases, ∗ Work supported by the U.S. Department of Energy, Office of Sci- 2011 by PAC’11 OC/IEEE — cc Creative Commons Attribution 3.0 (CC BY 3.0) which is one reason electron accelerators are so useful. Be- ence, Office of Basic Energy Sciences, under Contract No. DE-AC02- c ○ 06CH11357. fore explaining electron accelerators in more detail, we’ll † [email protected] briefly discuss some applications of SR. Light Sources and FELs Copyright 702 Accel/Storage Rings 20: Accelerators and Storage Rings (Other) Proceedings of 2011 Particle Accelerator Conference, New York, NY, USA TUOAS1 APPLICATIONS four quantities, namely, the energy spread and pulse dura- tion, along with the “transverse emittances,” x and y.In Following the initial observation of synchrotron radi- the simplified case used here, q = σqσq,whereq is x or y. ation, several essentially parasitic experiments were per- Typical light-source emittances are less than 10 nanometers formed in the 1950s and 1960s [5], leading to a dedicated to as low as a few picometers. facility, Tantalus I, in 1968. This facility demonstrated the Next, we review the basic components of an electron ac- utility of SR and encouraged world-wide growth of SR celerator, starting with the electron source or “gun.” These sources, so that today a full account of SR applications devices employ a thermally- or laser-excited “cathode” to would require many volumes. Here, we try to provide a release electrons, which are accelerated by pulsed DC or basic understanding of why it is so useful. radio frequency (rf) electric fields. In linear light sources We begin by describing the interaction of radiation (LLS), the gun’s beam quality is critical, and great effort with matter [3, 6]. When relatively low-energy (i.e., is expended on gun design, while for circular light sources low-frequency or long-wavelength) radiation interacts with (CLS), quantity is more critical than quality at this stage matter, it can excite vibrational modes of molecules and (see below). Typical gun beam energies are 100’s of keV hence tends to be absorbed or scattered. (For example, for DC guns to a few MeV for rf guns. At present, pho- in water this occurs between about 1 μeV and 1 eV.) As tocathode rf guns produce the highest quality beams since the frequency increases, this interaction diminishes and the the rapid acceleration reduces space charge effects. material may become transparent (as water largely does be- Following the gun, the electron beam typically enters tween about 1.7 and 3.5 eV). With increasing frequency, a linear accelerator (“linac”), which boosts the energy by radiation eventually interacts strongly with the light-weight several orders of magnitude using synchronized rf fields in electrons, and absorption rises dramatically again, only to a series of long, multi-cell rf cavities. For LLS, rapid ac- fall off rapidly as the energy increases to 10’s of keV and celeration and careful beam control is essential to preserv- beyond. Absorption is stronger for denser elements, which ing beam quality. For CLS, the beam is typically injected have more electrons per unit volume. Narrow absorption into a circular “booster synchrotron” as soon as practical, spikes are also observed at element-specificenergies,cor- at an energy of a few hundred MeV. In a booster, the beam responding to ionization of core electrons. This element- circulates past the same rf cavities repeatedly, acquiring a specificity leads to many applications. very large energy (typically several GeV) with compara- The radiation wavelength is λr(A)˚ = 12.39/Ep(keV), tively moderate expense. Because of radiation emitted in where Ep is the photon energy. Typical interatomic dis- the synchrotron, the final beam quality has little relation to tances in solids are several angstroms [7], corresponding to the beam quality entering the synchrotron, hence, the rela- photon energies of several keV. Hence, such radiation can tively relaxed attitude toward gun beam quality. give us information on the structure of solids, via Bragg The Lorentz equation states that, in addition to using diffraction, a field known as x-ray crystallography. electric fields E to accelerate electrons, we can use mag- Other SR applications include imaging and spec- netic fields B to bend their paths. For example, a locally troscopy. The common hospital x-ray is a simple example constant vertical magnetic field will bend to the left or right of imaging, making use of differences in x-ray absorption the path of a horizontally traveling beam. Such a field is of different materials. In spectroscopy, the variation in ab- created using a magnet with one north and one south pole, sorption or scattering with photon energy is used to probe i.e., a dipole magnet. Dipole magnets are used to bend the chemical bonds or the electronic structure of materials. beam path into a closed loop in a synchrotron or CLS “stor- age ring.” Since the degree of bending depends on the elec- BASICS OF ELECTRON ACCELERATORS tron energy, dipole magnets can be used for energy analysis and energy-dependent beam manipulation in linacs. We’ve seen that energetic radiation can be created using Of course, in addition to changing the beam path, dipole high-energy, free electrons. Electrons can be formed into magnets also cause the beam to radiate and thus lose en- a highly-directed, pulsed beam with low energy spread, ergy.