Simulations Uncover Obstacle to Harnessing Laser-Driven Fusion: Under Realistic Conditions, Hollow Cones Fail to Guide Energetic Electrons to Fuel
These images from their simulations highlight the trajectories of randomly-selected electrons for a thin cone (left) and 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. (Credit: Image courtesy of Ohio Supercomputer Center) Enlarge
Mar. 26, 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.
Researchers at The Ohio State University are evaluating a two-stage process in which a pellet of fusion fuel is first crushed by lasers on all sides, shrinking the pellet to dozens of times its original size, followed by an ultra-intense burst of laser light to ignite a chain reaction. This two-stage approach is called Fast Ignition, and there are a few variants on the theme.
In a recent paper, the Ohio State research group considered the long-discussed possibility of using a hollow cone to maintain a channel for the ultra-intense "ignitor pulse" to focus laser energy on the compressed pellet core. Drawing on both experimental results from studies at the Titan Laser at Lawrence Livermore National Laboratory in California, and massively-parallel computer simulations of the laser-target interaction performed at the Ohio Supercomputer Center (OSC) in Columbus, Ohio, the research team found compelling evidence that the cone-guided approach to Fast Ignition has a serious flaw.
"In the history of fusion research, two-steps-forward and one-step-back stories are a common theme," said Chris Orban, Ph.D., a researcher of the High Energy Density Physics research group at Ohio State and the 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."
Since the ultra-intense pulse delivers energy to the fuel through relativistic electrons accelerated by the laser interaction, the Ohio State study focused on the coupling of the laser light to electrons and the propagation of those electrons through the cone target. Rather than investigating how the interaction would work on a high-demand, high-cost facility like the National Ignition Facility (NIF), which is also based at Lawrence Livermore National Laboratory and one of the largest scientific operations in the world, the researchers considered experiments just across from NIF at the Titan laser, which is much smaller and easily accessible.
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