While most types of laser produce coherent light from electric charges bound within atoms, molecules, or solids, unbound charges are the light source in free-electron lasers. Lasers of this type can operate at higher frequencies than are easily achieved with bound-electron lasers. Various uses and designs of free-electron lasers are the focus of different projects sponsored through the Department of Energy.
Lasers, like any source of light or other electromagnetic waves, produce waves when some of the electric charges they contain go from having a higher energy to a lower one. The energy difference is emitted from the charges in the form of electromagnetic waves—vibrating electric and magnetic fields that spread out from their source—whose vibrational frequencies are proportional to their own energies. In stars, flames, and ordinary electric lamps, the charges’ energy emissions are spontaneous and almost completely independent, so the electromagnetic-wave quanta (or photons) that result have their vibrations mostly out of sync with each other. But in lasers, when the waves produced by the earliest-emitting charges run across other energized charges, they stimulate those charges to emit more waves in sync with themselves, so that the stimulated and stimulating waves are coherent. The way coherent and incoherent electromagnetic waves differ from each other is described by one writer as being like the way sound waves produced by a group of people differ when they either sing together as a choir or talk in small groups among themselves[Information Bridge (p. 3)].
In ordinary lasers, which radiate waves from electrons bound within atoms or molecules, the natural vibration frequencies range from those of microwaves (about 300 megahertz) through visible light (with highest frequency 790 terahertz).[Wikipedia] Producing ultraviolet light or x-rays, whether coherently in a laser or as out-of-sync waves, requires a source whose natural frequencies are correspondingly higher (790 terahertz to about 30 exahertz). Since, in heavy atoms, the electrons that stay closest to the atoms’ nuclei have very low energies, it’s possible to energize those electrons enough that they will produce photons with frequencies in the ultraviolet or x-ray range when they return to their original low-energy states. However, to get the emitted waves to vibrate in sync requires energizing lots of electrons, so that when the first spontaneous wave is emitted, there will be enough still-energized electrons nearby that the wave can stimulate other waves to be emitted in sync with itself. Energizing that many atomic or molecular electrons requires a great deal of power, especially to generate coherent waves at the upper end of this frequency range[Information Bridge (p. 4)].
But large energy differences between electron states don’t just occur in atoms and molecules. An electron, or any charged particle, that moves through a vertical magnetic field is deflected sideways with a force that depends on the strength of the field and the electron’s speed. If the magnetic field alternates between pointing upward and pointing downward, the electron’s deflection will alternate from left to right, giving it an undulating or wiggling motion. (The set of magnets producing the alternating field is thus known as an undulator or wiggler.) The energy of the electron’s left-right vibration will depend on the electron’s forward speed. For appropriate alternating magnetic fields, the possible electron vibration energies are large, with large differences between these possible energies—large enough that an electron losing energy can produce ultraviolet waves or x-rays. As with electrons bound in an atom or other piece of material, the energy losses of electrons in beams can either be spontaneous, or stimulated by a previously-emitted electromagnetic wave to produce another wave of equal frequency in sync with it. Devices that stimulate coherent electromagnetic radiation from electron beams moving through a magnetic field are known as free-electron lasers, or FELs.
The frequency of the waves produced by a free-electron laser depends on the wiggler’s magnetic-field strength, the distance within which the wiggler field completes one cycle of direction reversals, and the speed of the electron beam. In particular, the faster and more energetic the electron beam, the higher the laser frequency. High-frequency laser beams have high energy per photon, and also have short wavelengths. When shining on matter, photons interact more powerfully the greater their energy is. Also, illumination resolves material objects’ structural and temporal details down to the size of roughly one wavelength in space and one wavelength-period in time; shorter-wavelength laser beams can thus reveal finer details of whatever object or process they shine on.
Recent reports available in OSTI’s Information Bridge describe the designs of existing and proposed free-electron lasers as well as some of their uses.
The report “High Average Power UV Free Electron Laser Experiments at JLab”[Information Bridge] focuses on how the Thomas Jefferson National Accelerator Facility has added, alongside the laboratory’s infrared free-electron laser, a new ultraviolet laser. One interesting result is that the analyses that guided the laser’s design, which used not only standard one-dimensional simplifications of the laser’s features along the length of its electron beam, but more detailed three-dimensional descriptions as well, have actually underestimated the laser’s actual capabilities. According to the report authors, “[n]o single model captures all the observed details …. The performance of the UV FEL has greatly exceeded both 1D and 3D predictions; there is in no single case a fully consistent agreement of model with observation.” Whenever such discrepancies are found between theory and actual experiment, they mean that there’s something inaccurate or missing in the theoretical analysis, though it does seem uncommon for an actual device to surpass theoretical expectations instead of falling short of them. “This unusual circumstance is not yet understood, though it may possibly be due to an electron drive beam that is of ‘higher quality’ than that so far characterized by our preliminary empirical studies.” The report also says the lab is exploring options to produce free-electron laser beams of even higher frequency.
