A colorized image of a NIF “Big Foot” deuterium-tritium (DT) implosion, taken on Feb. 7, 2016. Image: Don Jedlovec
Scientists have brought the dream of nuclear fusion one step closer to reality with the first-ever demonstration of a “burning plasma” in the laboratory, a milestone that marks “a critical step towards self-sustaining fusion energy,” reports a new study.
Nuclear fusion occurs when atomic nuclei merge, a reaction that generates so much energy that it fuels the Sun and other stars. Harnessing fusion on Earth would unleash an abundant source of clean and renewable energy, which is why so many researchers have devoted their careers to pioneering this form of power.
However, the creation of human-made fusion is an enormous technical undertaking that will require overcoming many challenges. Now, at least one of those hurdles has been cleared by a team at the National Ignition Facility (NIF), a fusion device located at the Lawrence Livermore National Laboratory (LLNL) in California. After years of attempts, NIF scientists generated a burning plasma that was primarily heated by fusion reactions between two hydrogen isotopes, deuterium and tritium, that provided thermonuclear fuel for the experiment, according to a study published on Wednesday in Nature.
“To get fusion we have to get the fuel very hot—something like 100 million degrees—and historically people have done that by heating the fuel,” said Alex Zylstra, an experimental physicist at NIF who co-led the study, in a call.
“The significance of a burning plasma is that now the fusion itself is providing, actually, the majority of the heating of the fuel,” he continued. “Fusion is doing more heating than what we're doing, and that's a key scientific step on the way to getting the fuel to be able to become more self-sustaining.”
In other words, Zylstra and his colleagues demonstrated a process that will be essential to hypothetical future fusion reactors: a sustainable way to keep fuel in a hot plasma state so that it can, eventually, provide net positive yields of energy.
The team was able to achieve this breakthrough by meticulously fine-tuning an experiment that involves shooting 192 lasers at a two-millimeter capsule containing the thermonuclear fuel. This coordinated laser blast causes the fuel to heat up and implode; this sudden compression forces the deuterium and tritium to fuse into a helium atom, a reaction that releases the energy needed to kickstart a self-heating plasma.
A color-enhanced image of the inside of a NIF preamplifier support structure. Image: Damien Jemison
Scientists at NIF have been trying to achieve this milestone for years, but it wasn’t until they figured out how to enlarge the capsule so that it could hold more fuel, while also preventing asymmetrical fuel implosions that reduce the efficiency of heat production, that the experiment met the mark. An explanation of this key innovation is described in a separate paper published on Wednesday in Nature Physics.
Though this experiment was years in the making and took months to prepare, the implosion was over in a fraction of a second. Zylstra and his colleagues then rushed to look over the data and realized that they had generated a burning plasma for the first time.
“It was clear pretty quickly that it was a big success,” Zylstra said. “We start to see things like the amount of fusion production pretty quickly. The experiments that were reported in these papers are about a factor of two to three higher than the previous record for the amount of fusion production so we're pretty excited.”
The team was able to extract a maximum yield of 170 kilojoules from the fusion reaction, which is equivalent to about nine 9-volt batteries. While that is not a lot of energy in practical terms, it is a huge amount to extract from such a tiny amount of fuel in such a short amount of time. What’s more, the NIF breakthrough will enable scientists to produce bigger yields, which will move them closer to the goal of one day engineering a fusion reactor with a higher energy output than input.
“We've never been able to do experiments in this regime before,” Zylstra concluded. “There's a lot that we don't understand about how plasmas behave, and we’re continuing to make improvements. There's a number of ideas for continuing to get higher and higher performance and we're looking forward to, hopefully, those being successful.”