Nature reported this week that construction on the international fusion project ITER won't begin until next year, even though site preparation – clearing, leveling, and so on – was completed back in May. Turns out, there are still some details to be worked out about which countries are paying for what parts of the multi-billion-euro endeavor (whose construction costs, of course, have increased from initial estimates). Despite the delay, though, the machine is still projected to start operations by 2018.

ITER, for those not familiar with the project, is an international plasma physics experiment of an unprecedented scale.¹ The European Union and six other countries are working together to build the first magnetic nuclear fusion device (i.e., one that relies on magnetic fields to contain the hot plasma) that would put out more usable power than is required to run it. “Usable,” of course, is the key word here. Deuterium fusing with tritium is, to date, the “go to” fusion reaction because it has the most accessible parameters. Unfortunately, only about 20 percent of the energy it produces can be easily harnessed for use.

Incidentally, “accessible” in this case means operating at 130 million degrees C. Sure, the densities are still orders of magnitude lower than atmospheric pressure, but that still doesn't make it easy to achieve such high temperatures. (As an aside, the Americal Physical Society's Division of Plasma Physics has a handy chart that shows the relative temperatures and densities of some of the most well-known plasmas: http://apsdpp.org/plasma_brochure/page2.html. For reference, atmospheric pressure is a little over 10²⁵ particles per cubic meter.)

In fact, when you'd dealing with plasmas, our common concepts of “temperature” can became almost entirely inapplicable. Leaving thermodynamic definitions (involving entropy and free energies) for another time, consider the “standard” explanation for temperature, the one we usually first hear sometime in grade school:

In a solid, the atoms can vibrate, and the hotter they are, the more they vibrate, but they're pretty much stuck in place. In a liquid, those atoms start moving around faster, and with more freedom. Finally, in a gas, those atoms are zipping around at breakneck speed, completely independent of one another.

If you consider the individual atoms a little more carefully, you get a somewhat more sophisticated picture: although they're not all moving at the same speed, the distribution of their speeds (the Maxwell-Boltzmann distribution, to be specific) peaks around a central value, proportional to the square root of the temperature. The further you go from the central value, the fewer atoms you find with those speeds.

In a plasma, though, there's no guarantee that you'll have a Maxwellian speed distribution at all, because the particles might not collide enough to produce one. Also, you don't know if the particles will be as fast, on average, in one direction as they are in another. The interactions with electric and magnetic fields can cause particles to be “hotter” in, say, the X direction than the Y direction. Finally, even if the ions and electrons each have a well defined distribution, and hence a well-defined temperature, those temperatures aren't necessarily the same!  This is because electrons will collide more with electrons and protons with protons than either will with the other; protons also collide more than electrons do with any cold neutrons that might be around.  You can also have heating mechanisms that affect particles only above or below a certain mass.

Starts to look almost as complicated as ITER's budget, right?

Still, there are a lot of smart people working on both the subtleties of the financing and the subtleties of the science.  If it can all come together between now and 2026 (the planned date for the first power-producing experiments), it will be an impressive feat indeed.

Original report: http://www.nature.com/news/2009/091013/full/461855a.html

1.) By the way, in case you're wondering what the ITER stands for, it's worth noting that the organizers no longer identify the name as being an acronym for “International Thermonuclear Experimental Reactor,” as was the case at the outset. The history section of the project's website merely comments that the word “means 'the way' in Latin.” A contributor to the Wikipedia page about ITER suggests, rather eloquently, that “that title was dropped due to the negative popular connotation of 'thermonuclear,' especially when in conjunction with 'experimental.'”  Unfortunately, in the absence of other sources to confirm this, I'll have to classify it as a questionable, if seductive, explanation.