Jefferson Lab’s free-electron laser produces its ultraviolet light from an electron beam accelerated to just a few parts per million shy of the speed of light, giving the electrons a kinetic energy a few hundred times greater than the energy inherent in their rest mass. (An object moving at the speed of sound in air—fast, but scarcely more than a millionth of the speed of light—has a kinetic energy less than a trillionth of its rest mass.) Laser light of even higher frequency is produced at the first x-ray free-electron laser, the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory. To produce the much higher-frequency x-rays, LCLS uses SLAC’s two-mile long (3.2-km) linear accelerator to bring electron beams to within a few dozen parts per trillion of the speed of light, giving the electrons energy a few hundred thousand times greater than their rest mass. With these electron-beam energies, the laser x-rays have frequencies of a few exahertz and corresponding wavelengths around 0.1 nanometer, or 1 angstrom unit. The May 2010 report “The First Angstrom X-Ray Free-Electron Laser”[Information Bridge] describes the first user run of this laser in late 2009 as well as machine-development activities of that time, along with early results from preparations for the laser’s second user run.
These x-ray laser beams are especially intense. Just one pulse from the LCLS can concentrate more than a trillion x-ray photons into a spot just one micrometer across in less than 100 femtoseconds. When such an x-ray pulse strikes a piece of matter, the x-rays are scattered in a pattern that depends on the material’s internal structure; observing the scattering pattern with x-ray detectors thus allows the material’s structure to be determined. Once scattered, significant fractions of the original beam can strike the detectors in areas of a few hundred square micrometers—still concentrated enough to damage the detectors’ circuit chips. The report “High-Z Radiation Shields for X-ray Free Electron Laser Detectors”[Information Bridge] describes how, to protect against this, micro-patterned foils made of tungsten (a material of high atomic number Z) were designed to cover a portion of the circuit chip. The report also describes details of how the foils were made and installed.
X-ray scattering is not only useful for revealing the detailed structure of solids, but of dense plasmas[Wikipedia]. When x-rays undergo Thomson scattering[Wikipedia] in a plasma (meaning that the x-rays have the same energy after they’re scattered as they had before), how intensely the x-rays are scattered in different directions depends on how the electric charges within the plasma are distributed. Measuring a plasma’s x-ray scattering pattern thus indicates the temperatures and densities of the plasma’s electrons and ions. But accurate calculation of these temperatures and densities requires an accurate mathematical description of how they affect the x-ray scattering. That’s the subject of the Lawrence Livermore National Laboratory report “Using the X-FEL to understand X-ray Thomson scattering for partially ionized plasmas”[Information Bridge]. Whereas gases at ordinary temperatures consist mainly of molecules that have equal numbers of protons and electrons, once a gas is hot enough to become a plasma, many of its molecules are ionized, with their outermost electrons becoming energetic enough to escape the attraction of the molecules’ positive charges and disperse throughout the plasma. Plasmas thus contain free negatively-charged electrons and positively-charged ions. These negative and positive charges will scatter any x-ray photons that interact with them. The mathematical description of Thomson scattering discussed in the report improves on earlier ones by accounting in more detail for how the x-rays are affected by the electrons still bound within the ions.
A quite different use for free-electron lasers, to defend against supersonic cruise missiles, is the subject of the Los Alamos National Laboratory slide presentation “Navy Free-Electron Laser Program [Information Bridge]. Since free-electron lasers’ wavelengths can be adjusted, they could be used against cruise missiles in varying atmospheric conditions. The SLAC report “Sheet Beam Klystron for the Navy FEL”[Information Bridge] describes the design of an important component of such a free-electron laser: a 2.1-gigahertz, 200-kilowatt electron-beam accelerator (a klystron[Wikipedia]) for the Office of Naval Research. By accelerating the electrons as a sheet instead of a round beam, the klystron can be more compact and operate with lower current density, better power handling, higher efficiency, and lower voltage.
Other recent reports describe designs for new free-electron lasers and modification of an existing one.
“Design Alternatives for a Free Electron Laser Facility”[Information Bridge] from the University of Wisconsin-Madison, Michigan State University, the Thomas Jefferson National Accelerator Facility, and the University of Illinois at Urbana-Champaign describes a phased design for a next-generation free-electron laser facility, whose goal is to optimize value by minimizing cost while maximizing scientific capability. Constructing the facility in phases, with each phase raising the maximum frequencies of the laser beams produced, would allow research to begin without having to fund a complete facility; also, experience with using the facility in its earlier phases could inform the design of the later phases. Each phase’s higher-frequency lasers would add new capabilities. A phase 1 facility would generate laser beams with frequencies up to 435 petahertz, in the x-ray range, enabling (among other things) studies of chemical reactions in gases and on surfaces in real time, spectroscopic imaging of biological materials, and electronic effects in the fourth-period[Wikipedia] transition metals scandium through zinc. Phase 2’s 2.18-exahertz beams would allow analysis of soft matter and biological materials, as well as monitoring real-time changes to atomic structure. Phase 3’s 7.25-exahertz beams would reveal material features of interest in electronics.
