A muted milestone for the National Ignition Facility

Scientists at the National Ignition Facility (NIF) have achieved another milestone on the path to fusion ignition. A 28 September experiment marked the first time that a plasma of deuterium and tritium generated more fusion energy than the amount of energy that went into the plasma. The results put researchers a step closer to the goal of ignition, in which the amount of fusion energy is equal to or exceeds the energy of the 192-beam laser.

According to a 29 September internal memorandum from Edward Moses, then the principal associate director for lasers and photon science at Lawrence Livermore National Laboratory (LLNL), the experimental shot produced 5 × 10¹⁵ neutrons, or around 14 kilojoules, almost 75% more than the previous record output. About 4 kJ, roughly 1% of NIF’s 1.8-MJ laser, reached the plasma. About 99% of the laser’s energy is lost in the conversion of UV light to the x rays that drive the implosion of the peppercorn-size fuel capsule.

“More importantly,” Moses wrote in the memo obtained by Physics Today, “the self-generated energy of this target exceeded the input energy of the imploding D–T fuel. This is called scientific break-even.” The energy supplied to the plasma from alpha particle heating was nearly twice that imparted by the laser, a “clear demonstration of the mechanism that is needed to achieve ignition,” Moses said.

In addition, the hydrodynamic compression going into the hot spot—the small portion of the D–T fuel that is converted to plasma—was increased. Recent improvements in ignition experiments, which have mostly come from lowering the compression that is applied to the fuel capsule, have resulted in more symmetrical, stable implosions.

Announcement shutdown

The lab made no announcement of the results because of the government shutdown. The advance was first reported in the Independent, the local newspaper in Livermore, California. Although LLNL continued operations this week, the National Nuclear Security Administration has instructed officials at the weapons labs to not communicate externally while the shutdown continues. Sources at LLNL say employees may be sent home as early as next week, when the lab runs out of funds carried over from fiscal year 2013.

In August, LLNL reported then-record results of 3 × 10¹⁴ neutrons and 8 kJ of output. According to Moses’s memo, the yield of ignition experiments has increased more than fivefold since the current D–T series of shots began in May 2013. The results were “pretty nice and pretty significant,” says Riccardo Betti, assistant director at the University of Rochester’s Laboratory for Laser Energetics. Still, LLNL remains a long way from ignition. That goal will require around 2.5 times more compression pressure, which is no small task, Betti says.

Although NIF is operating near its peak design energy, a number of options can be pursued to attain ignition. Researchers can change the material of the shell surrounding the D–T fuel from plastic to high-density carbon or beryllium. That shell, known as the ablator, blows off at high velocity when struck by x rays, driving the implosion.