The hohlraum radiation cavity used to bath the fuel capsule in X-rays [Credit: Dr Eddie Dewald]

Nuclear fusion a step closer to ignition after breakthrough

Nuclear scientists are a step closer to the ‘holy grail’ of fusion energy after generating more energy from a fusion reaction than was deposited into the fuel.

The ultimate goal of inertial confinement fusion science is to reach ignition – the process of releasing fusion energy equal to or greater than the amount of energy used to confine the fuel – but a key step along the path is to have ‘fuel gains’ greater than unity, where the energy generated through the reaction exceeds the amount of energy deposited into the fusion fuel.

For the first time ever, a team at the Lawrence Livermore National Laboratory (LLNL) in the USA have reached the milestone of achieving fuel gains greater than one, in a series of experiments on the National Ignition Facility (NIF) that show an a factor of 10 improvement in yield performance over previous approaches.

The breakthrough was made by modifying the laser pulse used to compress the fuel to supress the instability that causes the break-up of the plastic shell that surrounds and confines the Deuterium-Tritium (DT) fuel as it is compressed, which was hypothesised as the source of degraded fusion yields observed in previous experiments.

By eliminating this instability a process known as ‘boot-strapping’ was allowed to occur, where alpha particles produced in the DT fusion process deposit their energy in the DT fuel rather than escaping, further heating the fuel and increasing the rate of fusion reactions, thus producing more alpha particles. This feedback process is the mechanism that leads to ignition.

“What's really exciting is that we are seeing a steadily increasing contribution to the yield coming from the boot-strapping process we call alpha-particle self-heating as we push the implosion a little harder each time,” said Dr Omar Hurricane, lead author of a paper on the research published in journal Nature.

But while the energy generated by the reaction is more than that deposited in the fuel, Hurricane was keen to point out that the laser energy delivered to the gold hohlraum radiation cavity used to bath the fuel capsule in X-Ray’s far outweighs the amount that was absorbed by the outer shell.

In turn, the fraction of this energy that is passed on to the DT fuel coated on the inside of the capsule is another order of magnitude lower.

“It’s important understand we have not achieved ignition,” said Hurricane. “Essentially we have two factors of 10 to make up before we get to the point of ignition.

“So it sounds very modest and it is, but this is closer than anyone has gotten before and it is very unique to finally get as much energy out of the fuel as was put into the fuel.”

The experimental results have matched computer simulations far better than previous experiments, providing an important benchmark for the models used to predict the behaviour of matter under conditions similar to those generated during a nuclear explosion, a primary goal for the NIF.

“This is a truly excellent paper that begins to get at the core problems that NIF has – instability of the capsule containing the fusion fuel as it is compressed by lasers,” said Professor Steve Cowley, Director of the Culham Centre for Fusion Energy, the site of the Joint European Torus (JET) Tokamak.

“The different measures of success make it hard to compare NIF’s results with those of ‘magnetic confinement’ fusion devices such as JET.  In 1997 JET made 16MW of power with 24MW into the device – approaching break-even. In principle better than NIF.

“But NIF is just beginning to understand what they need to do. We have waited 60 years to get close to controlled fusion, and we are now close in both magnetic and inertial confinement research. We must keep at it. The engineering milestone is when the whole plant produces more energy than it consumes – ITER, the successor to JET, will be the first experiment to do this. ITER is going slowly but progress is happening.”

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