Latest from the National Ignition Facility: More Neutrons, Less Hope

A paper with some recent ignition experiment results from the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) in California just turned up at Physical Review Letters.  The good news is that they’re seeing more of the neutrons that are released in fusion reactions than ever before, and the yield is in good agreement with computer code predictions.  The bad news is that they improved things by doing something that’s supposed to make things worse.  Specifically, they increasing the energy in the laser “foot” pulse that’s supposed to get the target implosion started.

The NIF was designed to achieve ignition via inertial confinement fusion (ICF), a process in which the fuel material is compressed and heated to fusion conditions so quickly that its own inertia holds it in place long enough for significant fusion to occur.  Scientists at LLNL  are currently using “indirect drive” in their experiments.  In other words, instead of hitting the BB-sized target directly, they mount it inside of a tiny, cylindrical can, or “hohlraum,” with holes at each end for the laser beams to pass through.  When the beams hit the inside of the hohlraum, they produce a powerful pulse of x-rays, which then hit the target, imploding it to extremely high density.  It’s harder to squeeze and implode hot objects than cold ones, so the laser beams are tailored to keep the target as “cold” as possible during the implosion process.  However, the fuel material must be very hot for fusion to occur.  According to theory, this can be achieved by launching a series of shocks into the imploding target, which must converge in the center at the moment of greatest compression, creating a central “hot spot.”  When fusion reactions start in the hot spot, they produce highly energetic helium atom nuclei (alpha particles), which then slam into the surrounding, still cold, fuel material, heating it to fusion conditions, producing more alpha particles, resulting in an alpha-driven “burn wave,” which moves out through the target, consuming the fuel.

So far, it hasn’t worked.  Apparently, hydrodynamic instabilities, such as the Rayleigh-Taylor and Richtmyer-Meshkov instabilities, are a big part of the problem.  They amplify tiny target surface imperfections during the implosion process, destroying the symmetry of the implosion, and quenching the fusion process.  There are some interesting simulations of the Rayleigh-Taylor instability on Youtube.  In the latest experiments, the LLNL team managed to control the growth of instabilities by using a bigger target “aspect ratio,” that is, increasing the thickness of the outer shell compared to the target radius, and driving it by dumping more energy into the “foot” pulse.  As a result, they drove the implosion process along a higher “adiabat,” which basically means that the fuel was hotter during the implosion.  Of course, absent instabilities, making the fuel hotter during the implosion is exactly what you don’t want to do.  In spite of that, LLNL is seeing more neutrons.

What this all boils down to is that LLNL has confirmed that the NIF has a big, potentially fatal problem with hydrodynamic instabilities using the current indirect drive approach to fusion ignition.  That doesn’t mean the situation is hopeless.  There are other approaches.  Examples include direct drive, in which the laser beams are aimed directly at the target, and fast ignitor, in which the cold, compressed fuel material is ignited on the outside, by another laser beam designed specifically for that purpose, rather than in a central “hot spot.”  In fact, the biggest potential problem here is probably more political than scientific.  You certainly have to get ignition if you plan to use inertial fusion as a source of energy, but, in spite of occasional hype to the contrary, the NIF was never intended as an energy project.  It was funded to support the weapons program in general, and to insure the continuing safety and reliability of the weapons in our arsenal in the absence of nuclear testing in particular.  It can do that extremely well, whether we get ignition or not.  The politicians whose support is needed to fund continued operation of the project need to realize that.

Regardless of whether it achieves ignition or not, the NIF is performing as well as or better than its design specs called for.  The symmetry and synchronization of its 192 laser beams is outstanding, and it has a remarkable and highly capable suite of diagnostics for recording what happens during the experiments.  The NIF can dump so much energy in a small space in a short time that it can generate physical conditions that can be reproduced in the laboratory no where else on earth.  Those conditions approach those that occur inside of an exploding nuclear device.  As a result, such experimental facilities give us a major leg up on the competition as long as there is no resumption of nuclear testing.  In other words, with the NIF and facilities like it we have a strong, positive incentive not to resume testing, potentially losing our advantage.  Without such facilities, the pressure to resume testing may become irresistible.  It’s really an easy choice.

The simulation of the Rayleigh-Taylor instability below was done by Frederik Brasz.

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