This was written shortly after the 1997 American Physical Society Plasma Physics conference in Pittsburgh:

I've been to the conference a few times, but this was the first year they actually had a special section on plasma thrusters for spacecraft. Sure, it was thrown into a tiny room on the far end of the convention center, and there were only ten short talks, but it was something.

Most of the talks, though, weren't really usable for interstellar spacecraft. People are building small plasma thrusters for use on satellites, and some people talked about bulding large versions to get to Mars in 3 months instead of 6. (Although one guy actually proposed building an enormous magnetic mirror spacecraft that, if everything worked perfectly, would still take 170 days to get to Mars!) A large magnetoplasma thruster prototype has been built and people are doing actual research.

However, there were a few talks that went into interstellar drives, and one in particular (from Penn State) sounded very interesting. (That is, interesting in the sense that it might be possible to use the idea in the next 50 years) The idea is to use small quantities of antiprotons to catalyze a hybrid fission/fusion pellet.

This idea, which has some experimental support (Phys Rev C, 45, p2332), is based on the observation that antiprotons can catalyze large fission and neutron yields in uranium pellets. If you can get 10¹¹ antiprotons in a small area, this could heat a small target to many keV and possibly create conditions that could ignite a fusion reaction.

Their suggested design was to use a series of small (42ng) DHe₃ (or DT) droplets, each doped with 2% U238. These are injected, one at a time, into an electromagentic trap that contains 10¹¹ antiprotons. 0.5% of the antiprotons annihilate, catalyze fission of the U238, which heats the DHe₃. Then they raise the voltage on the electromagnetic trap, which seperates out the negatively charged particles (the pellet electrons and unused antiprotons) from the positive ions (the hot D and He₃). The positive ions then fuse at a temperature of 100KeV, giving off 15kJ of net energy. The used 5 10⁸ antiprotons are then replenished, the antiprotons are put back in the original state, and the cycle repeats every 20ms with a net average power of 0.75MW. (Or 133MW for DT targets).

The radiation and fusion products occur inside a silicon carbide shell. The resulting plasma (Si and C ions) is channeled out of a hole to provide thrust.

They are currently designing a small, unmanned spacecraft based on the assumption that they could get this to work if they had enough antiprotons. The plan is not to make it to a star, but rather something more modest that will travel 10,000 A.U. in a 50 year flight. Their initial design parameters have a 100kg dry mass and 400kg of fuel. Of the fuel, most of it is the SiC shell, and only 5.7 micrograms are antiprotons, which they expect will be about a years supply from Fermilab 10 years from now (also they predict it wil be 0.14 micrograms/year by 2000; don't know what it is now). All the fuel is burned in the first 4.4 years; the rest of the time the ship coasts. The alternate design, using DT pellets, burns all the fuel in only 0.1years and still makes it out to the same distance in the same time, but it needs 26 micrograms of antiprotons. (Not sure why it needs more... they didn't go into the DT simulations in too much detail, so I'm not sure which of the numbers I've given apply for DT).

They also passed out a paper that was presented at a Propulsion Symposium in October, which had more details but was a different concept; it used fewer anitprotons (30ng), a 3-day burn, designed for outer solar system missions (Jupiter in 7.5 months). It also relied on ion beams to compress much larger fuel pellets, instead of the electromagnetic trap design they presented at the conference, so I'm not sure it's comparable at all. In the paper, though, they looked into engine radiation, and most of the dangerous stuff was neutrons; they put in 2.2 meters of LiH shielding to protect a crew. Less shielding is needed to protect the antiprotons themselves. They also had a scheme to vent an extra 60MW of heat; much more than in the DHe₃ interstellar proposal.

Anyway, I thought it was interesting. If anyone wants more info, that's really all I know, so I'm not the one to ask. The people at Penn State to hunt down are G.A. Smith, R.A. Lewis (Penn State Physics Dept.), B. Dundore, J. Fulmer (Penn State Aerospace Engineering Dept) and S. Chakrabarti (Penn State Mech. Eng. Dept).

I assume that with enough antiprotons, this scheme could be scaled up for an interstellar spaceship. But the key problem is that most of the fuel is the SiC shell that provides the thrust; less than 0.1% of the fuel is the DHe₃ pellets. So the ratio of the mass of the ejected fuel to the extractable kinetic energy from the fuel is huge; 250,000 rather than the the theoretical 250 that a pure DHe₃ engine could provide. If there was a better way to couple the fusion reaction into fewer, faster thrust particles, there would be a lot of room for improvement.