Freakin’ “laz-ers” November 17, 2009Posted by Maury Markowitz in fusion, nuclear.
Tags: bolognium, fusion power, nuclear power
So Edward Moses, the latest manager of the disaster that is the National Ignition Facility, continues to put a brave face on the latest efforts to suck up US taxpayer money. In spite of enormous budget overruns, continued delays, and the simple fact that there is no possible way it will ever be a new power producer, Moses recently claimed in a Newsweek article, with a straight face one presumes, that NIF will be a “endless supply of safe, clean energy.”
Here, let me just put this out there as simply as I possibly can: there is absolutely no way that the laser implosion technique used by NIF can ever possibly produce net output power. Period. Here, let’s do the math…
NIF is an “inertial confinement fusion” device. The “inertial” part means that it uses mechanical force to press together fuel to a super-high density, about 100 times that of lead. That pressure is supplied by shining laser light on the outside of the pellet, which heats it so quickly that a small portion explodes off the surface. Every action has an equal and opposite reaction, so the portion on the inside is driven inward.
As a side-effect of the ideal gas law, as the density increases you also dramatically increase the temperature. If everything is just right, the temperature in the very center will peak at levels that are hot enough for fusion to take place, very rapidly. Getting everything right is basically a function of the number and power of the laser beams.
We’ve been playing with these ICF designs since the 1970s, and each one is larger and more powerful than the last. NIF is by far the largest. Every laser pulse generated in the NIF uses 400 megajoules (MJ) of electrical power. The infrared laser flash it generates has a power of 4 MJ. That’s right, the lasers are 1% efficient. The output from the laser is then converted into 1.8 MJ of ultraviolet light that shines into the target chamber. So 0.5% end-to-end conversion efficiency.
When it gets into the target chamber, it hits a small pellet of fusion fuel. In the absolute best case scenario this will give off 25 MJ of fusion products. Some of that will be used to generate additional tritium fuel, although no one’s ever actually done this. Some more will be lost, escaping the reaction chamber as radiation. But most of it can be captured as heat. Now you use that heat to drive a turbine, like any other power plant. Generally you’re looking at 45% efficiency for combined-cycle systems. So maybe 10 MJ output.
So 400 MJ in, 10 MJ out. See the problem?
But we can do anything!
Invariably when I write an article outlining the completely ridiculous claims of the people promoting some new power source or other, the counter-claims involve some sort of “well we don’t know what the future will hold” line of reasoning. These are generally specious: if there is a miracle advance that will make this technology economical, then it is exactly as likely that there is some other advance we that will make some other technology even more economical, like coal fired power plants. In fact, given that there are a greater number of “technologies that are not your favorite” than the single one that is your favorite, then obviously chances are your favorite will loose the technology lottery.
So discounting flights of fancy, and considering the fact that we actually do understand a whole lot of science, what do we know now that will let us judge the future capabilities of these technologies? Well we know for a fact that a power source that requires 400 MJ to produce 10 is unlikely to be very useful. We’d need to produce at least 1000 MJ before things even start to get commercially interesting. And, given what we know, how likely is that?
The lasers are the big problem. If that 1% conversion efficiency was more like 10%, things get more interesting. And there’s actually a pretty good chance that we can do this, maybe into the 15 to 20% even, using new solid-state lasers. There’s some small-scale research on this going on, but nothing serious enough to justify Moses’ level of optimism. There’s also the possibility of improving the “laser coupling” with the fuel pellet, but much beyond 2 times is highly unlikely.
And that’s about it, those are the only obvious paths forward. We’ve worked on ICF for about 30 years now (hell, Shiva was in Tron), so we have a very good idea how these things work. Out best possible effort leaves us with 20 MJ in, 20 out.
But that’s NIF…
Now admittedly, that’s the NIF approach, which isn’t the only one. There’s a lot of activity in “fast ignition” going on that might offer another 10 times improvement, although at the cost of some of the coupling efficiency. Fast ignition machines might just be able to break even, end-to-end. But that’s just not good enough.
And then there’s the “whole other approach”, using magnets, which has always been the mainstream approach anyway. These systems are likewise chasing their tail, and show no signs of approaching the sort of densities where we can even consider commercial production.
But don’t take my word for it… the latest edition of Dr. Michael Dittmar’s series on energy futures is blunt:
We further postulate that, no matter how far into the future we may look, nuclear fusion as an energy source is even less probable than large-scale breeder reactors, for the accumulated knowledge on this subject is already sufficient to say that commercial fusion power will never become a reality.
Well, that’s about that.