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  • The NIF: No News is Bad News

    Posted on January 19th, 2011 Helian No comments

    For those who don’t follow fusion technology, the National Ignition Facility, or NIF, is a giant, 192 beam laser facility located at Lawrence Livermore National Laboratory.  As its name would imply, it is designed to achieve fusion ignition, which has been variously defined, but basically means that you get more energy out from the fusion process than it was necessary to pump into the system to set off the fusion reactions.  There are two “classic” approaches to achieving controlled fusion in the laboratory.  One is magnetic fusion, in which light atoms stripped of their electrons, or ions, typically heavy isotopes of hydrogen, are confined in powerful magnetic fields as they are heated to the temperatures necessary for fusion to occur.  The other is inertial confinement fusion, or ICF, in which massive amounts of energy are dumped into a small target, causing it to reach fusion conditions so rapidly that significant fusion can occur in the very short time that the target material is held in place by its own inertia.  The NIF is a facility of the latter type.

    There are, in turn, two basic approaches to ICF.  In one, referred to as direct drive, the target material is directly illuminated by the laser beams.  In the other, indirect drive, the target is placed inside a small container, or “hohlraum,” with entrance holes for the laser beams.  These are aimed at the inside walls of the hohlraum, where they are absorbed, producing x-rays which then compress and ignite the target.  The NIF currently uses the latter approach.

    The NIF was completed and became operational in 2009.  Since that time, the amount of news coming out of the facility about the progress of experiments has been disturbingly slight.  That is not a good thing.  If everything were working as planned, a full schedule of ignition experiments would be underway as I write this.  Instead, the facility is idle.  The results of the first experimental campaign, announced in January, sounded positive.  The NIF had operated at a large fraction of its design energy output of 1.8 Megajoules.  Surrogate targets had been successfully compressed to very high densities in symmetric implosions, as required for fusion.  However, on reading the tea leaves, things did not seem quite so rosy.  Very high levels of laser plasma interaction (LPI) had been observed.  In such complex scattering interactions, laser light can be scattered out of the hohlraum, or in other undesired directions, and hot electrons can be generated, wreaking havoc with the implosion process by preheating the target.  We were assured that ways had been found to control the excess LPI, and even turn it to advantage in controlling the symmetry of the implosion.  However, such “tuning” with LPI had not been foreseen at the time the facility was designed, and little detail was provided on how the necessary delicate, time-dependent shaping of the laser pulses would be achieved under such conditions.

    After a long pause, another series of “integrated” experiments was announced in October.  Even less information was released on this occasion.  We were informed that symmetric implosions had been achieved, and that, “From both a system integration and from a physics point of view, this experiment was outstanding,”  Since then, nothing.  

    It’s hard to imagine that the outlook is really as rosy as the above statement would imply.  The NIF was designed for a much higher shot rate.  If it sat idle through much of 2010, there must be a reason.  It could be that damage to the laser optics has been unexpectedly high.  This would not be surprising.  Delicate crystals are used at the end of the chain of laser optics to triple the frequency of the laser light, and, given that the output energy of the facility is more than an order of magnitude larger than that of its next largest competitor, damage may have occurred in unexpected ways, as it did on Nova, the NIF’s predecessor at Livermore.  LPI may, in fact, be more serious, more difficult to control, and more damaging than the optimistic accounts in January implied.  Unexpected physics may be occurring in the absorption of laser light at the hohlraum walls.  Whatever the problem, Livermore would be well advised to be forthcoming about it in its press releases.  After all, the NIF will achieve ignition or not, regardless of how well the PR is managed.

    All this seems very discouraging for the scientists who have devoted their careers to the quest for fusion energy, not to mention the stewards of the nation’s nuclear weapons stockpile, whose needs the NIF was actually built to address.  In the end, these apparent startup problems may be overcome, and ignition achieved after all.  However, I rather doubt it, unless perhaps Livermore comes up with an alternative to its indirect drive approach.

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