As the campaign to attain controlled fusion ignition at Lawrence Livermore National Laboratory moved into a second year, the US Department of Energy’s undersecretary for science Steve Koonin reported uneven progress from the nearly three dozen experimental shots performed in recent months at the National Ignition Facility (NIF). In an 8 November memorandum, Koonin cautioned that ignition, “an overriding goal” for the agency and NIF, isn’t certain by the end of the current fiscal year, when the two-year National Ignition Campaign (NIC) is due to run out.
NIC was launched shortly after the $3.5 billion NIF—seven years overdue and more than $2 billion over budget—was commissioned in 2009. The 192-beam NIF laser was commissioned as a 1.8-MJ device explicitly to produce fusion ignition and high energy gain conditions, primarily in support of the US nuclear weapons program. In September NIF reported new records for laser energy and neutron yield—1.6 MJ and 6 × 1014, respectively. The highest-energy NIC shot covered in Koonin’s previous review in June was 1.3 MJ. The record 1.6-MJ shot, he said, produced a density in the fuel “hot spot” one-third as much as, and a neutron yield an order of magnitude less than, that required for ignition.
In his memorandum, released the same day his resignation, effective 18 November, was announced, Koonin presented his fourth and last one-day review of NIC’s progress. The 35 experiments he reviewed up through 28 October were “well-diagnosed,” he said, and the laser has functioned well. The fusion targets are “well characterized,” “high quality,” and “behaving consistently,” but have been revealing some of the challenges ahead, some of which, he said, “are being addressed successfully, while others are so far proving more recalcitrant.”
From simulation toward success
Scientists at NIF have found large gaps in their ability to simulate experimental results and extrapolate them, Koonin said. Simulations of target performance show an inadequate understanding of energy deposition by the laser in the hohlraum and of the coupling of the x-ray field to the fuel. Even after ad hoc adjustments are made to simulations to compensate for deficiencies, the best experimental results have produced only 20% of the yields predicted by the codes, Koonin said.
Continued fine-tuning of the laser beams’ pulse shape and timing will be crucial to meeting the September 2012 NIC deadline, he said. The lab has increased the frequency of shots and has pushed a measurement known as the experimental Ignition Threshold Factor (ITFX) up fivefold since June. Still, the highest ITFX is just 10% of that needed for ignition. Although “roughly in accord with the campaign plan,” the ITFX mark was disappointing in light of the “excellent experimental control and laser parameters achieved.” If that measure can be increased by a factor of three or more in the coming months, he said, alpha deposition will begin to dominate the neutron yield and would mark “a heretofore unobtainable plasma regime.”
Koonin also expressed frustration that the ignition experiments, which he called “first and foremost a research project,” are being held to a firm timetable. Given the uncertain outlook for success by 30 September, he said it would “be prudent to devote some effort to understanding what might be the criteria for, and nature of, a ‘Plan B’ post FY-12.”
The prospects for obtaining funding for ignition experiments beyond 2012 are uncertain, given reductions in federal discretionary programs, mandated by the Budget Control Act that was enacted in August. The House-Senate super committee has until 22 November to recommend how to spread a minimum of $1.2 trillion in cuts to discretionary programs over the next decade. A failure by the super committee to reach an agreement will automatically trigger cuts of equal amounts from defense and nondefense programs. Still, DOE’s nuclear weapons programs, which pay for NIF and its experiments, have been treated favorably by congressional appropriators in recent years.
The complexity of achieving ignition, even in a lab of NIF’s three-football-field size, is enormous. A four-step, precisely timed increase in laser power must occur within a few nanoseconds in order to cause cylindrical hohlraums to emit an x-ray field that can uniformly implode a spherical fusion fuel pellet containing frozen deuterium and tritium. If either the timing or the power applied by the four shocks is slightly off, the pellet will implode unevenly into a pancake or sausage shape and fizzle. Recent experiments to tune the NIF pulses have revealed that the last pulse rises more slowly than had been anticipated, Koonin said.
A new hohlraum design fabricated from depleted uranium (DU) coated with gold has replaced the all-gold versions used in earlier shots. The DU devices, which were tested with the University of Rochester’s Omega laser, have yielded some improvement in the last-stage x-ray pulse, Koonin said.
The success or failure of the ignition campaign will have no direct relationship to nuclear weapons performance,“ Koonin said, although he added that NIF’s “already high value” for stewardship of the weapons stockpile would be “greatly enhanced” by experiments that ignition would enable.
David Kramer
