The significance of the latest NIF results
Points of View:
If you were hoping for the thermonuclear fusion breakthrough that would provide cheap electricity to every home this is not the one. However, the recent results from the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory are a very important and notable step forward in fusion research.
With regard to the scope of the NIF experiments, it is important to understand that NIF's mission is not to perfect fusion energy, but rather to establish the science related to the safety and reliability of the US stockpile of nuclear weapons. Fusion research is one element of that program. Since igniting thermonuclear fuel in the laboratory has been an elusive goal of the worldwide quest for fusion energy, the NIF ignition experiments have aroused interest within the fusion-energy community.
On 28 September 2013, a millimeter-sized plastic shell containing 0.17 mg of fusion fuel (deuterium and tritium, DT) was imploded inside a small gold can called the hohlraum by 1.8 megajoules of laser light incident on the hohlraum's inner walls. The laser energy is converted into x rays by the hohlraum plasma that then irradiates and implodes the capsule. At the start of the experiment, the fusion fuel was in the form of a solid DT ice layer frozen onto the inner shell wall. The fuel layer was imploded at a velocity of around 370 km/s and its total (kinetic) energy was around 10 kilojoules.
Note that less than 1% of the laser energy found its way to the fusion fuel. When the fuel was squashed into a tiny plasma ball 100 μm in size with 100 billion atmospheres of pressure, about 5000 trillion fusion reactions occurred. The total energy produced by those fusion reactions was about 14 kJ.
For the first time in the history of controlled fusion research, a DT plasma has produced more energy (14 kJ) than was supplied to it (10 kJ). That achievement has led to the first-ever positive fuel-energy balance achieved in a laboratory (defined as fusion energy exceeding input energy to the fuel).
Although the result is interesting physics, the fusion energy output is less than 1% of the laser energy delivered to the target. However, this is not the whole story.
What got the attention of laser fusion scientists was not simply the first-ever positive fuel-energy balance. To understand the importance of these results, one needs to understand that laser fusion works like a gasoline engine. The fuel is first compressed and a tiny spark is used to ignite the fuel. The spark is a product of the compression itself because an imploding shell causes the central plasma to become very hot (it reached around 50 million °C in the NIF shot).
To produce a large amount of fusion energy (many millions of joules), the spark must be strong enough to ignite the surrounding fuel. In the 28 September NIF shot, the spark was not strong enough and died without igniting the rest of the fuel. For the first time, however, that tiny spark increased significantly in strength on its own before dying (a process called "self-heating" or "alpha heating"). The spark received only about 5 kilojoules of energy; the remaining 5 kJ was stored in the surrounding compressed fuel that was not part of the "spark" plasma.
The spark alone produced 14 kJ of fusion energy and approximately 15% of that energy was retained by the spark, which increased in strength and produced more fusion reactions. The self-heating process of the spark is likely responsible for about half of the total number of fusion reactions.
Ignition and burn
The goal now is to improve the implosion to ensure that the spark keeps gaining more and more strength—enough to ignite the surrounding fuel. That milestone is called ignition and burn, very much like in a gasoline engine. When that happens, the tiny mass of DT fuel will produce more energy than the input laser energy.
Laser fusion is a rather inefficient process to implode fusion fuel. Most of the laser energy is used to heat up the gold can and the plastic shell. However, NIF scientists still have several options to improve the implosion performance, such as using more-efficient shell materials (pure carbon or beryllium instead of plastic), and improving both laser pulses and hohlraum shapes and fills (if viable, empty hohlraums without gas fill are very attractive).
The experimental campaign to achieve ignition on the NIF is a collaborative effort between the institutional partners Lawrence Livermore National Laboratory, the University of Rochester, Los Alamos National Laboratory, and Sandia National Laboratories, as well as other collaborators such as General Atomics, which makes the fusion targets. Ignition experiments are extremely complex and the NIF scientists have been very successful in identifying and correcting some of the causes that degrade implosion performance. The path to thermonuclear ignition and burn is not easy but surely the latest NIF results are encouraging and exciting for the scientists working on it.
Robert L. McCrory is a professor of physics and of mechanical engineering at the University of Rochester in New York. He also directs the university's Laboratory for Laser Energetics. Riccardo Betti is the Gowen Professor of Mechanical Engineering and a professor of physics at the University of Rochester.