Billions have been spent, with more budgeted. Currently a number of large governmental programs are working on the problem. Among these are the Z-pinch machine at Sandia Labs (as big as a large house; final cost more than $1 billion); the National Ignition Facility, or NIF (as big as a football stadium, cost around $4 billion); and ITER, the International Thermonuclear Experimental Reactor (housed in dozens of buildings on a 100-acre campus and projected to cost $20 billion).
And then there's Lawrenceville Plasma Physics (LPP), a startup company working out of a small lab in New Jersey. With a budget of about $0.001 billion, they have built an experimental fusion device that could fit inside your spare bedroom.
Care to guess which of these has achieved the hottest temperatures so far?
Yup, a few weeks ago LPP announced that they have measured ions at energies greater than 150 KeV, which works out to 1.8 billion degrees. That's more than 100 times hotter than the center of the Sun. LPP had previously measured electrons at 400 KeV, or 4.6 billion degrees, so we knew it was hot in there; but it's the much heavier ions that do the fusing, and getting them this hot is a new record for deuterium (heavy hydrogen) in a fusion device, at least as far as I'm aware. Their paper with these results has just been accepted by Physics of Plasmas.
The machine they use is called a Dense Plasma Focus (DPF), which uses currents in the plasma itself to create a magnetic field that collapses a knot of plasma into a tiny ball called a plasmoid. The DPF was invented decades ago, but had been thought unsuitable for fusion for technical reasons. Physicist Eric Lerner did some calculations which showed him that at the right energy levels those limitations might be overcome. Lerner and his colleagues at LPP have called their device "FocusFusion1", and have been working on it for over two years. (See my previous diaries on the project here and here.)
Fusion: How to do it
It's not easy to make a big atom out of two little ones. You have to fuse the atomic nuclei together, and the nucleus of an atom is protected by a cloud of electrons. So first you have to strip the electrons away, which turns the atoms into ions, and makes a state of matter known as plasma.It's actually not hard to make plasma; you just run some electricity through a gas. A neon light is plasma, and lightning is plasma. And if you've seen one of these doohickeys, you've seen plasma.
So making plasma is the easy part, but then comes the hard part. All of those ions are positively charged, and all those positive charges repel each other. And since the electromagnetic force is very strong, they repel each other very strongly. Now there's an even stronger force in the universe, called (let's see, what shall we call it?) the Strong Force. The Strong Force pulls ions (actually, their constituents, which are protons and neutrons) together very strongly, even stronger than the electromagnetic force pushes them apart. But the Strong Force only works at very, very short distances. So in order to get the Strong Force to pull those ions together so that they fuse, you have to figure out a way to overcome the electromagnetic force, which is pushing them apart at longer distances.
And the way you do that is to get the ions moving very fast, so that occasionally their kinetic energy can overcome the electromagnetic repulsion. ("Going very fast" is the technical term for "getting very hot".) And the other thing you have to do is to figure out a way to keep all those very fast-moving ions in the same place long enough to fuse together, without having them run away on you.
In FocusFusion1, and in a number of other devices like ITER, magnetic fields keep the ions together. In the NIF, they blast tiny pellets of hydrogen with brief laser pulses, and use the inertia of the matter itself to keep it together long enough for fusion to occur.
FocusFusion1 under construction. The blue boxes are 12 capacitor banks, each topped with a switch and a trigger. The heart of the machine, an electrode array inside a vacuum chamber, hangs below the circular plate in the center. Photo: Rezwan Razani
How close are they?
It's an oft-heard joke that fusion is 20 years away, and always will be. (I first heard that joke in the 1960's, which shows you how true it is.)It takes more than raw temperature to make fusion happen. Physicists use a figure-of-merit called the Lawson criterion to give them a rough idea of how close they are to achieving "break-even" (that is, the point where you get more energy out of it than you put into it). And just to make things confusing, there are two versions of the Lawson criterion in common use.
In the first version, you multiply temperature by the density of the plasma; and in the second version (called the triple-product version) you multiply temperature times density times the confinement time of the plasma. There are several different kinds of fusion reactions, and the break-even values of the Lawson criterion are different for each of them. But the easiest kind of fusion to achieve is called the "D-T" reaction, because it fuses together an ion of Deuterium (doubly-heavy hydrogen) with an ion of Tritium (triply-heavy hydrogen). The Lawson criterion for break-even with D-T is about 4 x 10^15 keV s/cm³.
The Lawson number for these temps hasn't been revealed yet, but in earlier experiments, Lerner has gotten to about 5 x 10^15 keV s/cm³.
That sounds like they're there already, but (as usual) things are more complex than a single number. First off, the temperatures LPP has achieved are actually too hot for ideal D-T fusion. At 150 keV they would need longer confinement times and/or higher densities than they have achieved (or so far announced) to reach break-even. So if they were looking for D-T fusion they would aim to make their plasma a little cooler and a little denser. In theory that's not all that difficult, but they haven't done that – and they probably won't.
That's because LPP isn't really interested in D-T type fusion. The problem with D-T fusion is that a lot of the energy in D-T comes in the form of high-energy neutrons, which are highly radioactive. In theory you can capture that energy the same way you do with a conventional nuclear reactor (that is, by using the radiation to get something hot and boiling water to run a turbine). And there's tritium itself, which is radioactive all on its own, and LPP doesn't want to get into all those radioactive-materials handling issues.
So LPP has their sights set higher. They want to bypass the D-T reaction in favor of aneutronic fusion. The type of fusion they want to create is called pB11, which uses conventional hydrogen (which becomes a bare proton, p, when ionized) and boron-11. In the pB11 reaction, most of the energy produced is in the form of ions, which carry an electric charge and can therefore be moved around by magnetic fields. So you avoid the radioactivity problem of neutrons. Even better, you can take those high-energy ions and funnel them through a coil of wire to generate electricity with no moving parts at all.
The problem with pB11 is that the Lawson criterion for making it happen is about 500 times higher than for D-T (in other words, pB11 is a lot harder to do). And while 1.8 billion degrees is actually too hot for D-T, it's still short of what's needed for pB11. That's the bad news.
The good news is, LPP still hasn't turned on FocusFusion1 up to full blast yet. Lerner's computations indicate that FocusFusion1 should be able to get to break-even for pB11, at least in theory, when it is. And although they're not yet hot enough for pB11, they're also not all that far away from it, either. They're taking things a step at a time, and since their funds are limited, they've run into some engineering challenges along the way.
And of course, as in any rapidly-moving field, new records can't be expected to last very long before someone else comes along to break them. But stay tuned, this could get interesting.
Links
Lawrenceville Plasma Physics press releaseThe Focus Fusion Society
Eric Lerner's Google Tech Talk about Focus Fusion
Four billion degree electrons
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