The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL) may have reached releasing fusion energy equal to or greater than the amount of energy used to confine the fuel.  The process has long been considered the “holy grail” of inertial confinement fusion science.

There is reason to believe a key step along the path to ignition where “fuel gains” are greater than unity and the energy generated through fusion reactions exceeds the amount of energy deposited into the fusion fuel.

NIF Hohlraum Fuel Target. Click image for more info.

NIF Hohlraum Fuel Target. Click image for more info.

The LLNL team reports the achievement of fusion fuel gains exceeding unity at the NIF using a ‘high-foot’ implosion method.  The method is a manipulation of the laser pulse shape in a way that reduces instability in the implosion.

The NIF has had some disappointing result over the past few years.  But these experiments show an order-of-magnitude improvement in yield performance over past deuterium–tritium implosion experiments.  The LLNL team also sees a significant contribution to the yield from α-particle (alpha particle) self-heating and evidence for the ‘bootstrapping’ required to accelerate the deuterium–tritium fusion burn to eventually ‘run away’ and ignite.

The LLNL press release notes the milestone of achieving fuel gains greater than 1 has been reached for the first time ever on any facility.  The team’s paper was published yesterday in the online issue of the journal Nature.

Lead author Omar Hurricane said, “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.”

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.

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.

Hurricane said, “There is more work to do and physics problems that need to be addressed before we get to the end, but our team is working to address all the challenges, and that’s what a scientific team thrives on.”

There’s quite a list of folks for co-authoring the paper.  Hurricane is joined by co-authors Debbie Callahan, Daniel Casey, Peter Celliers, Charlie Cerjan, Eduard Dewald, Thomas Dittrich, Tilo Doeppner, Denise Hinkel, Laura Berzak Hopkins, Sebastien Le Pape, Tammy Ma, Andrew MacPhee, Jose Milovich, Arthur Pak, Hye-Sook Park, Prav Patel, Bruce Remington, Jay Salmonson, Paul Springer and Riccardo Tommasini of LLNL, and John Kline of Los Alamos National Laboratory.

The worrisome research drought at the NIF has been ended.  For some time experiments didn’t match up with the predictions of simulations and it has been difficult to figure out what to change.  Now the laser ignition experiments appear to be behaving according to the predictions of current models.  That has to come as a great relief.

NIF Fusion Firing Graphic.  Click image for the largest view.

NIF Fusion Firing Graphic. Click image for the largest view.

For background the NIF experiment consists of a giant set of lasers that deliver a rapid (a few nanoseconds) pulse of about 2 megajoules (MJ) of energy to a spherical target of nuclear fuel (typically deuterium and tritium) about the size of a pea.  The idea is to use the laser to rapidly heat the pea sized spherical target. As the outside of the target expands, the fuel is compressed and heated, which drives a fusion reaction generating fast alpha particles and neutrons.

The concept for the process seems simple, but in practice it is surely not.  Time on target, the energy delivered and the energy intensity over the pulse are all being found to have considerable impact.  Then the fuel itself has questions, the fuel type, shape, the containment and other matters are being worked out.

To the surprise of many, the NIF team has worked through quite a set of issues that gotten the research back on track with the theory and simulations.  As each matter is improved the simulations can be upgraded.  Perhaps now the team will progress more quickly.

Keep in mind the research experiment happens in nanoseconds.  Much of the improvements so far have come from intuition and experienced supposition.  There is a fair bit of luck accompanying the intellect and skill.

The story is looking better.  Its taken a fair bit of courage to stay with the work, keep the funding coming and try over and over and over again much like the Edison method of work on every idea until you find the one that works.

Congratulations are in order for a team that’s well earned its pride.


2 Comments so far

  1. Matt Musson on February 13, 2014 9:36 AM

    Breakeven is a slippery concept. The press releases say that the fusion gives off as much energy ‘as it absorbs.’

    But, the fusion reaction only absorbs a small fraction of the total energy expended on it. The difference is around 3 orders of magnitude.

    So, some can say it is breakeven. Some will say it produced less than 1/1000th of the energy input into the experiment.

  2. jpstraley on February 13, 2014 2:43 PM

    Breakeven. But it makes — in the end — heat with which we boil water & etc. It is a Carnot machine. That’s a problem.

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