Centauri Dreams

Jim Benford’s article on particle beam propulsion, published here last Friday and discussed in the days since, draws from the paper he will soon be submitting to one of the journals. I like the process: By running through the ideas here, we can see how they play before this scientifically literate audience, with responses that Jim can use in tweaking the final draft of the paper. Particle beam propulsion raises many issues, not surprising given the disagreements among the few papers that have tackled the subject. Are there ways of keeping the beam spread low that we haven’t thought of yet? Does a particle beam require shielding for the payload? Does interplanetary particle beam work require a fully built infrastructure in the Solar System? We have much to consider as the analysis of this interesting propulsion concept continues. Dr. Benford is President of Microwave Sciences in Lafayette, California, which deals with high power microwave systems from conceptual designs to hardware.

by James Benford

Let me first say that I appreciate the many comments on my piece on neutral particle beam propulsion. With so many comments I can react in only a limited sense. I appreciate in particular the many comments and suggestions by Alex Tolley, swage, Peter Popov, Dana Andrews, Michael, Greg (of course), Project Studio and David Lewis.

Galacsi: The launch system as envisioned by Dana Andrews and Alan Mole would be affixed to an asteroid that would provide sufficient mass to prevent the reaction from the launch of the beam from altering the orbit of the Beamer and changing the direction of the beam itself. No quantitative valuation of this has been provided to date.

James Messick says we can have thrusters to maintain the Beamer in place, but the thrusters must have the same thrust as the Beamer in order to prevent some serious motion.

Rangel is entirely right; one has to start at lower power nearer objectives, as we have to do for all interstellar concepts.

Alex Tolley is quite correct that what is envisioned here is a series of beam generators at each end of the journey for interplanetary missions, which means a big and mature Solar System economy. That’s why I placed this in future centuries. And I agree with him that in the short term beamed electromagnetic or electric sails are going to be much more economic because they don’t require deceleration at the destination.

Adam: the Beamer requirement if the magsail expands as the pressure falls off probably doesn’t scale well, as B falls off very quickly- I don’t think the scaling justifies any optimism.

There are certainly a lot of questions about the solar wind’s embedded magnetic field. All these requirements would benefit from a higher magnetic field from the magsail, which unfortunately also increases the mass of the probe.

Alex Tolley correctly points out that deflecting high-energy particles produces synchrotron radiation, which will require some shielding of the payload. Shielded payloads are available now, due to DOD requirements. [Jim adds in an email: “Shielding is needed for the payload while the beam is on. Keep it, don’t discard, as there are cosmic rays to shield against on all flights].

Swage is correct in saying that we need to start small, meaning interplanetary, before we think large. Indeed lasers are far less efficient than the neutral beam concept. That’s because deflecting material particles is a much higher efficiency process than deflecting mere photons. Swage is completely correct about the economics of using beam propulsion.

And using multiple smaller beams doesn’t reduce divergence. ‘Would self focusing beams be an option?’ No. Charged beams don’t self-focus in a vacuum, they need a medium for that and it isn’t easy to make happen. Charged particle beams can be focused using their self-generated magnetic field only when some neutralization of charges is provided. There is also a large set of instabilities that can occur in such regimes. That’s a basic reason why charged particle beams are not being seriously considered as weapons and neutral beams are the only option.

Image: The divergence problem. A charged-particle beam will tend naturally to spread apart, due to the mutually repulsive forces between the like-charged particles constituting the beam. The electric current created by the moving charges will generate a surrounding magnetic field, which will tend to bind the beam together. However, unless there is some neutralization of the charge, the mutually repulsive force will always be the stronger force and the beam will blow itself apart. Even when the beam is neutralized, the methods used to neutralize it can still lead to unavoidable beam divergence over the distances needed for interstellar work. Image credit: Richard Roberds/Air University Review.

Peter Popov asked whether you could focus sunlight directly. You can’t focus sunlight to a smaller angular size than it fills in your sky. (That is because the sun is an incoherent source. The focusability of sunlight is limited by its incoherence, meaning that the radiation from the sun comes from a vast number of radiating elements which are not related to one another in a coherent way.) Therefore the ability to focus sunlight is limited, and is in no way related to the focusing of coherent light. However, you can increase the focusing aperture, collecting more light, making the power density higher, but the spot size doesn’t grow.

Dana Andrews’ comment that the neutral “atoms with any transverse velocity are eliminated before they are accelerated” means that you throw away all but one part in a million of the initial beam: Suppose this device, which separates particles out, reduces the divergence by 3 orders of magnitude. That implies, for a beam uniform in angular distribution, a reduction in Intensity of 1 million (because the solid angle scales with the square of the opening angle). Such a vast inefficiency is unaffordable.

