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.

Isn't the ponderomotive effect just radiation pressure? If so, the power density of that microwave beam is going to have to be pretty darn high to reach a significant fraction of the pressure of a fusion plasma. I assume you'd use a resonant cavity to get that high power density.

A couple of questios:

1: 2.45 GHz sounds kind of familiar. Was he using a magnetron from a microwave oven?

2: Is the effect reversible? Can I transfer energy from translational motion of plasma into the microwave field? Might be an interesting way to generate large amounts of microwave energy, sort of like a cross between a travelling wave tube and a free electron laser.

3: Seems like I could build a plasma jet in a microwave oven. I'm going to try getting a discharge going between two opposed 1/4 wave elctrodes (short nails, probably), try to get it into the right cross field orientation, and see what happens.

MMM -

1. I suspect you're right (see the McGraw-Hill Encyclopedia of Science and Technology reference).



2. I believe that magnetohydrodynamic generators use just this principle.



3. You don't live anywhere near me, do you? ;)

Whatever you do, MM, don't cross the beams...

I think we all want to know what county you're going to do this in.

Paul - the ponderomotive force isn't just radiation pressure. It's several orders of magnitude stronger.

MM -

(1) Yup. It's funny how often that number turns up in plasma physics experiments. Go to 3 GHz and the price of the magnetron goes up by a factor of ten or more.

(2) you can't run the effect backwards and produce radiation because the thermal motion of the plasma isn't coherent. Jay - MHD generators use the charge separation induced by flowing a plasma perpendicularly through a magnetic field, but it's a continuous process - the output is DC, not high frequency electromagnetic radiation.

(3) be extremely careful. If all you want is a bit of plasma in the oven you can do it by putting a small lighted candle in the oven at high power. The candle provides seed electrons to start the discharge. Do not under any circumstances defeat the oven door interlocks - microwave burns aren't immediately obvious until after a significant amount of damage is done. A cooked eyeball is nobody's idea of a good time.

Sorry, I was just thinking vaguely of getting power out of a moving plasma. Thanks to Andrew for the clarification.

MMM, post video of the experiment if you survive. ;)

Hi Andrew,
Great to hear that one of these alternative fusion schemes is making some progress. I was wondering about other groups working on IEC. In particular, the FINDS foundation (funded by Walt Anderson, I believe, and run by Rick Tumlinson) was funding IEC work at the Univ. of Wisconsin a few years ago, e.g. www.finds-space.org/He3.2000.html . Have you heard anything from that group recently? (I think the telecom downturn must have drained the money from FINDS since they seemed to have stopped making grants of any kind after 2001.)
- Clark

Dang! I always thought that would work. Too bad I've been too busy with other work to take a crack at calculating that.

MMM, you won't have to tell us where you are... When we see Keanu Reeves on a motorcycle outracing a massive explosion, we'll know where you were... ;-)

- Eric.

Can these IEF neutron sources be used for things like neutron diffraction and/or scattering experiments? Are the wavelengths comparable to thermal and cold neutrons that come out of a reactor (0.1 to 100 angstroms)?

Clark - The Wisconsin group is still working on IEC as far as I know.

Frank - The neutrons are high energy - 5 MeV for D-D fusion and 14 MeV for D-T fusion, so they aren't suitable for neutron diffraction experiments AFAIK

>Frank - The neutrons are high energy - 5 MeV >for D-D fusion and 14 MeV for D-T fusion, so >they aren't suitable for neutron diffraction >experiments AFAIK

I wonder if they could be moderated, or are they so energetic they would just plow through anything put in their path?

I'm somewhat out of my depth here, but this almost seems like it could form the basis for a "desktop" (very relatively speaking) neutron source. It would be a lot more convenient than having to work with a national user facility attached to a reactor or spallation source.

Andrew:
What, (if any :o) effect has things like "Z-Pinch" field, and other studies done under the Mini-Mag Orion concept:
>http://www.andrews-space.com/en/news/Pub-MMOJPLTalk.pdf

had on fusion research? (Or was it the other way around?)

Randy

Isn't the neutron energy from D+D -> 3He + n 2.45 MeV, not ~ 5 MeV?

Paul - you're right, it's 2.45 MeV. dunno where the 5 MeV number came from. Clearly I need a vacation:-)

Frank - it certainly can make a desktop neutron source, but the neutrons are fast, and moderating them down to spallation type velocities requires a lot of sheilding (I haven't done the numbers). Fast neutrons are a plus if you want to neutron assay a large volume of material, though. Schlumerger makes an IEC neutron source to drop down oil wells to do in situ assays, which lets you sample a volume of tens of cubic meters through a hole 10 cm in diameter. I tried to convince NASA to give me money to develop something similar for flight to mars, but no dice (for particularly irritating reasons, but I can't go into detail without burning bridges I might like to cross at some point in the future).

Randy - short answer: nearly none. Not enough work has been done.

Apparently this J.E. Brandenburg is the "Face on Mars" John Brandenburg.

Ouch. I think you're right, Bill. From

http://www.thespaceshow.com/guest.asp?q=191

"Dr. John Brandenburg is a Mars research scientist, he is aka Victor Norgarde, author of "Morningstar Pass: The Collapse of the UFO Cover-up” ... Dr. Brandenburg is a researcher at Florida Space Institute ... performed research on Fusion Propulsion ... author the “Dead Mars, Dying Earth”(1999) [etc.]"

I'd never heard of him, but he is apparently another "Face on Mars" proponent and says CO2 is leading to doomsday, among other things. I have no patience at all for the "Face on Mars" types. Science is the key, of course, but I wouldn't take anything he said very seriously unless independently verified.

Oh well. I have a soft spot for IEC. Maybe something like Bussard's "Polywell" scheme will do the trick.