Lawrence Livermore National Laboratory



Originally published October 2017

Since its introduction in the 1977 film “Star Wars,” the Death Star has remained one of science fiction’s most iconic figures. The image of Alderaan’s destruction at the hands of the Death Star’s superlaser is burned into the memory of millions of fans.

In the past, scientists and laser experts have maintained that this superbeam could never work due to the properties of lasers; theory says that rather than converging and combining their energy, the beams would just pass through one another.

That was true enough then. But now a team of LLNL researchers has added a plasma—a charged mixture of ions and free electrons—to the concept and successfully combined several separate NIF lasers into a “superbeam.” Their work, recently published in Nature Physics, is a next step in the Laboratory’s 50-year history of leadership in laser research and development.

While this superbeam isn’t quite as super as the one depicted in science fiction, it stands as an important achievement—for the first time, nine of NIF’s 192 laser beams were combined to produce a directed pulse of light with nearly four times the energy of any of the individual beams. Leveraging LLNL’s expertise in optics research and development, the team used a Livermore-designed plasma optic to combine the beams and produce this first demonstration of its kind.

In certain experimental configurations, targets can be driven only by a single beam. Each beam has a limit on the amount of energy it can deliver. By combining multiple beams into one, LLNL’s plasma beam combiner can break through that limit and push these experiments into new physics regimes. Beams with high energy and fluence are expected to advance a range of applications, including advanced x-ray sources and studies of physics at extreme intensities.

Illustration of Beamcombiner PrincipleIllustration showing the principle of energy transfer from several large pump beams to a central probe beam. Credit: Scott Wilks

“In high-energy laser systems which use conventional solid optics, the maximum fluence (energy density) is limited by the damage of the (optics’) material,” said Robert Kirkwood, lead author on the paper and programmatic lead for the experimental campaign. “Because a plasma is inherently such a high energy density material, you don’t destroy it. It can handle extremely high optical intensities.”

“Beam combining has recently been done with solid-state lasers, but was limited by typical standard optics,“ added co-author Scott Wilks, one of the campaign’s designers. “Because of this plasma optic, we can put a huge amount of energy into a very small space and time—serious energy, in a well-collimated (focused) beam.”

Laser research and development is pushing into new regimes of power and energy, which are limited by conventional solid-state optics. Using a plasma optic, however, might appear counterintuitive.

“Plasma is generally bad for lasers—it is the bane of our existence,” said co-author Brent Blue, program manager for NIF National Security Applications. “The team has turned that on its head and is intentionally harnessing plasmas for a benefit.”

Plasma generally creates instabilities when combined with intense laser beams. By controlling an instability that causes the transfer of energy when beams cross, however, the researchers were able to combine the energy from multiple beams into a single powerful beam.

“We’ve known that plasma can deflect light and change the direction of energy flow, but it’s been difficult to do it in a very precise way,” Kirkwood said. “Here we’ve shown that we can control optical instabilities in plasma so that rather than randomly scattering energy, they put it where we want it and do so with good collimation and high intensity, producing a bright beam that can be delivered to another target. We can now control and predict what the plasma does, quite accurately.”

Images from Superbeam Experiments(Left) Illustration of the NIF beam-combiner target. The gas-filled balloon target (10 millimeters in major diameter) is used to create a uniform plasma to amplify a single seed beam (red) by combination of eight pumping beams (yellow) via seeded stimulated forward scatter. In addition to the two groups of pumps crossing the seed at 14.7° and 20.7°, the gas is ionized and heated with forty heater beams at larger angle (not shown). A tantalum witness plate is used to diagnose the red-shifted seed beam energy as it emerges from the plasma via the relative brightness of the x-ray spots created by it and by a fiducial (reference) set of beams (green). (Right) Photograph of the balloon target and the plasma conditions produced in it by all incident beams. The C5H12 (pentane) gas is contained at the desired density by a thin membrane balloon mounted on a washer.

The emerging beam has an energy of four kilojoules (over one nanosecond) that is more than triple that of any incident (pumping) beam, and a fluence that is more than double. Because the optic produced is plasma and is diffractive, it is inherently capable of generating higher fluences in a single beam than solid-state refractive or reflective optics.

Transitioning to a new optic material with a much higher damage threshold than anything used before opens the door to higher laser intensities and energies. Looking forward, the team plans to scale up the experiment with the hope of combining up to 20 beams into one.

The campaign initially was funded by the Laboratory Directed Research and Development Program. Along with Kirkwood, Wilks and Blue, co-authors of the Nature Physics paper were LLNL’s Thomas Chapman, Mordecai Rosen, Richard London, Louisa Pickworth, William Dunlop, John Moody, David Strozzi, Pierre Michel, Laurent Divol, Nino Landen, Brian MacGowan, Bruno Van Wonterghem, and Kevin Fournier; and David Turnbull of LLNL and the Laboratory for Laser Energetics at the University of Rochester.

—Breanna Bishop

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