In the early morning of August 13, 2013, the National Ignition Facility (NIF) generated a record yield of nearly 3 × 1015 fusion neutrons from a cryogenic deuterium-tritium target. That's three times the previous record, and welcome good news from the giant laser at the Lawrence Livermore National Laboratory (Livermore, CA) after its failure to reach ignition by the target date of October 2012.
The immediate implications are for the National Nuclear Security Administration's Stockpile Stewardship program, which uses NIF to simulate the physics of nuclear explosions and assure the U.S. nuclear weapon stockpile is safe and functional. A Livermore release says, "The experiment attained conditions not observed since the days of underground nuclear weapons testing," and calls the results "an important milestone" in showing that further nuclear weapon testing is not required. It is part of the "path forward" program to reach ignition proposed by NNSA.
NIF's biggest problem has been getting experimental results to agree with theoretical models of fusion plasma behavior. The 192-beam laser has generated the 1.8 MJ pulses that computer models had predicted would be sufficient to ignite a fusion plasma, but the plasma did not reach the ignition threshold. To pin down what went wrong, Livermore researchers have been carrying out a series of tests to examine key issues in target compression.
The latest round of tests focused on improving control over the implosion of the imploding shell of the target capsule, and avoiding the break-up that had degraded target compression. In initial trials, the power in the initial part or "foot" of the laser pulse was reduced, producing higher target compression, but not improving the uniformity of the implosion. "High-foot" tests with higher power in the initial part of the pulse succeeded in improving control of the implosion, at the cost of reducing compression later in the pulse.
The successful tests used 1.7 MJ pulses with peak power of 350 TW; NIF pulses have exceeded 1.8 MJ and 500 TW. Lowering the pulse energy helped researchers "gain control and learn more about what Mother Nature is doing," said Omar Hurricane, lead scientist for the tests. "The results were remarkably close to simulations and have proved an important tool for understanding and improving performance."
Calculations show that fusion started to self-heat the burning core of the plasma, increasing yield by almost 50 percent and approaching "alpha burn," dominated by fusion reactions. That's important progress toward a self-sustaining target burn, says Ed Moses, principal associate director for NIF and photon science at Livermore.
The path to ignition remains formidable, but progress is welcome.
Jeff Hecht | Contributing Editor
Jeff Hecht is a regular contributing editor to Laser Focus World and has been covering the laser industry for 35 years. A prolific book author, Jeff's published works include “Understanding Fiber Optics,” “Understanding Lasers,” “The Laser Guidebook,” and “Beam Weapons: The Next Arms Race.” He also has written books on the histories of lasers and fiber optics, including “City of Light: The Story of Fiber Optics,” and “Beam: The Race to Make the Laser.” Find out more at jeffhecht.com.