Have you ever had that goosebump moment when you performed an experiment, and suddenly you got a result that exceeded your wildest expectations? It happened on August 8th at the Lawrence Livermore National Laboratory. Their National Ignition Facility NIF was built to find out under which conditions a super-strong laser system could ignite a nuclear fusion process, similar to the one our sun performs to turn hydrogen into helium. On August 8th NIF researchers conducted a laser fusion experiment which caught immediate attention of the science magazine and the New York Times.
Conditions for nuclear fusion are astronomical, as temperatures of several million degrees and high pressure are required to break through the barriers of nuclear repulsion. After making nuclear bombs, mankind succeeded quite quickly with the H-bomb, in which a conventional fission bomb compressed a core of the heavy hydrogen isotopes deuterium and tritium to ignite their fusion. That idea was tested successfully in 1952. Ever since, scientists were searching for a way to use nuclear fusion for power generation. In the age of decarbonization, this quest has even accelerated.
Getting two atoms into a fusion process is actually not that difficult. The challenge is the equivalent to a nuclear chain reaction, a self-sustaining fusion process, a nuclear burn. The start of this process is called ignition and refers to the point where the fusion reactions produce enough power to maintain the fusion going on without external heating. In a simplified picture this is achieved when the fusion produces more energy than it consumes.
Which may explain why the NIF scientists and their fellows around the globe are so excited about the result from the recent NIF shot: The experiment produced an unexpected high energy of 1.35 Megajoules. That is 70% of the 1.9 MJ laser energy that had been pumped in. A result that was eight times better than what they achieved before and four times of what calculations had predicted.
How did they get there?
Although NIF was built to perform ignition, the achievement was quite unexpected at this time. When NIF was completed in 2009 after delays and budget cuts and extensions it was by far the biggest and strongest laser in the world. In 2012 it reached its design specs and produced 1.9 MJ pulses at a wavelength of 351 nm. The actual laser pulse is combined from 192 separate beams. Using a pulse length of several nanoseconds, the laser reached a peak power of about 500 TW.
From 2010 to 2012 they conducted a “National Ignition Campaign”. The experts were very optimistic as ignition was expected at a level of 1.4 to 1.5 MJ. The NIC ended officially in September 2012 without reaching ignition. A final report reviewed the situation and came to an embarrassing conclusion” Barring an unforeseen technical breakthrough and given today’s configuration of the NIF laser, achieving ignition on the NIF in the near term (one to two years) is unlikely and is uncertain over the next five years. Although performance of NIF ignition targets continues to improve and simultaneously making contributions to the SSP, currently there is no known configuration, specific target design, or approach that will guarantee ignition on the NIF.”
The facility turned to tests for its main sponsor, the Department of Energy and their stockpile stewardship program. That is weapons and materials research.
Every PhD-student in experimental physics knows the moment, when an experiment doesn’t work. You have tried everything, and it simply does not work. You have to go on and try harder. It is this kind of persistence that makes a good physicist. And so did the researchers at NIF. Fusion research continued, although with less resources and less expectations. Targets were improved, the laser became ever stronger (2.15 MJ in 2018), and in 2018 a shot resulted in 54 kJ neutron energy from a 1.5 MJ laser pulse.
In 2020 the budget for inertial confinement fusion was increased, and in February 2021 a new record of 170 kJ energy was reached. Now imagine how the people felt when they saw 1.35 MJ output in August. Goosebumps. Tears. Champaign.
Due to several loss mechanisms, the actual energy delivered to the fusion target was much smaller than 1.9 MJ pulse energy from the laser. That shines a different light on the conversion: “Preliminary analysis estimates an energy gain of more than 5 times the energy delivered to the capsule from the laser-produced radiation drive,” says LLNL physicist Dr. Annie Kritcher. While the laser pulse was 9 ns long, the neutron emission took 90 ps. A scientific publication is still under review, but the numbers mentioned give strong arguments that NIF finally ignited a nuclear fusion fire. A little star was born.
The future is bright for star builders
Now the researchers must determine which of their recent tweaks was responsible for the unexpected breakthrough. These laser fusion experiments are conducted in a highly nonlinear regime, where the tiniest changes can lead to large results. An effect that is quite characteristic for plasma physics. Even after decades of development and ever larger computers it looks as if the simulations are still not good enough to predict the results reliably.
But the future looks bright for fusion research. This is the message of a book that has been published just in time: Plasma physicist Arthur Turrell wrote “The Star Builders” about the history and the current state of fusion research. According to this book (and its review in nature) there are some 25 startup companies pursuing fusion experiments. Bill Gates, Jeff Bezos or even Brad Pitt have backed startups, while governments pour billions into research facilities such as the European ITER in France. It seems as if ignition comes closer after decades of research.
Admittedly, most of research facilities and the startups focus on magnetically confined plasma for fusion, just a few startups embark on inertial-confinement-based laser fusion. Among them, Marvel Fusion bets on a hydrogen-boron fusion, a cleaner method which has yet to be proven feasible. The latest startup, Focused Energy, from Darmstadt, Germany, promises to be “Developing a Fusion Power Plant”. Their scientists have a record in fusion research at LLNL.
Now what can we expect? Certainly, progress in plasma physics, advanced targets, and better lasers. A laser fusion power plant? Although on the schedule of the startups, it is in the far future for scientists. It would require a quasi-continuous flow of power. NIF needs hours to prepare for the next shot. But laser physicists started already a relevant development.
In 2017 the European Extreme Light Infrastructure ELI received a laser from LLNL, the High-repetition-rate Advanced Petawatt Laser System HAPLS. With two Ti:Sa-amplifiers and a 200 J-level pump laser boosted by the world’s highest-peak-power pulsed laser diodes, the ultracompact system was installed at ELI-Beamlines in Prague, Czech Republic. That was a major step towards a highly repeating laser for fusion research.
Professor Constantin Häfner, director of the Fraunhofer Institute for Laser Technology ILT in Aachen, Germany, who himself served as director of the Advanced Photon Technologies Program at LLNL until 2019 commented on this system: “Even though HAPLS is a high-power laser built for science, it built on laser concepts LLNL developed for Inertial Fusion Energy laser drivers. We need to further mature and enhance laser technology, reduce cost, and equip experimental facilities with these rep rated high energy lasers to progress, simplify and test target and many other critical IFE [inertial fusion energy] reactor technologies.”
After decades of painstaking research, the experiment on August 8th has ignited a new round of laser fusion excitement. Will it lead to clean energy? It will surely lead to more sensational physics.