For Dana & Alex Tolley, re-ionizing the beam as it reaches the magsail will not be difficult. The reason is that they are in relativistically separated frames so that the magnetic field of the magsail will appear as an electric field in the frame of the atoms, a field sufficient to ionize the atom. No on-board ionizer is required.

Michael suggests going to ultrarelativistic beams, but that means much more synchrotron radiation when the beam deflects from the magsail. Consequently, very much higher fields are necessary for deflection. That would mean either much more current or much larger diameter in the magsail. My instinct is that that does not scale well. And the divergence I described is not changed by going ultrarelativistic, as it just depends on ratios of mass and energies of electron to ion. Also, using heavier atoms helps but, with a square root dependence, not enough.

ProjectStudio also advocates that an ultrarelativistic neutral beam would have a reduced divergence, for which see above. I note again the enormous amount of radiation they produce whenever they are either deflected by the magnetic field or collide with matter. In fact, going in the Andrews/Mole concept from 0.2 c to 0.9c means the synchrotron radiation increases by a factor of 2300! That bathes the payload, as the ions swing round.

Alex Jolie is also correct in saying that we need to look into the development of beam power infrastructure. Once it’s in place economics drives down the price of transportation; the same was true for the railroads.

David Lewis seems to get the concept entirely.

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.

We saw on Friday through Jim Benford’s work that pushing a large sail with a neutral particle beam is a promising way to get around the Solar System, although it presents difficulties for interstellar work. Benford was analyzing an earlier paper by Alan Mole, which had in turn drawn on issues Dana Andrews raised about beamed sails. Benford saw that the trick is to keep a neutral particle beam from diverging so that the spot size of the beam quickly becomes much larger than the diameter of the sail. By his calculations, only a fraction of the particle beam Mole envisaged would actually strike the sail, and even laser cooling methods were ineffective at preventing this.

It seems a good time to look back at Geoffrey Landis’ paper on particle beam propulsion. I’m hoping to discuss some of these ideas with him at the upcoming Tennessee Valley Interstellar Workshop sessions in Oak Ridge, given that Jim Benford will also be there. The paper is “Interstellar Flight by Particle Beam” (citation below), published in 2004 in Acta Astronautica, a key reference in an area that has not been widely studied. In fact, the work of Mole, Andrews and Benford, along with Landis and Gerald Nordley, is actively refining particle beam propulsion concepts, and what I’m hoping to do here is to get this work into a broader context.

Image: Physicist and science fiction writer Geoffrey Landis (Glenn Research Center), whose ideas on particle beam propulsion have helped bring the idea into greater scrutiny.

Particle beams are appealing because they solve many of the evident limitations of laser beaming methods. To understand these problems, let’s look at their background. The man most associated with the development of the laser sail concept is Robert Forward. Working at the Hughes Aircraft Company and using a Hughes fellowship to assist his quest for degrees in engineering (at UCLA) and then physics (University of Maryland), Forward became aware of Theodore Maiman’s work on lasers at Hughes Research Laboratories. The prospect filled him with enthusiasm, as he wrote in an unfinished autobiographical essay near the end of his life:

“I knew a lot about solar sails, and how, if you shine sunlight on them, the sunlight will push on the sail and make it go faster. Normal sunlight spreads out with distance, so after the solar sail has reached Jupiter, the sunlight is too weak to push well anymore. But if you can turn the sunlight into laser light, the laser beam will not spread. You can send on the laser light, and ride the laser beam all the way to the stars!”

The idea of a laser sail was a natural. Forward wrote it up as an internal memo within Hughes in 1961 and published it in a 1962 article in Missiles and Rockets that was later reprinted in Galaxy Science Fiction. George Marx picked up on Forward’s concepts and studied laser-driven sails in a 1966 paper in Nature. Remember that Forward’s love of physical possibility was accompanied by an almost whimsical attitude toward the kind of engineering that would be needed to make his projects possible. But the constraints are there, and they’re formidable.

