Press Release: The x-ray laser may soon become a reality

Stanford, CA

An experiment and a detailed design study recently have opened the way to the development of an x-ray laser.

Lasers, which have become commonplace in the last 30 years, operate in the visible, the infrared and even the ultraviolet range of the spectrum. They have a myriad of uses, ranging from playing music discs to producing light for scientific research. But so far no one has successfully developed a practical laser that operates at x-ray frequencies.

Now scientists at the Stanford Linear Accelerator Center (SLAC), the University of California-Los Angeles (UCLA) and other institutions believe that they have solved the physical and technological problems that have stood in the way of constructing such a device.

A recent experiment that confirmed the basic physics of an x-ray free-electron laser (FEL) was performed by a group of physicists from SLAC and the Stanford Synchrotron Radiation Laboratory, UCLA, Los Alamos National Laboratory (LANL) and the Kurchatov Institute in Moscow. They set up a 2-meter undulator - a device that creates an undulating magnetic field - at the Los Alamos Advanced Free-Electron Laser linear accelerator. When they passed a beam of electrons accelerated to 18 million electron volts through the undulator, they produced a laser beam of infrared light (12 micrometer wavelength) with an amplification, or gain, of 300,000.

This is the largest gain ever observed in the infrared region, report Max Cornacchia, assistant director of SLAC for the Stanford Synchrotron Radiation Laboratory (SSRL), and Claudio Pellegrini, professor of physics at UCLA, who participated in the experiment and design study.

The scientists also observed intensity fluctuations and structure in the radiation pulse that agree with theoretical predictions. Although the experiment was performed in the infrared, the basic physics is the same as that for an x-ray FEL so its results validate the computer codes that were used to design the new laser, the researchers say.

The computer codes were used by scientists at SLAC/SSRL, LANL, Lawrence Livermore National Laboratory (LLNL) and UCLA for the initial design of the Linear Coherent Light Source (LCLS). The LCLS is an x-ray FEL that would use the linear accelerator at SLAC to accelerate electrons to 15 billion electron volts and pass them through a 100-meter-long undulator. This would produce an x-ray beam with a peak power of 10 billion watts. The design, which was validated by an independent external review committee in November 1997, demonstrated the feasibility of constructing the device and provides an estimate of the costs involved, between $70 million and $90 million, depending on the number of users that it is designed to accommodate.

"Based on these developments, we are extremely confident that we can construct a 10 gigawatt x-ray laser," Cornacchia and Pellegrini say. A collaboration has been formed between SLAC, LANL, LLNL and UCLA to build the Linear Coherent Light Source. The group has submitted a proposal to the Department of Energy for the funding required to produce a final design of the LCLS. If this research and development effort is begun in 1999, then construction could start in 2002 and the new x-ray source could be completed in 2005.

The proposed x-ray laser would have a peak brightness 10 billion times that of the strongest x-ray sources now in existence. In addition, it would generate pulses as short as 100 femtoseconds - about the time that it takes a beam of light to travel one thousandth of an inch - 100 times shorter than the current state of the art. Most important, the device would produce the first x-ray beam that is coherent: The x-ray photons are emitted in phase with each other. In ordinary x-ray sources the photons are emitted randomly and so are out of phase.

The new design, first proposed by Pellegrini in 1992 at a workshop held at SLAC, is an extension of free-electron laser technology. FELs produce laser light by passing a rapidly moving beam of electrons through an array of magnets that create an undulating magnetic field. FELs have been built that can produce tunable beams of light in wavelengths ranging from the infrared to the ultraviolet. The scientists have figured out how to use this approach to produce x-rays.

The proposed design differs radically from the giant x-ray lasers proposed in the 1980s for the Strategic Defense Initiative program, which were conceived to destroy intercontinental ballistic missiles and would have been powered by nuclear explosions. The x-ray FEL is powered by an electron beam and is a tool for research.

X-ray lasers could open up a new era of research into the properties and structures of a wide variety of materials, the scientists say. Because x-rays are very short - with wavelengths about the size of atoms – the x-ray laser should provide new information about the atomic-level structure and dynamics of many substances that cannot be gained using other means. With its brightness, coherence and extremely short pulse length, the LCLS could be an important new research tool, says Prof. Ingolf Lindau of the University of Lund and Stanford, who is currently spending a sabbatical at SSRL. It has unique properties that include:

Brightness. With an average brightness 100 to 10,000 times greater than existing sources, the x-ray laser could be used to study the magnetic properties of extremely small magnetic domains, such as those used for computer memories, with greater precision. With a peak brightness 10 billion times greater than current sources, the device will be capable of determining how fast the increasingly small transistors on computer chips can function by providing new information about the electrical states of the interfaces between the different materials from which they are made.

Time structure. Its femtosecond pulses are short enough to catch chemical bonds in the act of forming and breaking. Researchers are currently using extremely fast lasers in the infrared range to study such processes. The penetrating capability of x-rays would allow researchers to extend these studies to structural changes in solid materials, and study metrical details of such processes.

Coherence. Laser light in the range of one angstrom should make it possible to create three-dimensional holograms of a wide variety of atomic-scale processes. It would also enable scientists to use a technique called intensity fluctuation spectroscopy to study phase transitions at the atomic level. Even in very simple systems, like water turning into ice, scientists do not understand what occurs in these transitions at the atomic level.

The work described above was supported by the Offices of Basic Energy Science and High Energy Physics in the Department of Energy.

Related material

Linear Coherent Light Source web page

	Figure shows average and peak birghtness calculated for the LCLS and for other facilities operating or under construction. (Download Illustrator 6.0 eps file - 8360A04.eps)
	Layout of the Linac Coherent Light Source (Download Illustrator 6.0 eps file - 8360a01.eps)