Nuclear Fusion Update

As I mentioned in a previous post about fusion progress, signs of life have finally been appearing in scientific journals from the team working to achieve fusion ignition at the National Ignition Facility, or NIF, located at Lawrence Livermore National Laboratory (LLNL) in California.  At the moment they are “under the gun,” because the National Ignition Campaign (NIC) is scheduled to end with the end of the current fiscal year on September 30.  At that point, presumably, work at the facility will be devoted mainly to investigations of nuclear weapon effects and physics, which do not necessarily require fusion ignition.  Based on a paper that recently appeared in Physical Review Letters, chances of reaching the ignition goal before that happens are growing dimmer.

The problem has to do with a seeming contradiction in the physical requirements for fusion to occur in the inertial confinement approach pursued at LLNL.  In the first place, it is necessary for the NIF’s 192 powerful laser beams to compress, or implode, a target containing fusion fuel in the form of two heavy isotopes of hydrogen to extremely high densities.  It is much easier to compress materials that are cold than those that are hot.  Therefore, it is essential to keep the fuel material as cold as possible during the implosion process.  In the business, this is referred to as keeping the implosion on a “low adiabat.”  However, for fusion ignition to occur, the nuclei of the fuel atoms must come extremely close to each other.  Unfortunately, they’re not inclined to do that, because they’re all positively charged, and like charges repel.  How to overcome the repulsion?  By making the fuel material extremely hot, causing the nuclei to bang into each other at high speed.  The whole trick of inertial confinement fusion, then, is to keep the fuel material very cold, and then, in a tiny fraction of a second, while its inertia holds it in place (hence the name, “inertial” confinement fusion), raise it, or at least a small bit of it, to the extreme temperatures necessary for the fusion process to begin.

The proposed technique for creating the necessary hot spot was always somewhat speculative, and more than one fusion expert at the national laboratories were dubious that it would succeed.  It consisted of creating a train of four shocks during the implosion process, which were to overtake one another all at the same time precisely at the moment of maximum compression, thereby creating the necessary hot spot.  Four shocks are needed because of well-known theoretical limits on the increase in temperature that can be achieved with a single shock.   Which brings us back to the paper in Physical Review Letters.

The paper, entitled Precision Shock Tuning on the National Ignition Facility, describes the status of efforts to get the four shocks to jump through the hoops described above.  One cannot help but be impressed by the elegant diagnostic tools used to observe and measure the shocks.  They are capable of peering through materials under the extreme conditions in the NIF target chamber, focusing on the tiny, imploded target core, and measuring the progress of a train of shocks over a period that only lasts for a few billionths of a second!  These diagnostics, developed with the help of another team of brilliant scientists at the OMEGA laser facility at the University of Rochester’s Laboratory for Laser Energetics, are a triumph of human ingenuity.  They reveal that the NIF is close to achieving the ignition goal, but not quite close enough.  As noted in the paper, “The experiments also clearly reveal an issue with the 4th shock velocity, which is observed to be 20% slower than predictions from numerical simulation.”

It will be a neat trick indeed if the NIF team can overcome this problem before the end of the National Ignition Campaign.  In the event that they don’t, one must hope that the current administration is not so short-sighted as to conclude that the facility is a failure, and severely reduce its funding.  There is too much at stake.  I have always been dubious about the possibility that either the inertial or magnetic approach to fusion will become a viable source of energy any time in the foreseeable future.  However, I may be wrong, and even if I’m not, achieving inertial fusion ignition in the laboratory may well point the way to as yet undiscovered paths to the fusion energy goal.  Ignition in the laboratory will also give us a significant advantage over other nuclear weapons states in maintaining our arsenal without nuclear testing.

Based on the progress reported to date, there is no basis for the conclusion that ignition is unachievable on the NIF.  Even if the central hot spot approach currently being pursued proves too difficult, there are alternatives, such as polar direct drive and fast ignition.  However, pursuing these alternatives will take time and resources.  They will become a great deal more difficult to realize if funding for NIF operations is severely cut.  It will also be important to maintain the ancillary capability provided by the OMEGA laser.  OMEGA is much less powerful but also a good deal more flexible and nimble than the gigantic NIF, and has already proved its value in testing and developing diagnostics, investigating novel experimental approaches to fusion, developing advanced target technology, etc.

We have built world-class facilities.  Let us persevere in the quest for fusion.  We cannot afford to let this chance slip.

Author: Helian

I am Doug Drake, and I live in Maryland, not far from Washington, DC. I am a graduate of West Point, and I hold a Ph.D. in nuclear engineering from the University of Wisconsin. My blog reflects my enduring fascination with human nature and human morality.

2 thoughts on “Nuclear Fusion Update”

  1. Hi Helian
    Thanks for the update. The article was “Received 8 October 2011; published 24 May 2012”. With this lag, who knows where the NIF currently stands?

    I concur that the NIF should Push On in spite of the approaching deadline. And the USA should support funding of this fusion research beyond the end of this fiscal year. Too much has been achieved to stop now. The progress made on the lasers, the crystals, the monitoring and diagnostics, and the incredible precision of the timing and control system, are mind boggling.

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