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Atomic physicists are using laser pulses to create conditions found in stars and planets.
In research that could evolve into a new light-source niche for ORNL, atomic physicists led by Dave Schultz are using laser pulses to create conditions found in stars and planets. The work involves extremes of time and energy in increments imperceptibly small and unfathomably large. The research described by Schultz involves zapping a solid or gas target inside a 10-micron focal spot with an incredibly energetic burst of light and then monitoring the resulting debris. How extreme is the process? Schultz says the pulses of light, generated by a technology called chirped pulse amplification, range in power from hundreds of terawatts to one petawatt. In comparison, the Tennessee Valley Authority's peak electricity production on the sultriest of summer days is a mere 32 gigawatts. A gigawatt is a billion watts; a terawatt, a trillion watts; and a petawatt, a thousand trillion watts. How can ORNL's atomic physicists harness more power than a large utility can generate? "We do it for an extremely short time," Schultz says. In fact, the duration of the pulses is broken down into femtoseconds, which is 10-15 of one second—well shy of how quick a cat can wink its eye. In fact, light can travel only 15 microns in 50-femtosecond-long bursts. The work involving these incredibly powerful bursts in incredibly brief intervals is performed at two university-based facilities—the Hercules facility at the University of Michigan and the Diocles facility at the University of Nebraska. About the only aspects of the ultrafast, ultra-intense laser experiments that are on an earthly scale are the facilities themselves. Both Hercules and Diocles are room-sized, crammed with specialized equipment. "ORNL researchers Jim Beene, Randy Vane and Herb Krause work with the laser center staff to fire the 50-femtosecond, hundreds-of-terawatt laser pulses into the targets and then detect and analyze the explosion debris to understand the conditions they have created," Schultz says. These "conditions" include gigapascal pressures and temperatures in the millions of degrees. The researchers also study the resulting turbulent plasmas that are scaled versions of astrophysical plasmas. "Light is a unique carrier of energy because photons are not charged, and a lot can be put in one place," Schultz says. "Electrons, which are negatively charged, are impossible to pull together because they repel each other. The number of uncharged photons in each short pulse is enormous." The researchers may also exploit the laser-produced plasma's ability to accelerate electrons (up to 400 megavolts in recent experiments), generate intense, ultrafast X-ray pulses and produce rare isotopes. The laser-produced plasma acts as a linear accelerator to generate electron beams that upshift scattered light pulses into high-energy X-rays. The laser linac is a mere few millimeters long, extremely short when compared with the 331-meter-long linac at ORNL's Spallation Neutron Source. Besides providing grist for the knowledge-of-the-universe mill, the chirped-pulse amplification studies could eventually enjoy a number of spin-offs. Intense, ultrafast X-ray pulses and rare isotopes for medical and basic science applications might become extremely valuable on Earth.—Bill Cabage
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