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Simulations Uncover Obstacle to Harnessing Laser-driven Fusion
Mar 2013
COLUMBUS, Ohio, March 27, 2013 — A once-promising approach for using next-generation, ultra-intense lasers to help deliver commercially viable fusion energy has been brought into serious question by new experimental results and first-of-a-kind simulations of laser-plasma interaction.

Investigators at The Ohio State University are evaluating a two-stage process, called Fast Ignition, in which a pellet of fusion fuel is crushed by lasers on all sides, shrinking the pellet to dozens of times its original size, followed by an ultraintense burst of laser light to ignite a chain reaction. Drawing on both results from studies at the Titan Laser at Lawrence Livermore National Laboratory (LLNL) in California and on massively parallel computer simulations of the laser-target interaction performed at the Ohio Supercomputer Center (OSC), they discovered compelling evidence that using hollow cones to maintain a channel for the “ignitor pulse” to focus laser energy on the compressed pellet core has a serious flaw.

“In the history of fusion research, two-steps-forward and one-step-back stories are a common theme,” said Dr. Chris Orban, a researcher in the High Energy Density Physics research group at the university and lead theorist on the project. “But sometimes progress is about seeing what’s not going to work, just as much as it is looking forward to the next big idea.”

Ultraintense pulses deliver energy to the fuel through relativistic electrons accelerated by the laser interaction, so the team focused its study on coupling the laser light to electrons and on the propagation of those electrons through the cone target. Rather than investigating how the interaction would work on a high-demand, high-cost facility such as LLNL’s National Ignition Facility (NIF), the researchers considered experiments using the smaller and more accessible Titan Laser.

Despite its size and lower total energy, for a brief moment the Titan Laser was many thousands of times more intense than NIF, making it an adequate stand-in as a second-stage ignitor pulse. The researchers focused pulses from the Titan on hollow cone targets attached at the tip to copper wires and observed the burst of x-ray photons coming from the copper as a measure of the laser energy to relativistic electron conversion efficiency.

The x-ray signal was much lower from the hollow cones with thicker cone walls.

“This was strong evidence to the experimental team that the typical approach to cone-guided Fast Ignition wouldn’t work, since thicker cones should be more realistic than thin cones,” Orban said. “This is because electrons are free to move around in a dense plasma, much like they do in a normal metal, so the thicker cone target is like a thin cone embedded in a dense plasma.”

Whereas earlier efforts to simulate the laser-target interaction were forced to simplify or shrink the target size to make the calculations more feasible, Orban used the large-scale plasma code to perform full-scale 2-D particle-in-cell simulations of the entire laser-target interaction using fully realistic laser fields.

These images from the Ohio State University researchers’ simulations highlight the trajectories of randomly-selected electrons for a thin cone (left) and a thick cone (right), each attached to a copper wire. Background colors show the strength of the electric fields pointing away from the cone and wire. For thin cones, the electric fields act to guide energetic electrons forward into the wire, while for thick cones — a more realistic case — these fields are too distant to be effective. Courtesy of Orban/Ohio State.

Simulations also included a sophisticated model for the preheating of the target from stray laser light ahead of the ultraintense pulse developed by collaborators at the Flash Center for Computational Science at the University of Chicago.

The scientists used the Flash code to provide realistic initial conditions for the simulations, said Don Lamb, director of the Flash Center.

The OSC’s flagship Oakley Cluster supercomputer system was used to conduct the simulations. The HP-built system features more than 8300 Intel Xeon cores and 128 NVIDIA Tesla GPUs. Oakley can achieve 88 teraflops, or 88 trillion calculations per second, or, with acceleration from the NVIDIA GPUs, a total peak performance of 154 teraflops.

“The simulations pointed to the electric fields building up on the edge of the cone as the key to everything,” Orban said. “The thicker the cone is, the further away the cone edge is from the laser, and as a result, fewer energetic electrons are deflected forward, which is the crucial issue in making cone-guided Fast Ignition a viable approach.”

Both the experiment and the simulations provided the same evidence: The cone-guided route to Fast Ignition is an unlikely one. Although other studies have come to similar conclusions, the group was the first to identify the plasma surrounding the cone as a severe hindrance. Fortunately, there are other ideas for the fusion pellet ignition with current or soon-to-be-constructed laser facilities.

Future efforts to spark fusion reactions using a two-stage fast-ignition approach must consider the neutralizing effect of the free electrons in the dense plasma.

The study appeared in Physical Review E (doi: 10.1103/PhysRevE.86.065402).  

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laser fusion
Optical confinement of matter with high field energies intended to induce a stable  nuclear fusion interaction.
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