Jim Benford’s work on particle beam propulsion concepts, and in particular on the recent proposal by Alan Mole for a 1 kg beam-driven interstellar probe, has demonstrated the problem with using neutral particle beams for interstellar work. What we would like to do is to use a large super-conductor loop (Mole envisions a loop 270 meters in diameter) to create a magnetic field that will interact with the particle beam being fired at it. Benford’s numbers show that significant divergence of the beam is unavoidable, no matter what technology we bring to bear.

That means that the particle stream being fired at the receding starship is grossly inefficient. In the case of Mole’s proposal, the beam size will reach 411 kilometers by the end of the acceleration period. We have only a fraction of the beam actually striking the spacecraft.

This is an important finding and one that has not been anticipated in the earlier literature. In fact, Geoffrey Landis’ 2004 paper “Interstellar Flight by Particle Beam” makes the opposite statement, arguing that “For a particle beam, beam spread due to diffraction is not a problem…” Jim Benford and I had been talking about the Landis paper — in fact, it was Jim who forwarded me the revised version of it — and he strongly disagrees with Landis’ conclusion. Let me quote what Landis has to say first; he uses mercury as an example in making his point:

[Thermal beam divergence] could be reduced if the particles in the beam condense to larger particles after acceleration. To reduce the beam spread by a factor of a thousand, the number of mercury atoms per condensed droplet needs to be at least a million. This is an extremely small droplet (10-16 g) by macroscopic terms, and it is not unreasonable to believe that such condensation could take place in the beam. As the droplet size increases, this propulsion concept approaches that of momentum transfer by use of pellet streams, considered for interstellar propulsion by Singer and Nordley.

We’ve talked about Cliff Singer’s ideas on pellet propulsion and Gerald Nordley’s notion of using nanotechnology to create ‘smart’ pellets that can navigate on their own (see ‘Smart Pellets’ and Interstellar Propulsion for more, and on Singer’s ideas specifically, Clifford Singer: Propulsion by Pellet Stream). The problem with the Landis condensed droplets, though, is that we are dealing with beam temperatures that are extremely high — these particles have a lot of energy. Tomorrow, Jim Benford will be replying to many of the reader comments that have come in, but this morning he passed along this quick response to the condensation idea:

Geoff Landis’ proposal to reduce beam divergence, by having neutral atoms in the particle beam condense, is unlikely to succeed. Just because the transverse energy in the relativistic beam is only one millionth of the axial energy does not mean that it is cool. Doing the numbers, one finds that the characteristic temperature is very high, so that condensation won’t occur. The concepts described are far from cool beams.

Where there is little disagreement, however, is in the idea that particle beam propulsion has major advantages for deep space work. If it can be made to work, and remember that Benford believes it is impractical for interstellar uses but highly promising for interplanetary transit, then we are looking at a system that is extremely light in weight. The magsail itself is not a physical object, so we can produce a large field to interact with the incoming particle stream without the hazards of deploying a physical sail, as would be needed with Forward’s laser concepts.


Image: The magsail as diagrammed by Robert Zubrin in a NIAC report in 2000. Note that Zubrin was looking at the idea in relation to the solar wind (hence the reference to ‘wind direction’), but deep space concepts involve using a particle stream to drive the sail. Credit: Robert Zubrin.

Another bit of good news: We can achieve high accelerations because unlike the physical sail, we do not have to worry about the temperature limits of the sail material. The magnetic field is not going to melt. Although Landis is talking about a different kind of magsail technology than envisioned by Alan Mole, the point is that higher accelerations come from increasing the beam power density on the sail, and that means cruise velocity is reached in a shorter distance. That at least helps with the beam divergence problem and also with the aiming of the beam.

Two other points bear repeating. A particle beam, Landis notes, offers much more momentum per unit energy than a laser beam, so we have a more efficient transfer of force to the sail. Landis also points to the low efficiency of lasers at converting electrical energy, “typically less than 25% for lasers of the beam quality required.” Even assuming future laser efficiency in the fifty percent range, this contrasts with a particle beam that can achieve over 90 percent efficiency, which reduces the input power requirements and lowers the waste heat.

But all of this depends upon getting the beam on the target efficiently, and Benford’s calculations show that this is going to be a problem because of beam divergence. However, the possibility of fast travel times within the Solar System and out as far as the inner Oort Cloud make neutral particle beams a topic for further study. And certainly magsail concepts retain their viability for interstellar missions as a way of slowing the probe by interacting with the stellar wind of the target star.

I’ll aim at wrapping up the current discussion of particle beam propulsion tomorrow. The image in today’s article was taken from Robert Zubrin and Andrew Martin’s “The Magnetic Sail,” a Final Report for the NASA Institute of Advanced Concepts in 2000 (full text). The Landis paper is “Interstellar flight by particle beam,” Acta Astronautica 55 (2004), 931-934.