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  • Another Fusion Tease?

    Posted on October 10th, 2013 Helian No comments

    It has always seemed plausible to me that some clever scientist(s) might find a shortcut to fusion that would finally usher in the age of fusion energy, rendering the two “mainstream” approaches, inertial confinement fusion (ICF) and magnetic fusion, obsolete in the process.  It would be nice if it happened sooner rather than later, if only to put a stop to the ITER madness.  For those unfamiliar with the field, the International Thermonuclear Experimental Reactor, or ITER, is a gigantic, hopeless, and incredibly expensive white elephant and welfare project for fusion scientists currently being built in France.  In terms of pure, unabashed wastefulness, think of it as a clone of the International Space Station.  It has always been peddled as a future source of inexhaustible energy.  Trust me, nothing like ITER will ever be economically competitive with alternative energy sources.  Forget all your platitudes about naysayers and “they said it couldn’t be done.”  If you don’t believe me, leave a note to your descendants to fact check me 200 years from now.  They can write a gloating refutation to my blog if I’m wrong, but I doubt that it will be necessary.

    In any case, candidates for the hoped for end run around magnetic and ICF keep turning up, all decked out in the appropriate hype.  So far, at least, none of them has ever panned out.  Enter two stage laser fusion, the latest pretender, introduced over at NextBigFuture with the assurance that it can achieve “10x higher fusion output than using the laser directly and thousands of times better output than hitting a solid target with a laser.”  Not only that, but it actually achieved the fusion of boron and normal hydrogen nuclei, which produces only stable helium atoms.  That’s much harder to achieve than the usual deuterium-tritium fusion between two heavy isotopes of hydrogen, one of which, tritium, is radioactive and found only in tiny traces in nature.  That means it wouldn’t be necessary to breed tritium from the fusion reactions just to keep them going, one of the reasons that ITER will never be practical.

    Well, I’d love to believe this is finally the ONE, but I’m not so sure.  The paper describing the results NBF refers to was published by the journal Nature Communications.  Even if you don’t subscribe, you can click on the figures in the abstract and get the gist of what’s going on.  In the first place, one of the lasers has to accelerate protons to high enough energies to overcome the Coulomb repulsion of the stripped (of electrons) boron nuclei produced by the other laser.  Such laser particle accelerators are certainly practical, but they only work at extremely high power levels.  In other words, they require what’s known in the business as petawatt lasers, capable of achieving powers in excess of a quadrillion (10 to the 15th power) watts.  Power comes in units of energy per unit time, and such lasers generally reach the petawatt threshold by producing a lot of energy in a very, very short time.  Often, we’re talking picoseconds (trillionths of a second).

    Now, you can do really, really cool things with petawatt lasers, such as pulling electron positron pairs right out of the vacuum.  However, their practicality as drivers for fusion power plants, at least in their current incarnation, is virtually nil.  The few currently available, for example, at the University of Rochester’s Laboratory for Laser Energetics, the University of Texas at Austin, the University of Nevada at Reno, etc., are glass lasers.  There’s no way they could achieve the “rep rates” (shot frequency) necessary for useful energy generation.  Achieving lots of fusions, but only for a few picoseconds, isn’t going to solve the world’s energy problems.

    As it happens, conventional accelerators can also be used for fusion.  As a matter of fact, it’s a common way of generating neutrons for such purposes as neutron radiography.  Unfortunately, none of the many fancy accelerator-driven schemes for producing energy that people have come up with over the years has ever worked.  There’s a good physical reason for that.  Instead of using their energy to overcome the Coulomb repulsion of other nuclei (like charges repel, and atomic nuclei are all positively charged), and fuse with them, the accelerated particles prefer to uselessly dump that energy into the electrons surrounding those nuclei.  As a result, it has always taken more energy to drive the accelerators than could be generated in the fusion reactions.  That’s where the “clever” part of this scheme comes in.  In theory, at least, all those pesky electrons are gone, swept away by the second laser.  However, that, too, is an energy drain.  So the question becomes, can both lasers be run efficiently enough and with high enough rep rates and with enough energy output to strip enough boron atoms to get enough of energy out to be worth bothering about, in amounts greater than that needed to drive the lasers?  I don’t think so.  Still, it was a very cool experiment.

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