Science: Record-breaking laser pulse paves way for fusion
In terms of laser energy, it was the biggest bang yet. On 15 March, the National Ignition Facility at the Lawrence Livermore National Laboratory in California fired a single laser pulse containing 1.875 million joules of energy, exceeding its design energy of 1.8 megajoules for the first time.
“This is very exciting, like breaking the sound barrier,” says NIF director Edward Moses. Next on the agenda for the building-sized laser is firing a pulse that energetic at a target, thereby igniting nuclear fusion.
The milestone comes $4 billion and 15 years after construction began on the massive laser, which started operating in 2009. Since then Moses has gradually been turning up the power, so the new record is only a small step above NIF’s previous record of 1.6 MJ. Still, no other laser in the world comes close to such energies – the 192-beam NIF delivers dozens of times more energy than the 30-kilojoule pulses from the world’s next-largest operating laser, the 60-beam Omega laser at the Laboratory for Laser Energetics in Rochester, New York.
NIF’s ultimate goal is to reach fusion’s “ignition threshold”, where fusion reactions generate more energy than the laser pulse contains. This will require delivering lots of energy in a very short time to heat and squeeze a tiny hydrogen target into such a hot, dense mass that the nuclei fuse, releasing energy.
The energy required is relatively modest – 1.8 megajoules is about the energy released by a half kilogram of high explosive. But NIF delivered that energy in only 23 billionths of a second, so that its power during that period reached 411 trillion watts, 1000 times more power than the US electric grid generates on average.
It has been a long, hard slog to achieve this goal. Livermore took its first shots at fusion targets in the 1960s with much smaller lasers, and NIF itself had a troubled birth in the 1990s. Though it has taken more time than expected to reach the energy required for igniting fusion, Livermore hopes to reach ignition by year’s end.
Meanwhile, the lab is already planning an even bigger laser called LIFE (Laser Inertial Fusion Energy).
By the end of this year, the National Ignition Facility will try to focus nearly 2 million joules of ultraviolet laser energy at a tiny target at the tip of this “pencil” (Image: Lawrence Livermore National Security, LLC/Lawrence Livermore National Laboratory/Department of Energy)
Jeff Hecht, New Scientist