Optics Recycle Loop

Every time the world’s most powerful laser fires, it leaves behind tiny cracks and damage that could bring the entire system to a halt. Instead of replacing costly components, scientists at Lawrence Livermore National Laboratory have engineered a high-tech repair system—like a Formula 1 pit stop for lasers—to keep it running at peak performance.

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  Lawrence Livermore National Laboratory is hiring. If you're passionate about tackling real-world challenges in science, engineering, business, or skilled trades, there's a place for you at the lab. Right now, positions are open for a Senior Guidance, Navigation, and Controls Engineer and a Senior Business Operations Specialist. These are just a few of the more than 100 exciting roles available. At Lawrence Livermore, you'll work on projects that matter, from national security to cutting-edge scientific advancements. Join a team that values innovation, collaboration, and professional growth.

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  A Formula One race car hurtles down the track at blistering speeds. Every second counts, and every part of the car must perform flawlessly. But over time, tires can wear thin, engines strain, and components face extreme heat and pressure. When the car pulls into the pit stop, a precisely coordinated team springs into action. In a matter of seconds, they replace worn parts, make adjustments, and send the car back to the race, ready to perform at its maximum potential. Now imagine this level of precision on a cosmic scale.

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  Inside the National Ignition Facility at Lawrence Livermore National Laboratory, 192 laser beams work together, harnessing immense power to replicate the energy of the stars. And like a Formula One car, the system faces constant challenges. Tiny cracks and pits, as small as 1 twentieth the diameter of a human hair, can form on the optics due to laser-induced damage every time NIF fires. These imperfections, if left unchecked, can grow exponentially, thus scattering light, reducing efficiency, and jeopardizing the entire laser system.

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  Fortunately, scientists and engineers at Lawrence Livermore National Laboratory have developed solutions to these challenges. Today, we're exploring the optics recycle loop, a cutting-edge process akin to a high-tech pit stop, where damaged components are repaired and returned to service with precision. We'll meet the researchers solving the mysteries of laser damage, the engineers designing innovative tools, and the technology supporting it all.

• 2:53Speaker 00

  Welcome to the Big Ideas Lab, your weekly exploration inside Lawrence Livermore National Laboratory. Hear untold stories, meet boundary-pushing pioneers, and get unparalleled access inside the gates. From national security challenges to computing revolutions, discover the innovations that are shaping tomorrow, today.

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  Lawrence Livermore National Laboratory is hiring. If you're passionate about tackling real-world challenges in science, engineering, business, or skilled trades, there's a place for you at the lab. Right now, positions are open for a Senior Guidance, Navigation, and Controls Engineer and a Senior Business Operations Specialist. These are just a few of the more than 100 exciting roles available. At Lawrence Livermore, you'll work on projects that matter, from national security to cutting-edge scientific advancements. Join a team that values innovation, collaboration, and professional growth. Explore opportunities at llnl.gov/careers, where your next career move could make history.

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  Lawrence Livermore National Laboratory's National Ignition Facility, or NIF, began operations in 2009. Its mission is to replicate the extreme conditions where fusion ignition naturally occurs, such as in the cores of stars. NIF is the most energetic and precise laser system on the planet. It pushes the boundaries of science and technology, generating temperatures of 100 million degrees and pressures more than 100 billion times Earth's atmosphere. This unique facility plays a critical role in advancing national security and high-energy density physics. By conducting experiments that ensure America's nuclear arsenal

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  remains safe, secure, and reliable, without the need for underground testing, NIF provides essential data to modernize and uphold our defense systems. But beyond its Earth-bound contributions, NIF opens a window into the cosmos. By replicating the extreme states of matter found in the center of planets, stars, and other celestial objects, NIF helps scientists unlock the mysteries of the universe. In December 2022, Lawrence Livermore National Laboratory made history by demonstrating fusion ignition for the first time ever in a laboratory setting. For the smallest fraction of a second,

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  they replicated a star on Earth. And they've repeated this achievement over and over and over. As we've explained in previous episodes, fusion ignition is the process where two atomic nuclei combine to form a single, heavier nucleus. This releases an immense amount of energy.

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  At the National Ignition Facility, scientists are turning this cosmic phenomenon into a groundbreaking reality. Each of NIF's 192 laser beams, roughly 40 centimeters in size, travel nearly a kilometer through the facility. Along the way, they pass through more than 7,000 large optics that amplify, transmit, reflect, shift wavelengths, or focus the light. All of this precision engineering converges energy from the lasers onto a tiny fuel pellet, compressing it to extreme temperatures and pressures.

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  The purpose of an optic is to manipulate the light that is transitioning through it.

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  Tayeb Suratwala is the Program Director for Optics and Material Science at Lawrence Livermore.

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  So either you're trying to reflect it, you're trying to focus it, you're trying to use it where you increase the intensity of the light.

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  The NIF spans three football fields, housing a labyrinth of machinery, precision optics, and advanced systems.

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  NIF is going in a new paradigm space in terms of operations of lasers. The real important question is, why? Why are these lasers so large? And the reason is that there is a limitation today of how much light you can put through a material before you will destroy it. And because of that limitations, we have to make bigger optics and try to look at strategies to get more and more light through those components.

