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A new age for science?

Nature Photonics volume 2, pages 35 (2008) | Download Citation

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What would you do if you owned the world's most powerful laser? The US government is hoping to use it to achieve the ignition of thermonuclear fusion in the lab for the first time. Nature Photonics spoke to Edward Moses of the National Ignition Facility to find out more.

A NIF scientist working inside the target chamber. Image: NIF

Three years until a new age for science. At least that's how Ed Moses, the director of the National Ignition Facility (NIF) puts it. The NIF, which is based 45 miles (72 kilometres) east of San Francisco, boasts the most energetic laser that has ever been built, and is due to be fully operational by 2010. And once it is turned on, Moses believes that a whole host of scientific avenues will open up.

About the size of two football fields (roughly 65,000 square metres in area), the NIF is based at the Lawrence Livermore National Laboratory (LLNL) in northern California. It is a highly ambitious, rather controversial project funded by the US government — specifically, the Department of Energy's National Nuclear Security Administration — with the aim of achieving inertial confinement fusion (ICF).

Inertial confinement fusion involves heating and compressing a fuel target (usually heavy hydrogen) to such a degree that nuclear fusion reactions are set off. The heating process can be set in motion by high-energy laser beams that are fired onto the fuel target or target capsule, and which heat the target's outermost layers. The heated layers then explode outwards, causing the rest of it to collapse into a very small, very dense point.

The explosion drives shockwaves into the innermost layers of the target, which causes further heating and compression of the fuel. If the temperature and pressure conditions inside the target become extreme enough — which requires an extremely symmetrical compression process — efficient fusion can occur, and a significant amount of energy can be released. 'Ignition' is said to occur when the heat of the fusion reaction can sustain further burning of the target fuel. Controlled ignition is considered to be critical if fusion energy is ever to become a viable prospect. According to Moses, “Our goal is to do our first ignition experiments in 2010.”

An evolving science

The NIF is the culmination of a long line of large, powerful glass laser systems built to study ICF at LLNL, but it is much more powerful than anything that has come before.

Moving the target chamber into place within the NIF. Image: NIF

As Moses explains, “We're taking 192 laser beams, each one of them the world's most energetic laser, and focusing them onto a very small target — only about 2 mm in diameter — so that we can create temperatures and pressures high enough to force hydrogen atoms to fuse together into helium, releasing a huge amount of energy in the process.”

The initial laser light is provided by an ytterbium-doped optical fibre known as the Master Oscillator. This light is directed into 48 pre-amplifier modules and passed through each module four times. The amplifiers, which operate in the infrared at 1,054 nm, boost the energy of the beams from a few microjoules to about 10 J. Further amplification then takes place in a series of glass amplifiers, where the energy is increased from 10 J to about 4 MJ.

Following amplification, the light is precisely focused using diagnostic and pulse-shaping equipment, and sent on its way to the target chamber. Before reaching the chamber, however, the light is converted into UV radiation (a wavelength of 351 nm) using potassium dihydrogen phosphate crystals because UV light is more efficient at heating the target than infrared radiation. All of the beams must reach the target chamber within a picosecond of each other for efficient fusion to occur.

In the target chamber, the main beams will deliver over 2.2 MJ of energy and about 500 TW of power to a cryogenically cooled deuterium–tritium mixture. When fully up and running, the NIF will be 40–50 times more energetic than the next leading competitor — the OMEGA laser that is now operational at the University of Rochester.

Only 15–20% of the facility's time will be dedicated to basic science and fusion-energy research. Livermore Lab, which hosts the NIF, was founded in 1952 to further US national security interests, and the major motivation behind the NIF is actually defence. The Department of Energy (DoE) sees the NIF as a key player in its Stockpile Stewardship Program, which was established in the mid 1990s to ensure that the US nuclear capabilities do not diminish as its nuclear weapon stockpile ages. The NIF will be able to study the fusion stage of hydrogen bombs in detail, in the absence of full nuclear testing, but scientists and anti-nuclear advocates have brought into question just how useful these findings will be in maintaining the stockpile. According to Moses, stockpile stewardship work will take precedence at the start of operations in 2010.

Troubled waters

To get to this point, the NIF's path has been a rocky one. Over-budget, behind schedule and bogged down by controversy over its nuclear weapons goals, the NIF brought Moses on board as Project Manager in 1999 to redefine the budget and schedule and get the experiment back on track. As Moses recalls, “People had this great vision of how to set up the NIF. But three years into the project it became very clear that there were significant issues of budget, schedule, logistics and engineering that had to be faced. I was one of the people brought in to take a look at what was going on.”

In 1999 and 2000 Moses and colleagues re-baselined the project and drew up new projections of the overall cost and budget. “After a year of really being put under the magnifying glass, the DoE and US Congress re-approved the project in 2000,” he explains. The result is that the cost of the NIF ballooned from $1.8 billion to $3.5 billion. This $3.5 billion will see the project through until 2009 and covers the cost of research and development, building, commissioning and turning the facility over for operations.

The project re-baselining gave the NIF people room to breathe, but senior figures at institutes such as Sandia and Los Alamos National Labs in the US were critical of what they saw as an out-of-control project. In May 2000 Tom Hunter, the vice-president for Sandia's Nuclear Weapons Programs, argued that the spiralling costs of the NIF would eat out of other labs' budgets and that the project's size should be toned down. “The apparent delay and significant increase in cost for the NIF...will disrupt the investment needed...at the other laboratories...by several years. This causes us to question what is a reasonable investment in the NIF.”

