Posted on 09/24/2013 8:56:27 PM PDT by LibWhacker
The dream of igniting a self-sustained fusion reaction with high yields of energy, a feat likened to creating a miniature star on Earth, is getting closer to becoming reality, according the authors of a new review article in the journal Physics of Plasmas.
Researchers at the National Ignition Facility (NIF) engaged in a collaborative project led by the Department of Energy's Lawrence Livermore National Laboratory, report that while there is at least one significant obstacle to overcome before achieving the highly stable, precisely directed implosion required for ignition, they have met many of the demanding challenges leading up to that goal since experiments began in 2010.
The project is a multi-institutional effort including partners from the University of Rochester's Laboratory for Laser Energetics, General Atomics, Los Alamos National Laboratory, Sandia National Laboratory, and the Massachusetts Institute of Technology.
To reach ignition (defined as the point at which the fusion reaction produces more energy than is needed to initiate it), the NIF focuses 192 laser beams simultaneously in billionth-of-a-second pulses inside a cryogenically cooled hohlraum (from the German word for "hollow room"), a hollow cylinder the size of a pencil eraser. Within the hohlraum is a ball-bearing-size capsule containing two hydrogen isotopes, deuterium and tritium (D-T). The unified lasers deliver 1.8 megajoules of energy and 500 terawatts of power—1,000 times more than the United States uses at any one moment—to the hohlraum creating an "X-ray oven" which implodes the D-T capsule to temperatures and pressures similar to those found at the center of the sun.
"What we want to do is use the X-rays to blast away the outer layer of the capsule in a very controlled manner, so that the D-T pellet is compressed to just the right conditions to initiate the fusion reaction," explained John Edwards, NIF associate director for inertial confinement fusion and high-energy-density science. "In our new review article, we report that the NIF has met many of the requirements believed necessary to achieve ignition—sufficient X-ray intensity in the hohlraum, accurate energy delivery to the target and desired levels of compression—but that at least one major hurdle remains to be overcome, the premature breaking apart of the capsule."
In the article, Edwards and his colleagues discuss how they are using diagnostic tools developed at NIF to determine likely causes for the problem. "In some ignition tests, we measured the scattering of neutrons released and found different strength signals at different spots around the D-T capsule," Edwards said. "This indicates that the shell's surface is not uniformly smooth and that in some places, it's thinner and weaker than in others. In other tests, the spectrum of X-rays emitted indicated that the D-T fuel and capsule were mixing too much—the results of hydrodynamic instability—and that can quench the ignition process."
Edwards said that the team is concentrating its efforts on NIF to define the exact nature of the instability and use the knowledge gained to design an improved, sturdier capsule. Achieving that milestone, he said, should clear the path for further advances toward laboratory ignition.
It’s an easy mistake to make. The key thing is that the protons don’t care about where they are. They’re just not that smart. Classically, all they need is enough kinetic energy to get close enough to overcome the Coulomb barrier. Quantum mechanically, they don’t need quite that much energy, just enough for their proximity to be close enough that their wave functions have significant overlap (or equivalently, that their KE can “tunnel” through the Coulomb Barrier.)
And this happens at the same energy whether the impetus is gravity, lasers, or Maxwell’s Demons.
That's nowhere near "ignition", which is being sought in the ICF experiments. I only recently learned of a type of supernova caused by "pair production instability" which does actually represent a star exploding like an H-bomb, with near 100% efficiency due to its size. Mind-boggling.
ICF is striving for a pinhead portion of one of these, not the slow simmering sun. In fact, reading the ICF article, it's hard not to scoff.
This is exactly right. We are no closer to fusion today then we were 30 years ago. The problem is and always has been containment — or how not to melt a hole through the Earth. I don’t think actual ignition has ever been the hard part of the problem.
We’ll probably be building non-orbiting stationary power generation factories in space and beaming the power down well before they solve the fusion puzzle.
And cold fusion? Cold fusion is a myth, it’s not fusion at all, it’s a chemical reactor.
Massive stars that can undergo pair production instability are not using fusion mechanisms that are envisaged in the ICF. These stars, I believe, are triple-alpha-cycle degenerating stars. At the very least they are carbon-cycle stars. Their energy is not coming from the fusion of isotopes of hydrogen.
This is correct.
The problem is and always has been containment — or how not to melt a hole through the Earth.
This is not correct.
I don’t think actual ignition has ever been the hard part of the problem.
This is also not correct.
There is no "containment" issue. "Confinement" refers to the issue of confining sufficient fusion material for sufficient time in a small-enough region at sufficient temperature to maintain a self-sustaining fusion reaction -- achieving what is colloquially referred to as "ignition." It is the only issue there has ever been. And we are not much closer to doing it than we were in the 1950's; we only know more things that don't work.
Re: “It seems that every article I’ve read on fusion since I was a teenager has said we’re 20 years away from fusion.”
I agree.
Europe, Russia, and the USA have been doing commercial fusion research since at least 1960.
It would not surprise me if the world wide price tag for this technology has reached $100 billion.
