Question: What does it take to prepare a giant laser to routinely perform fusion experiments at record-breaking levels of energy and power?
Answer: Patience and commitment, coupled with innovative and disciplined hard work by a highly talented multidisciplinary team.
On July 5 of this year, the Department of Energy National Ignition Facility (NIF), the world’s largest and highest-energy laser system, fired its first two-megajoule, 500-trillion-watt-plus shot of ultraviolet laser energy. Two megajoules is 100 times more energy than the next highest-energy laser, the Omega Laser at the University of Rochester, and 500 terawatts is 1,000 times more power than the electrical power produced in the United States at any instant in time. The shot met the performance goals set for NIF in the late 1990s and marked a major step forward in the decades-long quest for fusion energy.
NIF, a project of DOE’s National Nuclear Security Administration, is located at Lawrence Livermore National Laboratory. Its missions are to support NNSA’s Stockpile Stewardship Program and to further fundamental science and fusion energy by creating in the laboratory conditions never before accessible on Earth. NIF’s biggest challenge is to use that laser energy to compress and heat a tiny target capsule filled with the hydrogen isotopes deuterium and tritium until the hydrogen nuclei fuse in a self-sustaining, but very short-lived, thermonuclear reaction—producing the same kind of energy that powers the sun and the stars. “Ignition” is defined as the capsule releasing as much (or more) energy than the energy delivered to the target by the lasers. Over time, NIF’s goal is to multiply the input energy by a factor of ten.
By conducting experiments on NIF, scientists will be able to mine the resulting data for valuable information about the reliability and safety of the nation’s nuclear stockpile. The unprecedented heat and compression in the NIF target—more than 100 million degrees centigrade and billions of times Earth’s atmospheric pressure—also will enable researchers to gain new insights into the evolution and makeup of the stars and planets; ignition will significantly advance the pursuit of safe, limitless fusion energy.
“NIF has demonstrated steady progress and is becoming everything scientists planned when it was conceived more than two decades ago,” said NIF Director Ed Moses. “Thanks to the hard work and innovation of the NIF team and our partners in the National Ignition Campaign, NIF is now fully operational. Scientists are taking important steps toward achieving ignition and providing experimental access to user communities for national security, basic science and the quest for clean fusion energy.”
The science and engineering that went into NIF’s record-breaking shot on July 5 wasn’t just a matter of turning up the energy of NIF’s 192 giant lasers. For NIF to reach the performance goals set for it in the late 1990s, every aspect of the complex laser system had to be analyzed, evaluated, upgraded, realigned and fine-tuned as never before.
Power is energy divided by time; two megajoules is the amount of energy consumed by 20,000 100-watt light bulbs in one second. When that much energy is focused and delivered in a four-nanosecond laser burst, the result is 500 trillion watts of peak power. The July 5 shot was actually the third NIF experiment in which total energy exceeded NIF’s design specification of 1.8 megajoules on the target.
Original concerns about achieving these levels of extreme laser performance on NIF centered in part on the ability of optics existing in the late 1990s to withstand such intense laser light. NIF Optics Program Manager Tayyab Suratwala, Optics Science and Technology Group Leader Jeff Bude and other researchers pursued a ten-year research and development program to understand the fundamentals of optics laser damage. They worked closely with their industrial partners to improve manufacturing methods and drastically reduce the number of defects. Under the direction of now-retired Chief Technology Officer Mary Spaeth, Livermore scientists also developed a new strategy for handling laser damage called the “NIF Loop Strategy.” Instead of discarding a damaged optic, it is removed and recycled on the LLNL site. The loop strategy is designed to assure that NIF economically operates at maximum energy by limiting the likelihood of damage initiation and acting quickly to mitigate further damage when it does occur.
Also contributing to optics damage prevention was the development of innovative, R&D 100 award-winning “programmable filters” that can block pinpoints of laser light from reaching and enlarging potential damage sites downstream.
But that’s only part of the story. At lower power levels, the NIF laser beams were small enough to comfortably fit inside the meter-sized optical amplifiers that boost their energy from a few billionths of a joule to several megajoules. Increasing the dimensions of the beams, however, meant they had to be precisely rotated and positioned so they wouldn’t touch the edges of the optics and lose energy, and possibly damage the optics.
“Achieving full energy and power meant making maximum use of the (optics) apertures,” Burkhart said. Early this year the NIF team, led by Optics and Targets Director Paul Wegner, began an experimental program to determine how much larger the beams could be made while still ensuring safe propagation through the apertures. Different masks (filters in the preamplifier modules, or PAMs, that tailor the beams to the desired size and shape) were designed and tested, and the beam size was increased in small increments until the optimum size was reached.
Additional beam area was gained by optimizing the number and position of the “corner blockers” installed in one or more corners of each beam aperture to block “counter-propagating” light returning through the beamlines from the NIF target chamber. Beam “clipping” at the beam edge, even at very low intensity levels, causes light to radiate back into the main beam. This causes interference fringes or intensity ripples. Above a certain point the result could be damaging to the optics, and even to the metal brackets holding the optics in place. NIF engineering and technician teams fine-tuned the beam size for each of the 192 beams, optimizing beam size to the available aperture.
Finally, the team used NIF’s powerful final optics damage inspection system, or FODI, to image the beamlines from the center of the target chamber and verify that the apertures were the correct size and the beams were properly centered in the final optics. The FODI produced huge data files that had to be painstakingly analyzed.
The result of all these adjustments, Burkhart said, was that the NIF beams are now not only bigger and able to deliver more energy, they are more spatially and temporally uniform, enhancing the laser system’s precision and reproducibility. “By doing this,” he said, “we were able to bring the laser intensity down so that the intensity doesn’t exceed the damage threshold of the optics.”
Charles Osolin is a writer at Lawrence Livermore National Laboratory.
The Boy Who Played With Fusion
Taylor Wilson stood at the front of the auditorium, shifting from foot to foot and adjusting his suit sleeves like any other 18-year-old boy. His constant physical energy mirrors his ceaseless pursuit of net-energy fusion, a topic not many other teenage boys could discuss with any level of prowess. At age 14, Wilson built