The reason I’ve been a little quiet these past few days is that I’ve been preparing a talk for presentation at the IEEE International Conference on Plasma Science, held in Baltimore this year. I presented yesterday, and it was generally well received. The topic was technical and boring, so I won’t gn into details here. The talk that ended the session I was at was particularly interesting, though, so I thought I’d blog about it.
The talk in question was presented by J. E. Brandenburg of the Florida Space Institute, titled Microwave Enhancement of Inertial Electrostatic Confinement of Plasma for Fusion: Theory and Experiment. Inertial Electrostatic Confinement (IEC) uses two (or more) nested spherical grids charged to a high relative voltage to accelerate ions towards the common center of the grids, where they collide and fuse. Philo Farnsworth patented an IEC concept he called the Fusor, and there are all the usual conspiracy theories about suppression of his research surrounding the history of the Fusor, though I suspect the truth of the matter has a lot to do with the fact that it didn’t really work very well, at least for power generation.
Anyway, back to the point. IEC has seen a resurgence of interest lately (for an overview of what people are up to check out the presentations at the 2002 US-Japan workshop on IEC). Various problems are slowly being worked out and the prospects for IEC for power generation are improving. I talked to Brandenburg after his presentation and he claimed that some experiments were getting within (relative) spitting distance of break-even, bearing in mind that for fusion spitting distance is about a factor of ten or so away.
From a purely technological standpoint IEC is attractive because it does not use magnets, so the power requirements are a lot lower than many other fusion schemes. IEC de-vices are also compact (grid sizes are 1 to 15 cm in radius), which makes experiments much easier to perform. More interesting to me is that IEC de-vices are evolvable along an economically viable path. IEC de-vices are already being sold commercially as neutron sources (see the overview pdf from the US-Japan conference I linked to above for one example). If the market for neutron sources expands (which it may well, since neutron assay is a very convenient way of remotely detecting the elemental composition of things, particularly convenient if you are looking for nuclear contraband), then companies doing IEC can have a near-term revenue stream to fund further development.
Another nice feature of IEC is that the startup costs are relatively small, so that even amateurs can build primitive IEC de-vices. There is a site devoted to exactly that here. It’s not necessarily a great idea to build neutron sources at home, but intelligent people tinkering with IEC in their spare time may help move the ball down the field. There are important safety precautions that need to be taken, both for high voltage and for radiation, which drive the costs up. Those who take shortcuts with safety will earn their just darwinian reward – I’m not encouraging anyone to try this at home, but if you do, consider yourself warned.
The point of Brandenburg’s talk was that he’d tried using ponderomotive forces (see below) to improve confinement in an IEC de-vice, with apparent success. The results were preliminary, but it certainly looked like there was an improvement in the focus of the ions. The experiment was conducted in Argon, so there was no fusion, but the bright spot at the center of the grids got smaller with the application of microwaves. This is a good thing, because IEC depends on having all the ions coming in to a single high density focal point.
The ponderomotive effect is a neat little nonlinear plasma phenomenon that arises when an electromagnetic wave interacts with a charged particle. The EM wave consists of electric and magnetic fields oscillating in synchrony. The electric field accelerates the particle at right angles to the wave, and the magnetic field deflects the accelerated particle orbit. Half a cycle later, the electric field has flipped direction but: so has the magnetic field. The upshot is that the particle is deflected in the same direction during the second half of the cycle as during the first half. The electric field slams the particle from side to side, and the magnetic field distorts the sideways oscillation into a slight motion in the direction in which the EM wave is propagating. This is true regardless of the charge of the particle because the electric field effect flips sign with the change in particle charge, but so does the effect of the magnetic field, so the two sign changes cancel. The upshot is that electromagnetic waves push charged particles along their path.
The innovation that Brandenburg applied to IEC was to use this ponderomotive effect to enhance the confinement of the steady state IEC discharge. He injected 2.45 GHz microwaves into an IEC de-vice, where the ponderomotive force acted to shove the plasma in further towards the core, driving the density up. Obviously there’s a lot more that needs to be done to see if this will actually drive up fusion yields, but it’s an interesting development. I’ve always been kind of fond of the ponderomotive effect, in part because it’s such a neat nonlinear effect with all these minus signs cancelling out just right to give a net force, and in part because as soon as I saw it I started trying to figure out how to use it in a confinement scheme. It’s nice to see an application, and I’ll keep tuned for further developments.