If we had put that money into photovoltaic, battery, and hydrogen research instead, I think we might be at break even or better on those three technologies.
6: Fryer, C.L.; Woosley, S. E.; Heger, A. (2001). "Pair-Instability Supernovae, Gravity Waves, and Gamma-Ray Transients". The Astrophysical Journal 550 (1). arXiv:astro-ph/0007176. Bibcode:2001ApJ...550..372F. doi:10.1086/319719
But my point was, it's not a nuclear fusion from what are ordinarily considered nuclear reactant candidates in terrestrial fusion projects.
My understanding is that pair-production collapse only occurs in post-Helium stars. A citation by two of the same investigators of more recent vintage, I believe, says oxygen fusion chain reactants are in play at this point in the star's life:
a b Kasen, D.; Woosley, S. E.; Heger, A. (2011). "Pair Instability Supernovae: Light Curves, Spectra, and Shock Breakout" (pdf). The Astrophysical Journal 734 (2): 102. arXiv:1101.3336. Bibcode:2011ApJ...734..102K.
He proposed that it might be possible through a staging process similar to a thermonuclear weapon, where about 30-65% of the energy came from fusion and other 70 to 35% came from the fission used to initiate the staging reaction. Of course, that type of fusion wouldn't satisfy the Enviro-Nazis, and that kind of reactor -- to my knowledge, and I am no expert -- has never been significantly researched.
As for the other kind of fusion? "It might happen," he said, looking around the seminar room, "within the lifetimes of a few of the people in this room." I was then one of the youngest grads in the room, a 26 year old. 30+ years have passed since then. I believe Teller's prediction might have very well be too optimistic.
OK, but MY point was that the "ignition" being referred to in ICF is a chain reaction, and not just "burning" at some low rate of consumption, as in the sun:
The energy released by these reactions will then heat the surrounding fuel, and if the heating is strong enough this could also begin to undergo fusion.The aim of ICF is to produce a condition known as "ignition", where this heating process causes a chain reaction that burns a significant portion of the fuel.
"These reactions" refers to the fusion caused by the laser heating, which plays the role of the fission "trigger" in an H-bomb, as I understand it. This is thermonuclear ignition, like an H-bomb, and like a pair instability supernova, but not like the sun.
Your body actually generates more heat per cubic inch than the sun.
Simply because only ~300W/m^3 are being produced, that doesn't mean the reaction is not sustaining itself! Remember even though only ~300W/m^3 are produced per second, a cubic meter of matter in the center of the sun has an energy density of around 10^16 J/m^3. That's ignited plasma, no question about it.
And keep in mind, when you consider the 'Q' that various projects are envisioning, fusion reactors will be using most of their energy to maintain confinement (fed back into the lasers or magnetic fields); they are not going to be exploding like a thermonuclear weapon. They will have many times better yields than the P-P fusion chain, but the definition of ignition doesn't change. It's just the difference between burning incense or gasoline. At some point, either fuel "catches" and the oxidation becomes hot enough to maintain the kindling temperature for more oxidation to occur. Only difference is, it's nuclear chemistry instead of atomic chemistry.
This isn't right. A thermonuclear reaction is unstable, the reaction rate accelerates explosively as result of the heat it is generating.
The sun is like a furnace held together by gravity. Its stability is in stark contrast to a pair instability supernova, which consumes its fuel on a time scale of seconds.
I know in astronomy they refer to Hydrogen ignition and Helium ignition for the beginning of the different burning phases, but the ICF article puts "ignition" in quotes and carefully explains the positive feedback they are looking for. They have a link for "chain reaction" where the definition is given: "A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events."
Note: "self-amplifying", i.e. a runaway reaction.
““Cold” fusion is real”
So far no one has produced a cold fusion system. Every last one claimed to work has been a scam.
Sorry, but this isn't correct. There's no more "instability" in the ignition of a thermonuclear weapon than there is in the sun. The reaction kinetics are different because the reactants and confinement mechanisms are different. That's all. There is nothing fundamentally different about the physics which involves a release of binding energy from the strong nuclear force. The criticality, or runaway, reaction is entirely in the fission part of the bomb.
The "instability" in all the cases you're talking about is all about the containment's ability to confine fuel long enough to fuse; that's all. In a PPI supernova, most of the fuel is not instantaneously consumed by some sort of "instability." In fact, most of the fuel isn't consumed at all. The core is entirely consumed and blows most of the hydrogen/helium atmosphere (about 75% of the star) off into space. The same thing happens in a thermonuclear device. Ablation of the shell and x-ray pressure confines the reactants long enough for significant consumption of the reactants, but there isn't any special magic sauce there that isn't present in the sun. It's just a more energetic reaction because it isn't the P-P chain.
Like I said, not viable. Ok maybe should have said for even small power gen, too...
No magic sauce, just feedback. It’s a very general concept, epitomized by the HS auditorium audio feedback, heard as an earsplitting screech. It accounts for supernovae of all sorts, I believe.
As I read the LHC wiki article, this is what they’re trying to achieve, but they’re having trouble getting the microphone close enough to the speakers :-)
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