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William H. Goldstein

William H. Goldstein
Associate Director of Physics and Advanced Technologies

Let There Be Light Sources

LAWRENCE Livermore continues to be a leader and innovator in the development and application of light sources, building the most powerful lasers in the world and demonstrating the first x-ray laser. As light sources become brighter, faster, and more energetic, their role in the Laboratory’s future is as important as ever. In fact, the Laboratory’s long-range science and technology plan identifies new light sources as crucial to progress in many of our core science and technology areas: stockpile stewardship; high-energy-density physics; nuclear and radiative science; and chemical, biological, and materials research.
Two articles in this issue look at applications of the new generation of intense light sources. The first, An Extraordinarily Bright Idea, discusses the Linac Coherent Light Source (LCLS), an x-ray laser being built at the Stanford Linear Accelerator Center by a consortium of institutions that includes Livermore. The second article, Using Proton Beams to Create and Probe Plasmas, describes exciting developments using the Laboratory’s Janus-pumped ultrashort-pulse (JanUSP) laser.
The LCLS is what is called a single-pass, free-electron laser. A very short bunch of high-energy electrons is injected into an undulating magnetic field where they emit bremsstrahlung—literally, braking radiation—as they are accelerated. Under carefully designed conditions, the emitted radiation interacts with the electron bunch and builds in intensity. The resulting x-ray beam is 10 billion times brighter than currently available light sources. Its copious photons are coherent, with energies more than 10 times that needed to ionize any atom. Interactions between this beam and atoms are different from those produced by even the most intense optical lasers. X-ray pulses are tunable from 0.8 to 8 kiloelectronvolts, may be less than 100 femtoseconds long, and may have wavelengths as small as 0.1 nanometer. These photon intensities, pulse lengths, and wavelengths will allow scientists to make measurements on atomic scales.
One particularly exciting use of the LCLS will be to examine the structure and function of such large biomolecules as proteins. With current x-ray light sources, structure can be determined only for those molecules that can be formed into a crystal pattern, a process that invariably destroys the protein’s functionality. With its ultrabright, ultrashort pulses, the LCLS will be used to image single molecules, without the need to crystallize or immobilize them. Many challenges remain to meet this goal, but the payoffs for understanding the mechanisms of life are enormous.
We will also be able to use LCLS’s x-ray pulses to heat material to conditions replicating those inside weapons, stars, and planets. By splitting the x-ray beam, we can use part of it to heat a material and the other part to take measurements. Once the beam is split, one or more of its parts can be delayed with respect to the others, allowing us to look at what happens as a function of time. Using this technique, we can perform dynamic studies of materials fast enough to see molecular motion taking place during chemical reactions. We will also be able to measure the interactions of complex systems, such as protein folding and crystalline phase transitions.
JanUSP produces laser pulses as short as those produced by the LCLS but in visible light. When JanUSP’s laser energy is focused onto a thin metal target, a plasma forms. Electrons in this plasma are accelerated and escape, which sets up an intense electric field that pulls a short-pulse, high-energy proton beam out of the target. This effect, discovered at Livermore, has created an entirely new field of science at laser laboratories around the world.
The JanUSP proton beam can be focused to heat material just as the x rays from the LCLS will be. The proton beam also can be used for radiography, providing time-frozen pictures with spatial resolution of 1 micrometer. Because the protons are charged, they respond to electric and magnetic fields, so they can be used to measure these fields on very small time and space scales.
This new, exciting proton beam may even find its way into the National Ignition Facility, providing a novel way to ignite inertial confinement fusion capsules.

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UCRL-52000-03-12 | December 3, 2003