X RAYS are handy for examining all kinds of materials, from our bones and lungs to high explosives and other ingredients in a nuclear weapon. If the x rays are intense enough and come in short enough pulses, they can supply information about the dynamic processes in many forms of condensed matter, such as solid materials, liquid crystals, and extremely dense plasmas. Using the Linac Coherent Light Source (LCLS)-an x-ray machine with unprecedented brilliance being considered for construction at the Stanford Linear Accelerator Center-researchers will be able to measure, for the first time, melting, recrystallization, and light-induced structural change on time scales down to a quadrillionth of a second.
The extraordinarily bright, short pulses of the LCLS have the potential to open new areas of science that are unimaginable given current scientific knowledge. The LCLS will make visible dynamic processes that can only be guessed at now. Upon completion in 2004, the new facility is certain to help solve problems in ultrahigh-energy-density physics, structural biology, fundamental quantum electrodynamics, warm dense matter, and high-field atomic physics, among others. The extreme brightness of the LCLS also means that results will be available much faster than before and will offer a level of detail that has been impossible to obtain with existing tools.
"For decades, we have studied nonlinear phenomena at optical wavelengths," says physicist Art Toor, who is leading the Livermore work on the LCLS. "But we've never had the tools to study nonlinear multiphoton processes in the x-ray region. That is tremendously exciting and opens the door to whole new regimes of research in physics, biology, and chemistry."
Protein crystallography, used to study the structure of proteins, is just one example of the research techniques that will benefit from the new x-ray source. Livermore and other biological research facilities use third-generation light sources to obtain images of molecules in a process that takes many hours of exposure time for each image. The shorter, brighter pulses of the LCLS will produce enough flux to image a molecule in a single pulse.
Livermore is part of the collaboration that is conducting research and development leading to this fourth generation light source. The LCLS is the next step beyond third-generation synchrotron radiation light sources, such as the Advanced Light Source at Lawrence Berkeley National Laboratory and the Advanced Photon Source at Argonne National Laboratory. Third-generation light sources rely on storage rings where electrons traveling at nearly the speed of light are forced into a circular path by magnets. When the electrons pass through a magnetic structure called an undulator, they emit soft x rays that shine down beamlines to experimental stations.
The LCLS, in contrast, will use a linear accelerator rather than a circular one. It will also be home to the first x-ray free-electron laser, made possible by recent progress in undulator technology and in forming high-brightness, short-duration electron bunches in accelerators. The light from the LCLS will come in wavelengths smaller than the size of an atom. These hard x rays can be superior to longer-wavelength soft x rays for studying matter. The laser light will be fully coherent across the beam and 10 billion times brighter than the x-ray beams produced at the Advanced Light Source and its third-generation cousins. (Brightness is a measure of photon density, as shown in the figure below.). The pulses will also be 100 times shorter than those of today's machines.
The Key to Success|
The LCLS will be built around the portion of the Stanford Linear Accelerator that is not being used by the B Factory. (See S&TR, "The B Factory and the Big Bang," January/February 1997, pp. 4-13.)
Its first major component is the photoinjector, which produces tiny bunches of electrons traveling at almost the speed of light. Next is a 1,000-meter-long linear accelerator that pushes the electrons' energies up to 14 gigaelectronvolts. Compressors along the accelerator path reduce the length of each bunch by a factor of 30 to increase their peak current. Then the electrons enter an undulator, a vacuum chamber just 5 millimeters across and about 125 meters long that is lined with 7,000 magnets arranged in alternating poles. In this narrow channel, the magnetic fields push and pull on the electron bunches, causing them to emit x rays that in turn force the electrons into even tinier microbunches that release x-ray photons in a bright, coherent beam. Optical devices beyond the undulator manipulate the direction, size, energy, and duration of the x-ray beam and carry it to whatever experiment is under way.
Key to making this machine work is the low emittance of the electron beam injected into the accelerator. Emittance is a function of the diameter and divergence of a beam. A small beam with a wide spread has been easy to achieve, but a small beam with narrow spread has typically been difficult to produce. New photoinjector technology can produce a narrow, bright beam of electrons with emittance several times lower than previously achieved.
When the accelerated beam enters the undulator, interaction with the magnetic fields there causes x rays to appear. As the electron bunches move down the undulator, the electron beam and the growing amount of x radiation interact more and more. More x rays produce more bunching, which produces more x rays, which makes the microbunches smaller and smaller, and so on. This chain reaction finally results in saturation of the x-ray beam to produce a narrowband, coherent beam of light, or laser, that is about 10 billion times brighter than the light from any other light source today. Also present is broadband spontaneous radiation about 10 thousand times brighter than that from any other light source.
Most other free-electron lasers store the light from many passes of the electron beam through the undulator in an optical cavity before putting it to use. The LCLS will require just a single pass by the electron beam through the undulator, thanks largely to the low emittance of the electron beam at the front end of the system.
Optics by Livermore|
Livermore is part of a consortium with the Stanford Linear Accelerator Center, the University of California at Los Angeles, and Los Alamos, Brookhaven, and Argonne national laboratories that is developing the LCLS. Each institution is responsible for a different part of the overall project.
In one project, Livermore scientists are working with colleagues at Brookhaven National Laboratory to demonstrate the new technology for LCLS and produce the first free-electron laser in the visible wavelength. The Livermore team designed the 4-meter undulator, vacuum system, and other portions of the project.
For the LCLS at Stanford, Livermore will design and fabricate the x-ray optics downstream of the undulator. Says Toor, "The high peak power, full transverse coherence, and very short pulse lengths combine to make the optics for the LCLS a real challenge. We also are designing optical systems to accommodate a variety of experiments. Some require submicrometer focus at very high intensity, others require only coherence, and still others require illuminating large areas at much lower light levels."
A critical element in the optical system that Livermore scientists are working on is the absorption cell, which intercepts the beam after it leaves the undulator. The cell attenuates the beam's power to levels manageable with conventional optics and provides a transition to power densities that match the needs of the various experiments. The cell can also completely remove the free-electron laser light for experiments that use only the spontaneous radiation. Both the spontaneous and coherent x radiation pass through an ultrahigh-vacuum system to the experimental areas, which may ultimately be as much as a kilometer away. Shielding will protect personnel and experiments from bremsstrahlung, the gamma radiation that results from high-energy electrons interacting with matter.
This work is opening new territory. Virtually no information exists now on the interaction of extremely high levels of hard x rays with matter. If the Department of Energy approves construction of the LCLS, the beginning of testing and experimentation in 2004 will herald a brave new world in physics.
Key Words: free-electron laser, Linac Coherent Light Source (LCLS), linear accelerator, Stanford Linear Accelerator Center.
For more information contact Art Toor (925) 422-0953 (firstname.lastname@example.org).