Fuel gain over unity in an inertially confined fusion implosion

For the first time anywhere, we’ve gotten more energy out of the fuel than what was put into the fuel” for a nuclear fusion experiment. This is reported by the Wall Street Journal and other sources from a paper published in the Journal Nature by researchers at Lawrence Livermore National Lab

“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 lead author Omar Hurricane.

Boot-strapping results when alpha particles, helium nuclei produced in the deuterium-tritium (DT) fusion process, deposit their energy in the DT fuel, rather than escaping. The alpha particles further heat the fuel, increasing the rate of fusion reactions, thus producing more alpha particles. This feedback process is the mechanism that leads to ignition. As reported in Nature, the boot-strapping process has been demonstrated in a series of experiments in which the fusion yield has been systematically increased by more than a factor of 10 over previous approaches

Nature – Fuel gain exceeding unity in an inertially confined fusion implosion

Ignition is needed to make fusion energy a viable alternative energy source, but has yet to be achieved. A key step on the way to ignition is to have the energy generated through fusion reactions in an inertially confined fusion plasma exceed the amount of energy deposited into the deuterium–tritium fusion fuel and hotspot during the implosion process, resulting in a fuel gain greater than unity. Here we report the achievement of fusion fuel gains exceeding unity on the US National Ignition Facility using a ‘high-foot’ implosion method which is a manipulation of the laser pulse shape in a way that reduces instability in the implosion. These experiments show an order-of-magnitude improvement in yield performance over past deuterium–tritium implosion experiments. We also see a significant contribution to the yield from α-particle self-heating and evidence for the ‘bootstrapping’ required to accelerate the deuterium–tritium fusion burn to eventually ‘run away’ and ignite.

The experimental series was carefully designed to avoid breakup of the plastic shell that surrounds and confines the DT fuel as it is compressed. It was hypothesized that the breakup was the source of degraded fusion yields observed in previous experiments. By modifying the laser pulse used to compress the fuel, the instability that causes break-up was suppressed. The higher yields that were obtained affirmed the hypothesis, and demonstrated the onset of boot-strapping.

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

A metallic case called a hohlraum holds the fuel capsule for NIF experiments. Target handling systems precisely position the target and freeze it to cryogenic temperatures (18 kelvins, or -427 degrees Fahrenheit) so that a fusion reaction is more easily achieved. Photo by Eduard Dewald/LLNL

Scientific American also has some information

The fuel pellet itself is a perfectly spherical capsule of plastic, roughly two millimeters in diameter and precisely shaped (at a cost of roughly $1 million per pellet) to ensure the best performance.

Roughly 500 megajoules of electricity feed lasers that then pump out 1.9 megajoules worth of energy. Those lasers take a long, power-boosting trip through amplifying optics and shoot into the hohlraum, which is made of gold and measures 5.75 millimeters in diameter and 9.425 millimeters long.

Employing 1.9 megajoules in slightly more than a nanosecond, the lasers deliver 500 terawatts of power inside the hohlraum (a terawatt is a trillion watts). A cloud of helium gas holds back the gold plasma that would otherwise intrude as the laser power is translated into x-rays by the hohlraum. These x-rays hit the plastic shell of the capsule, which absorbs roughly one tenth of the energy put into the lasers to begin with.

Only about 1 percent of the energy from the lasers ends up in the fuel, which then pumps out 17,000 joules’ worth of energy, or less than the energy needed to make the DT fuel in the first place. All of it lasts for 150 picoseconds, or 150 trillionths of a second, before the fusion zone blows itself apart.

Nature – Plasma physics: A promising advance in nuclear fusion

SOURCES – Nature, Lawrence Livermore National Labs, Wall Street Journal, Scientific American

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