The slide presentation “The Regenerative Amplifier Free-Electron Laser (RAFEL)[Information Bridge], presented by Los Alamos National Laboratory and describing work at Los Alamos in collaboration with researchers at the Naval Research Laboratory, Science Applications International Corporation, the University of Twente in the Netherlands, and Thomas Jefferson National Accelerator Facility, explores the advantages of regenerative amplification for high-power free-electron lasers and possibly x-ray free-electron lasers. In many free-electron lasers (e.g., SLAC’s Linac Coherent Light Source), electromagnetic waves emitted by one electron get one chance to stimulate the emission of other photons on their way out of the electron-beam wiggler. A regenerative amplifier captures emitted photons between mirrors for a while before they leave the wiggler, giving them multiple passes through it and thus an opportunity to stimulate even more electrons to emit in-sync waves. The presentation describes designs of infrared and x-ray free-electron lasers designed to operate on this principle.
The SLAC report “Design and Start-to-End Simulation of an X-Band RF Driven Hard X-Ray FEL with LCLS Injector”[Information Bridge] discusses a modification of SLAC’s Linac Coherent Light Source that would equal or surpass its present performance while reducing its overall length by 850 meters. The modification involves the components that accelerate electrons to the high speeds and high kinetic energies they require to emit x-rays when they pass through the laser’s wiggler. Whereas radio waves traveling unconstrained through space accelerate any charged particles they meet along the axis of the waves’ electric fields, which are perpendicular to the waves’ forward motion, radio waves that travel through the near-vacuum of a particle accelerator’s waveguides have electric fields that point along the axis of their forward motion, which push any charged particles in the waveguide along that axis. The Linac Coherent Light Source currently uses S-band radio waves to accelerate the electron beams; using higher-frequency X-band accelerators instead would accelerate them over a much shorter distance.
A quite different way to accelerate electrons is examined in “A compact soft x-ray free-electron laser facility based on a dielectric wakefield accelerator”[Information Bridge] from Euclid Techlabs LLC, Argonne National Laboratory, and Northern Illinois University, and in “Compact X-ray Free Electron Laser from a Laser-plasma Accelerator using a Transverse Gradient Undulator”[Information Bridge] from SLAC and Lawrence Berkeley National Laboratory. Instead of the electrons being pushed to higher speeds and energies with radio waves moving through a waveguide, the electrons are accelerated by the much stronger electric fields of a plasma whose own negative and positive charges are separated. Plasmas can sustain much stronger electric fields than a waveguide, which offers the potential advantage of making the electron accelerator much smaller. On the other hand, because of the way each bunch of electrons in the beam interacts with the plasma, electrons in the front of the bunch get accelerated more than those in the rear, thus getting different energies. Since electromagnetic waves can only be in sync if they have the same frequency, the electric charges that produce them have to undergo energy changes of the same size—a quite unlikely occurrence if the different electrons in each bunch have different energies to begin with. So to have electrons accelerated by plasma for a free-electron laser, some method of reducing the electrons’ energy differences is needed.
The main topic of “A compact soft x-ray free-electron laser facility based on a dielectric wakefield accelerator”[Information Bridge] is a proposal to make a free-electron laser facility more economical when its most expensive component is the waveguide-based accelerator that supplies the electron beam: have a single, low-energy waveguide accelerator feed multiple higher-energy plasma accelerators that supply electron beams to multiple free-electron lasers. This way, many more experiments can be accommodated at one time. But using plasma accelerators means dealing with the energy differences among the accelerated electrons. The authors show that these energy differences can be reversed if, instead of sending the electrons through wigglers whose magnetic fields alternate between the same maximum strength in the same way every X centimeters along their length, the electrons are sent through wigglers whose alternating fields have either increasingly large maximum strength along the electrons’ path, or else take increasingly longer distances to alternate between maximum strength upward and maximum strength downward. Passing through wigglers of this type reduces energy differences between electrons at the front and back of the same bunch, thereby maximizing the chances that any electromagnetic wave emitted by one electron will stimulate another electron to emit another wave in sync with it.
The authors of this report mention in their abstract that such variations in the wigglers’ magnetic fields may not be the only way to deal with the variation in plasma-accelerated electrons’ energies. This appears to be confirmed by “Compact X-ray Free Electron Laser from a Laser-plasma Accelerator using a Transverse Gradient Undulator”[Information Bridge]. This report shows how a sideways variation in the wiggler’s magnetic field can reduce the energy variation. Electrons with different energies vary in their side-to-side motion through the wiggler, not just their forward motion. If the wiggler’s magnetic field is stronger toward one side and weaker toward the other, the electrons that initially have more energetic side-to-side motions will be exposed to greater magnetic-field variation, which will again reduce the energy differences between electrons in the same bunch and increase the coherence of the electromagnetic waves they emit.
SLAC’s Linac Coherent Light Source
Prepared by Dr. William N. Watson, Physicist
DOE Office of Scientific and Technical Information