The 192-beam laser at the National Ignition Facility (NIF) in California has set a new energy record, delivering a 2.15 MJ pulse to its target chamber.
Officials at Lawrence Livermore National Laboratory (LLNL), where the giant system is located, say that represents more than a 10 per cent increase on the previous best of 1.9 MJ – a milestone set back in March 2012.
“This demonstration shot successfully meets a National Nuclear Security Administration (NNSA) Level 2 milestone for 2018,” stated the laboratory, adding that the demonstration could see NIF operate at substantially higher energies in the future – and well beyond the original design specification of 1.8 MJ.
NIF director Mark Herrmann said in an LLNL release: “NIF’s users are always asking to use more energy in their experiments, because higher energies enhance the science NIF can deliver in support of the stewardship program.
“These results mark a major step toward increasing NIF’s energy and power capability. This demonstration serves as the first step on a path that could allow NIF to operate at substantially higher energies than ever envisioned during NIF’s design.”
Remix to ignition?
As well as extending the energy limits to which NIF can be used to carry out so-called “stockpile stewardship” experiments – where the laser system is used in place of live nuclear weapons testing – the latest result could provide some impetus towards the tantalizing goal of achieving ignition.
Although the 351 nm wavelength source - produced by frequency-tripling a Nd:glass laser - has generated nuclear fusion conditions for a couple of nanoseconds, it has not yet delivered the hoped-for runaway combination of fusion with a net energy gain; the definition of ignition.
Achieving ignition was originally thought possible with the 1.8 MJ pulse energies called for in the NIF design specification, and the “ignition campaign” was a key mission focus for the LLNL team after the huge system was first switched on in 2009.
But after discovering discrepancies between experimental data and the mathematical models used to simulate the conditions required for fusion with ignition, the past five years or so have seen NIF’s priorities switch back to stockpile stewardship and other scientific experimentation.
Other difficulties encountered have included damage to the large optical components used in the NIF system, caused by phenomena like neutron bombardment when the extraordinary star-like conditions are recreated in the lab.
However, after recent work by LLNL scientists and engineers to study and extend the performance limits of the giant ultraviolet laser, as well as reduce the level of damage initiated when the laser fires and find better ways to mitigate existing damage to optics, it seems that ignition is back on the agenda.
“Based on this successful demonstration, NIF is working with LLNL’s ignition program to execute the first ignition experiments that utilize this enhanced energy capability later this summer,” announced LLNL. “Looking ahead, this is the first major step toward extending NIF’s energy and power output through technology development and laser research to extend the NIF mission space and its contributions to the SSP (stockpile stewardship program).
Optics damage mitigation
NIF’s associate director Jeff Wisoff added: “The successful 2.1 MJ demonstration is the result of a sustained science and technology investment in NIF and fundamental understanding of optical damage, much of which has been supported by Laboratory Directed Research & Development (LDRD) and other institutional programs.”
The work to mitigate optics damage has included development of new anti-reflective coatings and chemical processing techniques – particularly for the “grating debris shield” components that sit just outside the NIF target chamber and protect other key components from bombardment.
Such was the extent of the damage prior to the coatings work that up to 40 optical components had to be removed from service for repair every week.
The NIF system uses tens of thousands of large precision optical components, including lenses, laser glass slabs, mirrors, and frequency-tripling crystals to amplify and guide each of the 192 beams towards a tiny target housed in the 10 m-diameter target chamber.
“Continuous research and development efforts have put these optics at the cutting edge of material science and technology and play a crucial role in raising the laser’s energy and power thresholds,” says LLNL.