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  The immense scale of NIF's lasers is crucial for achieving the conditions necessary for fusion. However, this extraordinary combination of power and energy pushes the optics to their absolute limits, leading to potential damage. When light interacts with delicate glass components, laser beam intensity is so extreme that the light can physically remove material from the surface of the glass optics, carving away microscopic layers with each pulse. Laura Masiyo Kegelmeyer led the team at NIF for optics inspection and data management.

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  NIF was designed and built to shoot laser light at energies that we knew would damage the optics. If you think of an optic as a piece of glass and damage as what happens if a rock hits your windshield.

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  Oh, man.

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  You get a pit of damage to that glass. And it's hard to imagine that laser light does the same thing as a rock, but it is so intense that the laser light actually puts little pits of damage in the glass that we use for many purposes to get the laser light from where it starts to where it hits the target.

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  The NIFs optics are made from various types of glass.

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  We have crystals. People know what crystals are because you could put a crystal in your window and see all the different colors come through. We have amplifiers. You put in a certain amount of light energy, and more light energy is going to come out of an amplifier. Our amplifier glass is made out of a very special material that accepts light from an ordinary flashlight, like your camera flash, and it donates that light to the laser. And that's how it gives the laser more energy. So when our laser starts out as a very weak pulse of light, like a laser pointer that you would shine, it starts out weaker than a laser pointer. But by the time it goes through amplifiers,

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  it has so much energy that after we change the color of the light, it can damage the optics that are the most expensive optics in the whole system. We don't want to just let our optics damage and then throw them out.

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  Because of the high costs associated with these optics, the lab developed what's known as the Optics Recycle Loop, a carefully designed system to efficiently repair and reuse the laser's most delicate components.

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  When the laser was being designed and built, if you just build your tunnel solid, but you have a bunch of pieces of glass inside your tunnel, it's going to be really hard to replace those pieces of glass. But if you build your tunnel with little modules that come in and out from the side, then you can swap your optic by taking out one module and replacing it. So these beam lines were designed with line removable units, which a clean room robot can slide one out without adding any debris or contamination in a very clean manner, slide another one in. We take the one that came out and we repair it. It goes through the recycle loop. And then, again, it's ready to be exchanged online with another one.

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  So we had to design the laser with this concept in mind so that the optics could be exchanged and repaired and reused. People didn't know exactly if this was possible, and we didn't know exactly all the details of how it would work. But they built the laser to allow for these exchanges.

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  These removable units allow for seamless optic exchanges, ensuring that maintenance can be done without risking contamination. And the ingenuity didn't stop there. NIF was also equipped with advanced diagnostic systems and cameras to monitor the optics in real time, adding another layer of precision to its operations.

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  We find every single pit of damage using cameras when the optics are in this gigantic laser facility. And then, those camera systems and the software that we've worked on for decades lets us know exactly where every damaged site is, so we know when to remove an optic, so we can repair all the damage, and we can put it back to work and extend the usability of that optic. We can use it many, many times with this recycle loop concept.

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  To meet the demanding firing schedule, roughly every day, damaged optics are removed and replaced with a freshly refurbished set while the originals are repaired. This exchange allows NIF experiments to continue uninterrupted without delays caused by the intricate repair process.

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  The laser is firing at higher and higher energy, so the damage is changing its nature. The optics are suffering in different ways. And it's always a tug of war, because, of course, the people doing the ignition experiments want to just hit the target with more energy. And the people protecting the optics are saying, "Well, wait, can we hold back?" And so, we're constantly making the optics tougher.

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  So how does the repair and recycle process work? And why is it critical to NIF's operations?

• 12:58Speaker 00

  Lawrence Livermore National Laboratory is hiring. If you're passionate about tackling real-world challenges in science, engineering, business, or skilled trades, there's a place for you at the lab. Right now, positions are open for a Senior Guidance, Navigation, and Controls Engineer, and a Senior Business Operations Specialist. These are just a few of the more than 100 exciting roles available. At Lawrence Livermore, you'll work on projects that matter, from national security to cutting-edge scientific advancements. Join a team that values innovation, collaboration, and professional growth.

• 13:38Speaker 00

  Explore opportunities at llnl.gov/careers, where your next career move could make history.

• 13:54Speaker 01

  For the optics recycle loop, the basic concept is we're going to pull the optic off, we're going to repair the damaged side and return it. The ability for the NIF to operate at a certain rate, how many shots that they take, the accumulated power and energy that they shoot, is strongly linked to the rate at which we can recycle the components and return them back to the facility. So this is rate balance. In fact, we made up our own terminology. We call it a currency, which is termed log growth, which describes the rate at which we can shoot NIF.

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  The recycle rate depends on the shot rate, or log growth, and continued improvements in the laser damage resistance of the optics. Optics improvements enable higher energy and power, or a higher shot rate. After every few NIF shots, a special camera system captures high-resolution images of the optic components near the target area. These cameras can detect damage as small as 50 microns. This allows scientists to pinpoint exactly where damage is occurring on each optic. Because NIF fires almost daily, it's not practical to immediately remove an optic every time damage is detected. Instead, the team uses a unique technique.

