Beaming to a Magnetic Sail

by Paul Gilster on August 26, 2014

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.

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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.

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{ 8 comments… read them below or add one }

Eric Hughes August 26, 2014 at 15:18

>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 [...]

I think it’s important to recall that Benford’s article last Friday only addresses one class of methods for making a neutral particle beams. He acknowledges that himself in the last sentence of the article, when he speaks of “much more advanced beam divergence technology than we have today”. It’s also worth recalling van der Meer’s work on stochastic cooling, for which he won the Nobel.

The design space here is large. Rather than speaking of particle beams or droplet beams or pellet beams as separate kinds of things, it seems better to consider matter beams as a whole in the first instance and to consider distinctions only after a common analysis applicable to the class as a whole.

Ole Burde August 26, 2014 at 16:01

Focusing a beam of charged particlers is a tecnical problem that might have number of very different solutions , like most other tecknology bottlenecks .
My first thought is that the electrical charge is the key to a solution , a way of manipulating the particle which should only be neutralized after the beam is stabilized . I seem to remeber that solar wind particles somhow flow paralel to magnetic fieldlines …perhabs we need a mixed beam composed of ALMOST ecual charges …The second thought is that i may be easyer with many small low intensity particle accelerators than with a single big one , reducing the ”explosive” efect of same-charge proximity . The multible beams would only meet up at the target , and would not have to be of the same velocity . ..Why would the particles not be gradually cooled by emitting infrared radiation , for possible condensation ?

Craig Watkins August 26, 2014 at 17:44

Maybe this is a dumb question, since much of the technical information is beyond me, but if the main problem is divergence over distance, why couldn’t you have another beam generator placed out at a further distance? Once the first beam generator’s beam wasn’t efficient enough, another one might be able to continue to accelerate the craft. It seems like you could continue this pattern of placing more generators along the intended flight path until the craft approached a more practical interstellar speed.

Andrew Palfreyman August 26, 2014 at 17:45

Obviously the magsail, with all its areal mass density advantage over a light sail, will do us no good with neutral beams, and so if we wish to emit a neutral beam, we’ll need a pre-ioniser attached to the craft upstream of the magsail. Have any designs been thought out to implement said ioniser?

ljk August 27, 2014 at 9:59

About placing those generators along the flight path: Ignoring for the moment the fact that you would need a mode of transportation to get them there in the first place, how would one keep them staying put? What about maintenance? And what if you want to go in more directions in the galaxy? This is going to get awfully expensive to build, maneuver, and maintain – and yes, so long as humans remain human money will be a big deciding factor in the future.

Brett Bellmore August 28, 2014 at 6:28

Suppose it were refocusing stations, not generators? How far apart could they be, and still keep the beam columated enough for interstellar use?

And could they be built with holes in the middle a star ship could thread along it’s way?

Craig Watkins August 28, 2014 at 13:58

ljk,

After thinking it through, I think the real problem is that the stations would get diminishing returns due to the velocity of the vehicle taking it out of range faster and faster. Even the second time, it would get less than 2 minutes of acceleration before it passed out of range. Getting to something like 10% the speed of light would be nigh impossible. That’s the sort of speed I’d like to see for interstellar work. For interplanetary work, the extra expense and trouble of even one more station is hardly justified.

Brett Bellmore August 28, 2014 at 20:25

Considering this, suppose your neutral particle beam, before it has diverged horribly, were to pass through a zone where Doppler cooling was used to reduce it’s “temperature” perpendicular to be beam axis, and perhaps to narrow the beam as well? It would resume diverging at a reduced rate, and at some point, another beam cooling and focusing station would clean it up some more.

The penumbra of the beam could be used to push these stations along, while new stations were assembled around the beam and launched. Eventually you’d have a long, long string of these stations, at ever wider intervals, keeping the beam contained. It seems to me that the sideways velocity of the particles in a beam could be reduced to almost trivial levels compared to their relativistic velocity along the beam. Maybe even to the point where the beam could gravitationally self-focus, in the limit.

Since the only thing present in the beam itself for this concept is electromagnetic radiation, were the beam cleanup stations large enough in diameter, a star ship could ride the beam right through the middle of one.

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