Despite the completion date being seven years behind schedule, Moses is quick to defend the NIF's recent performance. “Since [the re-baselining] seven years ago, we have met all of our schedules, milestones and budgets without fail. Yes, we did have a rocky start, nothing that we are proud of or that we are hiding either. But I think the world has come to an understanding that this is a successful endeavour at this point. We have a good hold on how to do things now.” At present the project is 93% complete, with 104 of the 192 beams operational. The size of the staff is gradually being cut back as the facility moves towards coming fully online. Once up and running, the NIF will employ about 500 engineers and scientists to run the experiment in three shifts per day, seven days a week.

New frontiers

The gold-plated capsule that will house the deuterium–tritium fuel for thermonuclear fusion. To approach the target from a range of directions, 192 high-energy laser beams are reflected off a variety of mirrors. Image: NIF

From an optics point of view, the NIF is by far the largest and most complex optical system ever built (see Box 1). First, there is the sheer scale of the engineering project: the active optical area used by the NIF is 50 times larger than that of the Keck Observatory in Hawaii. The facility contains about 7,500 large optics (on the scale of about a metre), 30,000 small optics (about 10–20 cm in size), 60,000 control points, and an integrated computer system that orchestrates complicated beam-shot sequences over periods of several hours. “When we started out, and we looked at the world's ability to make optical components, the NIF alone required five times more capacity than existed internationally at that time for large-scale, high-quality optics,” Moses recounts. So US scientists partnered with colleagues in France who were working on Laser Mégajoule (a French competitor of the NIF that is scheduled to be completed by 2010), as well as suppliers in Asia, the USA and Europe, to develop the manufacturing capabilities needed to make the laser glass, the mirrors, the transmissive optics and lenses, and the crystals used to turn the colour of the laser light from infrared to UV.

Box 1: Ingredients for the world's most powerful laser

192 laser beams

48 independent pulse-shaping systems

48 pre-amplifier modules with four beams per amplifier

7,500 large optics components

30,000 small optics components

60,000 control points in more than 6,000 electro–optical assemblies

430 tonnes of ultrahigh-purity laser glass

1 integrated computer control system

Optical components precisely aligned over a 300-m optical path

Laser pulses one billionth of a second in duration, in lock-step to trillionths of a second, targeted with 10-micrometre accuracy

The second major challenge has been to make sure that the optical equipment can withstand the extremely high energy densities at the heart of the NIF. Most of the project's predecessors, including the LLNL's decommissioned Nova laser and Rochester's OMEGA laser, produced radiation fields with fluences of less than one joule per square centimetre. In contrast, the NIF will create fluences ten times larger, enough to easily damage standard optics on the first shot. The team has therefore devoted a lot of effort to fundamental research into surface and bulk material damage in high-energy-density environments and developed manufacturing techniques to produce more resilient equipment. Moses believes these techniques will benefit the entire optics community in the future.

So how much energy do we expect to get out of the NIF? Of the original 4 MJ of laser energy fed in through the beamlines, 1.8 MJ remains after conversion from infrared to UV energy, and about half of this is then lost during X-ray conversion in the target chamber. The NIF is actually an indirect fusion device: incident laser energy heats up the walls of a small metallic cylinder containing the target fuel, which in turn emit an even distribution of intense X-rays onto the fuel that subsequently induce the fusion reaction. Overall, about 10–20% of the initial laser energy is transferred to the target fuel. “Our initial goal,” explains Moses, “is to get about 10 MJ [of energy] out. Over the next few years as we continue to tune our process and push the lasers to higher powers, we could get over 100 MJ of energy out. So we're looking at [energy] gains of 50 to100.”

Power plants based on ICF could potentially offer new, clean sources of energy, which would involve delivering a successive stream of fusion targets to the ignition chamber, and harnessing the resulting energy to drive a conventional steam turbine. “Just think of how much energy has to be created over the next 25 years, then the next 50 years and project that to the year 2100,” comments Moses. “We will need approximately 10,000 1-GW nuclear or coal-fired reactors in the next 80 years, which is an incredible demand. While I don't know if there will be a single solution to this problem, fusion energy could play an important role.”

Despite the teething problems, Moses believes that the NIF has now won over the world. “Interest in the NIF as a science facility is growing very rapidly as the scientific community becomes aware of the capabilities we have developed and are developing. There is huge interest from around the world.” The French, British and Japanese all have similar projects in the pipeline, and China plans to build a NIF-scale laser by around 2018–2020.

The NIF will be able to recreate unprecedented scientific environments — those of extreme astrophysics, for example — in the lab. It will generate temperatures of more than 108 K and matter densities of more than 1,000 g cm−3, conditions that are extreme enough to enable new studies of high-energy-density science and insights into processes that occur, for example, in core-collapse supernovae.

An artist's impression of the laser beams targeted at the deuterium–tritium fuel inside a gold capsule. Image: NIF

Moses admits it's been tough getting this far. “There is a continuing need to prove yourself when you're spending as much money as we are.” At present the NIF has secured funding — at least on paper — until 2012 from the US Congress, and three US groups are already proposing basic science experiments for the years 2009–2011. The eventual goal is to open up the NIF to researchers all around the world so that they can borrow time on the facility much like researchers use telescopes and particle accelerators, although of course the funding and logistical details of this still need to be ironed out. As Moses explains, “We have no guarantees but if it all works out the way it's supposed to, the pay off could be huge: national security, energy security and basic science. I hope some bright young scientist finds a way to do Nobel-prize-winning science at the NIF in the years ahead. It would just be terrific in my old age to look back and think that I had something to do with that.”

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https://doi.org/10.1038/nphoton.2007.256

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