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  They place a shadow over the damaged area in the beam. Imagine a very small umbrella placed in the pathway of the laser beam to cast a small shadow over the damaged spot to protect it from further harm. By eliminating the energy in that area, it prevents the damage from spreading.

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  That damaged site won't get bigger and bigger. It's like a protection mechanism. Every time we do that, we're turning the beam into kind of Swiss cheese, right? Because you're putting all these little black spots all over it. So there's a limit to the number of those that we can apply.

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  When it's time to remove an optic, the component is carefully extracted from the system. The optic is transported to a clean room facility where it undergoes an intricate repair process. Scientists first inspect and clean the optic to avoid contamination. Next, they place it under an automated microscope, which catalogs all the optic's features. The level of detail is astounding. Each optic is scanned with magnification so precise that even features as small as five to 10 microns are documented. This requires up to 20,000 images per optic. Advanced AI software processes these images, sorting through thousands of features: damaged

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  sites, debris, scratches, and other minor imperfections, and generating a detailed map of the optic containing every flaw. Most are benign, but those requiring repair are sent to the next stage.

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  We use another laser. This is called a CO2 laser, a carbon dioxide laser, and we shoot the optic at the position where that damaged site is, and we essentially do laser surgery at that location and we remove the damaged site. And what we leave behind is a tiny little divot on the surface of that optic. And we do this on essentially all the damaged sites that get created on these optics. And the repair rate is on the order of about 10,000 to 20,000 sites per month.

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  But what about the tiny little divot that's left after the repair?

• 17:22Speaker 01

  What it does is it basically scatters the light a little bit, and it's not that problematic. There's a limit to the number of these divots you can put onto a part. And at that point, once it gets to a certain level, which represents about 1% of the beam area, we retire the optic.

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  Here's Ren Kar.

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  I'm here for laser damage.

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  He's the science and technology leader for the optical and material science and technology group at the lab.

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  When you look at the damage sites after everything has settled down, you see molten regions, you see fibers of previously molten glass that have sprayed all over the place and big fractures. It's really very interesting set of physical phenomena that occur every time you get laser damage. For the laser, it's just a big nuisance, but scientifically, it's just fascinating.

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  These damage sites might appear as tiny sparkles on the surface of the optic, but under an electron microscope, the heat and energy create molten cores surrounded by intricate fractures, each telling the story of the laser's immense power.

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  From standing three feet away and looking at an optic, you just see a tiny little sparkly thing. But with an electron microscope, you see all these different details about it. The previously molten core and the fracture that surrounds it are damage sites grow exponentially. If you shoot 10 shots on the NIF, the damage site that might start out as a tenth of a human hair on the first shot, it could be 100 human hair diameter by the 10th shot, and it just grows more and more quickly. We have found ways to repair these damage sites, but only if we don't let them get too big. They start small, and they grow, and they grow, and they grow, but because it's exponential growth, the rate at which they get bigger speeds up as they grow.

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  However, not all damaged optics can be fixed. Some optics reach a point where repair is no longer possible, and their role in the system shifts. This is especially true for the bottom beams at NIF, where gravity introduces unique challenges.

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  We now have 128 beams that permanently have these few circle debris shields in, and they're protecting the gradient debris shield. We have found additional problems in the bottom 32 beams of NIF. And what happens is you blow the target up, and you get damage on these other things, and then gravity just pulls it down to the bottom.

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  This accumulation of debris in the bottom beams creates what the team jokingly calls "the ashtray of NIF."

• 19:55Speaker 03

  All the gunk from blowing the targets up and all the damage from all the other optics, it just finds its way down and just kind of rattles down and gets around the fuselage debris shield. And so when we did an experiment where we put the fuselage debris shield in the bottom 16 beams of NIF, it helped, but it didn't help enough for the cost of them. So we've punted on that.

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  Rather than abandon these beams entirely, the team found a creative solution, turning them into composting beams.

• 20:26Speaker 03

  We have turned the bottom 16 beams of NIF to our composting beams. And by what I mean by that is when we've pretty much used up our optics on the other beams, they don't damage as fast now because of the protections we've given them, but eventually they just kind of run out of life. And so the last time we think we're going to be able to install them, we put them in the bottom 16 beams. We know we're not going to be able to repair these no matter what. And so we put them down there and we just let them damage.

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  This resourceful approach maximizes the lifespan of optics and provides valuable data for future innovations. Once they've extracted every last bit of life out of an optic, it must be replaced. The bottom beams remind us that even the toughest obstacles can yield opportunities for discovery and adaptation.

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  As the team continues to make optics improvements and as NIF goes to higher energies, new damage mechanisms will emerge. As the saying goes, the reward for good work is more work. This cycle continues to push NIF to operate at higher and higher energy levels. Like every pit stop in a Formula One race demands perfect timing and coordination, every repair and innovation at NIF ensures the laser system performs at its peak. From designing a facility ready for the future to restoring optics damaged by the world's most energetic laser, each challenge sets the stage for the next loop in innovation.

• 22:11Speaker 00

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