This was the last day of scientific presentations, and it ended on a high note with a banquet, about which more later. L. J. Perkins did an excellent overview of fusion physics, and mentioned a couple of things in passing that caught my attention. The most significant is that p-B11 is viable as a fuel in fast ignition ICF. In ICF a fuel pellet is compressed by depositing energy symmetrically on a spherical capsule, blowing off the outer layer. The resulting reaction force collapses the pellet to fusion relevant densities, heating in the process. Fast ignition is a scheme where you hit the compressed pellet at or just before the moment of maximum compression with an additional energy source (ion beam or laser) focused on a small spot. Ignition of the fusion fuel is initiated at the spot, and this serves as a spark plug which sends a shock front through the high density fuel, triggering fusion throughout the volume. The nice thing about ICF is that the fuel density is really high, so the mean free path for photons is really short, smaller than the size of the pellet. This means that bremstrahlung, the traditional enemy of p-B11, is less of a problem, since bremstrahlung photons are captured within the pellet, rather than escaping as they do with the lower density plasmas used in magnetic confinement.

There was a talk on Deuterium-Helium 3 fusion, coming out in favor of it as a power production scheme. I remain skeptical, but I'd love to be proved wrong. There was the usual invocation of lunar He3 production, and a mention of the possibility of obtaining He3 from the gas giants. It'd be interesting to see if the energy cost of extracting the He3 from the gravity well is worth it.

In comments to my previous post Phil Fraering asked about electrostatic confinement. I had a chat with Greg Piefer, who is working on the UW IEC device. The upshot is that inertial electrostatic confinement cannot scale to a commercial reactor, even in theory, unless there are major breakthroughs in grid materials or a way to get rid of the grid altogether. The problem is that the grid simply erodes iunder ion bombardment. There are people working on dealing with these issues, but limited success so far. The major item of good news from IEC is that there is commercial interest in it as a neutron source or a proton source (using Deuterium-Helium3).

The highlight of the conference for me was the banquet, where I got to hang out with some people who've forgotten more than I'll ever know about fusion. One of my tablemates is a senior scientist at a national lab, and he was able to name $750 million worth of fusion experiments where significant hardware was built but zero data produced before the plug was pulled. I'm not sure he'd want his name widely associated with this number, which is why I'm leaving it out. Anyway, the experiments in question weren't over budget, just killed because DOE decided to change the direction of the program, often as a result of lobbying by people hoping to switch the funding over to their pet projects. This sort of crap is why large scale government funded research has to be approached very carefully. Without hawk-like oversight politics ends up beating both good science and good sense. The analogies in the space program are left as an excercise for the reader.

I thought IEC has been in use for several years commercially as a neutron source.

Would it help for IEC to build the whole inner grid really large, like a few 100m in diameter? After all, the grid does not have to be very strong, so you could build it very thin so that the area of the grid relative to the area of the openings is very small.

Of course you could only build such a device in space...

I thought somebody (bussard?) had found a way to get rid of the grid completely. Has this concept worked out?

Sam - IEC has been in use as a neutron source for a while, but this is the first I've seen of uses as a proton source.

tuttle - IEC scaling gets better as you go smaller, so really big grids just won't work.

And all of these goings on are taking place a mere hours drive from my house.

I keep thinking I should have ditched work - sounds much more enjoyable than what I've got in front of me. On the other hand, Andrew is surely dumbing down some of the content in his postings - I might get one word in five after the topic summary.

The delta-V to reach low orbit around Uranus from its 'surface' is about 20 km/s. This is very small compared to the speed corresponding to the energy released by fusion of 3He, so I don't think energy considerations alone would rule it out. It might require a multistage solid core nuclear thermal rocket to reach that orbit, which may or may not be practical.

One other potential source of 3He I've mentioned elsewhere is a (as yet undiscovered) planet in the Kuiper Belt or beyond. My very rough BotE calculation is that a Mars-size KBO could retain a helium atmosphere if sufficiently far from the sun (depends on the temperature at the exobase and the rate at which the solar wind or passing interstellar medium erodes the atmosphere; Pluto and Triton are unfortunately too small). Getting out to this body would be challenging, but a low thrust fusion rocket would do the trick, and we're already assuming D-3He fusion is workable.

I look at He3 fusion as interesting but extreme speculation. The problem is that too much of what I see printed doesn’t discuss the questions at all. I’ve seen more than a few bits that seem to suggest all we need is to send a couple of landers to the moon and start pumping the stuff up.

We don’t know if it will be possible to build economically viable d-he3 reactors. It will likely be many decades before we know. We don’t know if He3 can be practically extracted from the lunar surface. If it can, it will require a huge investment and major space infrastructure. There is no use for He3 unless we have the reactors, and vice versa: Classic chicken and egg. Failing the moon, we can go mine a gas giant – and Uranus is probably the best choice with the lowest escape velocity. But that means we have to have regular transportation to the outer solar system, develop atmospheric extraction systems, have extremely advanced automated systems or a large human presence there as well, and a practical system to get the stuff out of Uranus’s atmosphere into space. And even if there are small Kuiper Belt/Oort Cload "planets" with helium “air” that would be even further out yet.

It may happen, but so may many other things. If it does work out, it will probably be long after we have developed economical fusion power and a major space infrastructure. It won’t be at the beginning.