Lab reports big advance in laser fusion quest

Researchers at Lawrence Livermore National Laboratory say they have tripled the number of neutrons produced by fusion in tiny capsules of deuterium and tritium and thus have moved the National Ignition Facility a step closer to its goal of sustained nuclear fusion. The 13 August firing of NIF’s 192-beam laser yielded 3 × 10¹⁵ neutrons, whose total energy reached 8 kilojoules. That output was nearly twice the 5 kJ of energy that produced the plasma in the peppercorn-sized sphere of fusion fuel, says Ed Moses, the lab’s principal associate director for NIF.

The result puts NIF a factor of four to five away from ignition, says Moses; last fall the Department of Energy reported that NIF was an order of magnitude away from its goal. Only a factor-of-two increase in plasma energy will be needed to attain alpha heating, an intermediate milestone at which alpha particles from fusion reactions contribute twice as much energy to the plasma as the laser does.

One to two more kilojoules reaching the plasma from the laser should be enough to yield the 15 kJ of fusion energy and from 6 × 10¹⁵ to 8 × 10¹⁵ neutrons that will produce alpha heating, Moses says. “If you have temperatures and pressures long enough to enable fusion reactions, and also to trap the alpha particles so they start to heat the hot spot, now the rate of reaction goes up as the temperature goes up. You have more fusion reactions and it starts to bootstrap its way up.”

The NIF experiment was designed to prevent the breakup of the ablator—the plastic shell surrounding the D–T fuel—that has been a problem in previous experiments. By changing the temporal shape of the laser pulse, researchers lowered the temperature at which the target was compressed. The result was a lessening of the hydrodynamic instabilities that have both caused the fuel capsules to implode asymmetrically and driven material from the ablator to mix with the fuel.

“What’s very interesting is [the results] agreed with our preshot calculations very nicely. By taking out these instabilities, things seem to be acting predictably and nicely,” Moses says.

“It’s a significant result from the physics point of view,” says Riccardo Betti, assistant director for academic affairs of the Laboratory for Laser Energetics at the University of Rochester. The experiment marked the first time that a NIF plasma showed significant self-heating from alpha particles, which elevated the plasma temperature by 10–15%, he notes.

The NIF laser focuses 1.7 megajoules of UV light onto the cylinder, or hohlraum, that contains the fuel. Only a few kilojoules of that energy actually winds up in the plasma. “Most of the energy that the laser transfers is wasted on heating up the gold hohlraum, then heating up the plastic, and only a very small fraction of the energy actually heats up the thermonuclear fuel,” says Betti. “It’s a very complicated way to transfer energy, but it’s the best way so far.” The sequence mimics on a laboratory scale the secondary stage of a thermonuclear weapon, and the NIF experiment produced conditions that haven’t been observed since underground nuclear weapons testing ended in 1992.

Since the two-year-long campaign to achieve ignition officially ended in September 2012, ignition experiments have continued at a somewhat reduced pace, interspersed with other weapons-related research topics that include equations of state and radiation transport.

Only the hot spot, a portion of the fusion fuel, becomes plasma in NIF experiments. A denser, colder outside layer of D–T serves to surround and confine the plasma. Fusion, says Betti, will need to propagate to the outer D–T fuel layer to produce the megajoules of energy required for laser fusion to become a viable energy source.