World's most powerful X-ray laser to be built

Keith Hodgson of Stanford University flew to Pittsburgh last Monday to deliver the Buhl Lecture at Carnegie Mellon, an annual physics presentation held at the Mellon Institute. The lecture was about the emergence of a new technology that will enable detection of atomic transitions and chemical reactions that take place on the order of femtoseconds.

Hodgson is the director of Stanford’s LINAC Coherent Light Source (LCLS) project and is in the process of building the world’s most powerful X-ray laser. Yet for all its power, the purpose of the laser is not to explode molecules, but to take pictures of them before the explosion. In 2009, Hodgson and his team at the Stanford LINAC — short for Linear Acceleration Center — will fire a 10-gigawatt photon beam into a single molecule and take a picture before it explodes.

Coincidentally, Stanford University’s founder was also interested in high-speed photography. In 1872, Leland Stanford hired photographer Eadweard Muybridge to confirm his belief that all four hooves of a galloping horse leave the ground together. Using tricks very clever for 1872, the photographer was able to take short enough exposures to confirm Stanford’s belief.

Hodgson’s goal is the same, but on a different scale. He hopes to use the laser to take pictures so fast that he can see the motion of atoms.

To learn about the properties of atoms, one needs photons of comparably small wavelength. A typical atom diameter is one tenth of one billionth of a meter. While the wavelength of visible light is over 1000 times too large for such imaging, the wavelength of an x-ray is just right.

Not only does the wavelength of the LCLS photons need to be very short, the time scale of the photon burst must be extremely fast and intense. Finally, the photon beam should be coherent, meaning that the photons are all polarized in parallel.

The new X-ray laser will be based on Stanford’s three-kilometer linear accelerator, the world’s longest and highest-energy electron linear accelerator. Short, intense bunches of electrons will be injected into the accelerator and then passed through compressors, which pack them into even shorter bunches. The electrons are then excited (energized) as they pass through an undulator magnet, where they emit X-ray radiation as they oscillate in the alternating magnetic field.

To fully comprehend the capability of this machine, it’s important to understand the meaning behind the phrase “order of magnitude.” One order of magnitude denotes a scale factor of 10. The LCLS will be 15 orders of magnitude brighter than our sun. And while ordinary lasers emit about 1012 photons per second, the LCLS will emit that many photons in a single femtosecond. There are about as many femtoseconds in a minute as there are minutes in the entire history of the universe.

“The powerful combination — an X-ray beam with extreme brightness (a trillion X-rays in a needle-thin beam), short wavelength (on the scale of atoms), and short pulse duration (a few femtoseconds) — makes the LCLS a revolutionary machine,” stated Hodgson in an e-mail.

A great many scientific discoveries will be possible with the LCLS. Many atomic transitions and chemical reactions take place on the order of femtoseconds, and the ability to image such a process will likely lead to something Hodgson calls “femtochemistry.” It takes 10 femtoseconds for a hydrogen atom to attach itself to a molecule. A valence electron completes an orbit of its nucleus in about one femtosecond, while a gas molecule can move about one angstrom in a few femtoseconds. Thus, with the LCLS, many physical events that until now could only be theorized about can now be experimentally observed.

“The LCLS has tremendous discovery potential,” Hodgson stated. “I’m personally most excited about the potential to image at or near atomic resolution non-periodic materials, including single large biomolecules. If the LCLS single molecule imaging experiments work as expected, it will really give us a completely new approach to imaging on the atomic scale.”

Given the extreme intensity of the X-ray laser, “brighter than a quadrillion suns,” anything the beam encounters will be destroyed. Since the pulse is so brief, it will hopefully be able to image its target before the target explodes.

The U.S. Department of Energy has given approval for construction to begin, and Hodgson hopes for a completion date in late 2008 and operation by 2009. However, this new technology is not unknown to researchers in other countries, and according to Hodgson, the “Japanese and the Europeans are getting on board and are planning even more aggressive programs.”

For the time being, Hodgson and colleagues are very optimistic about this new technique. “We are fortunate to be leading the world in this field,” he said. “Only when we gain full access to this remarkable light source in 2009 will we really begin to explore and understand their potential. But with some confidence, one can predict a new revolution in X-ray science.”