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Next-gen optical coatings could boost NIF laser power

07 Apr 2017

New anti-reflective material on grating debris shield component also helps to protect other optics from debris and will cut need for replacement parts.

Optics engineers at the National Ignition Facility (NIF) in California have developed a new anti-reflective coating for the giant laser system’s optics that should both boost the pulse energy delivered to target chamber and cut the rate at which damaged components need to be replaced.

NIF chiefs say that the new colloidal silica particle coating will help cut the need for replacement optics by more than half, and the optics team at the site is currently three-quarters of the way to completing installation of the new technology.

Sol-gel process
Fabricated using a sol-gel chemical process, the coating was developed to reduce reflections from the rear surface of NIF’s grating debris shields (GDS). The GDS is the penultimate optic before the 192 laser beams enter the target chamber. As well as generating a diagnostic beam that NIF engineers use to monitor the system, it serves to protect other optics from the hostile target chamber environment.

Engineers at the Lawrence Livermore National Laboratory, where NIF is located, say that a patented fabrication method, which they call the “advanced mitigation process” (AMP), further protects the optics by making their surfaces more damage resistant through removing impurities and reducing microfractures.

“These imperfections, when exposed to laser light, create tiny damage craters on the surface, which grow with repeated laser shots and limit the life span of the optic,” they explain. “Both the AMP process and the coating are required to realize these reductions to the damage rate and the possibility of higher energy on NIF.”

Both of the technologies have been developed at LLNL over the past decade – a decade in which the NIF team has generated immensely powerful beams that have enabled some unprecedented physics experiments, but has thus far also failed to achieve the ultimate goal of laser fusion with ignition (i.e. with a net energy gain).

User facility focus
Speaking at the SPIE Photonics West conference earlier this year, NIF representatives said that while the lab’s focus had switched more towards nuclear stockpile stewardship, alongside a difficult transition from a research facility to a user facility operation, it remained “committed to the pursuit of ignition” with the giant laser system.

Tayyab Suratwala, program director for Optics and Materials Science and Technology (OMST) at LLNL, explained in a lab announcement how the new optics would aid with that transition:

“Maximizing the amount of energy and shots reaching NIF targets is a critical factor in the facility’s efforts to serve the needs of its users and laboratory missions. By minimizing the damage rate to NIF’s optics, we can save considerable time and expense involved in recycling or replacing optics that become unusable, and thereby increase the number of shots available to experimenters.”

In fact, the number of laser “shots” completed by NIF has doubled in recent years, with a current run-rate of around 400 shots per year now being carried out.

However, that increased demand also takes its toll on the system’s optics, which suffer damage each time the colossal ultraviolet laser powers are fired at tiny hohlraum targets, generating powerful X-rays and other hostile phenomena in the process.

GDS stability
Each of NIF’s laser shots relies on a complex series of amplifiers, mirrors and wavelength converters, to scale up and guide laser light into the target chamber, where inertial confinement fusion takes place through a rapid compression.

The GDS component is a key one: it diffracts a small amount of laser light for beam diagnostics, helping NIF researchers balance the energy of all 192 beams as they enter the target chamber - crucial for creating conditions for laser fusion. But it is reflections from the uncoated GDS that ultimately lead to much of the damage to other optics.

Marcus Monticelli, LLNL’s process engineer lead, said: “These gratings have to be very stable. The way to keep it stable, historically, was to leave it uncoated, because the index of refraction of the coating can change with time. That will affect diffraction efficiency significantly, and that would cause power balancing issues on NIF.”

However, the knock-on effect is that, left uncoated, the fused silica optic reflects nearly 4 per cent of the ultraviolet power back up the beamline. And while "only" a reflection, this still represents a huge amount of optical power that must be directed to a beam “dump” to minimize any collateral damage.

Unfortunately, some of the reflected light ends up in the integrated optics module (IOM) containing the final optics, creating a highly focused beam of light that is sufficiently intense to damage the IOM’s stray-light-absorbing glass, just above the exit surface of the GDS.

Debris
According to LLNL, it means that every NIF laser shot both damages the IOM and “spews” debris all over the GDS.

That debris then creates thousands of potential damage sites on the GDS. And when they are exposed to NIF’s high-energy laser beams, they grow and eventually render the component useless.

As a result, up to 40 optics need to be removed from service each week, so that damage sites can be locally repaired through a process known as the “NIF Recycle Loop.” The loop is designed to ensure that NIF economically operates at maximum energy by limiting the likelihood of damage and acting quickly to mitigate further damage when it does occur. Now the LLNL-developed colloidal silica particle coating looks like it should make a huge difference to that rate of replacement.

To accommodate the coatings, NIF’s optics team needed to modify the lithographic process for imprinting the holographic gratings on the exit surface of the GDS.

Controlled experiments using both standard and coated GDS, combined with other laser improvements such as reducing beam contrast, have shown a more than 50-fold reduction in the number of problematic damage sites, claims the lab.

Tests have now been under way for two years and, as of last month, the NIF team was said to be about three-quarters of the way through replacing uncoated GDS with the new model as the older versions outlive their usefulness.

They predict that the recycle rate for damaged optics will be cut in half – to between 10 and 20 each week – when all the coated GDS components are in place. At the current shot rate, the number of new GDS components required should drop from about 130 a year to just 40, said to represent a significant cost saving.

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