Landis, in fact, finds three liabilities for beamed laser propulsion:

• The energy efficiency of a laser-beamed lightsail infrastructure is extremely low. Landis notes that the force produced by reflecting a light beam is no more than 6:7 N/GW, and that means that you need epically large sources of power, ranging in some of Forward’s designs all the way up to 7.2 TW. We would have to imagine power stations built and operated in an inner system orbit that would produce the energy needed to drive these mammoth lasers.
• Because light diffracts over interstellar distances, even a laser has to be focused through a large lens to keep the beam on the sail without wasteful loss. In Forward’s smaller missions, this involved lenses hundreds of kilometers in diameter, and as much as a thousand kilometers in diameter for the proposed manned mission to Epsilon Eridani with return capability. This seems highly impractical in the near term, though as I’ve noted before, it may be that a sufficiently developed nanotechnology mining local materials could construct large apertures like this. The time frame for this kind of capability is obviously unclear.
• Finally, Landis saw that a laser-pushed sail would demand ultra-thin films that would need to be manufactured in space. The sail has to be as light as possible given its large size because we have to keep the mass low to achieve the highest possible mission velocities. Moreover, that low mass requires that we do away with any polymer substrate so that the sail is made only of an extremely thin metal or dielectric reflecting layer, something that cannot be folded for deployment, but must be manufactured in space. We’re a long way from these technologies.

This is why the particle beam interests Landis, who also looked at the concept in a 1989 paper, and why Dana Andrews was drawn to do a cost analysis of the idea that fed into Alan Mole’s paper. Gerald Nordley also discussed the use of relativistic particle beams in a 1993 paper in the Journal of the British Interplanetary Society. Here is Landis’ description of the idea as of 2004:

In this propulsion system, a charged particle beam is accelerated, focused, and directed at the target; the charge is then neutralized to avoid beam expansion due to electrostatic repulsion. The particles are then re-ionized at the target and reflected by a magnetic sail, resulting in a net momentum transfer to the sail equal to twice the momentum of the beam. This magnetic sail was originally proposed to be in the form of a large superconducting loop with a diameter of many tens of kilometers, or “magsail” [7].

The reference at the end of the quotation is to a paper by Dana Andrews and Robert Zubrin discussing magnetic sails and their application to interstellar flight, a paper in which we learn that some of the limitations of Robert Bussard’s interstellar ramjet concept — especially drag, which may invalidate the concept because of the effects of the huge ramscoop field — could be turned around and used to our advantage, either for propulsion or for braking while entering a destination solar system. Tomorrow I’ll continue with this look at the Landis paper with Jim Benford’s findings on beam divergence in mind as the critical limiting factor for the technology.

The Landis paper is “Interstellar flight by particle beam,” Acta Astronautica 55 (2004), 931-934. The Dana Andrews paper is “Cost considerations for interstellar missions,” Paper IAA-93-706, 1993. Gerald Nordley’s 1993 paper is “Relativistic particle beams for interstellar propulsion,” Journal of the British Interplanetary Society 46 (1993) 145-150.

Is it possible to use a particle beam to push a sail to interstellar velocities? Back in the spring I looked at aerospace engineer Alan Mole’s ideas on the subject (see Interstellar Probe: The 1 KG Mission and the posts immediately following). Mole had described a one-kilogram interstellar payload delivered by particle beam in a paper in JBIS, and told Centauri Dreams that he was looking for an expert to produce cost estimates for the necessary beam generator. Jim Benford, CEO of Microwave Sciences, took up the challenge, with results that call interstellar missions into doubt while highlighting what may become a robust interplanetary technology. Benford’s analysis, to be submitted in somewhat different form to JBIS, follows.

by James Benford

Alan Mole and Dana Andrews have described light interstellar probes accelerated by a neutral particle beam. I’ve looked into whether that particle beam can be generated with the required properties. I find that unavoidable beam divergence, caused by the neutralization process, makes the beam spot size much larger than the sail diameter. While the neutral beam driven method can’t reach interstellar speeds, fast interplanetary missions are more credible, enabling fast travel of small payloads around the Solar System.

Neutral-Particle-Beam-Driven Sail

Dana Andrews proposed propulsion of an interstellar probe by a neutral particle beam and Alan Mole later proposed using it to propel a lightweight probe of 1 kg [1,2] The probe is accelerated to 0.1 c at 1,000 g by a neutral particle beam of power 300 GW, with 16 kA current, 18.8 MeV per particle. The particle beam intercepts a spacecraft that is a magsail: payload and structure encircled by a magnetic loop. The loop magnetic field deflects the particle beam around it, imparting momentum to the sail, and it accelerates.

Intense particle beams have been studied for 50 years. One of the key features is that the intense electric and magnetic fields required to generate such beams determine many features of the beam and whether it can propagate at all. For example, intense charged beams injected into a vacuum would explode. Intense magnetic fields can make beam particles ‘pinch’ toward the axis and even reverse their trajectories and go backwards. Managing these intense fields is a great deal of the art of using intense beams.

In particular, a key feature of such intense beams is the transverse velocity of beam particles. Even though the bulk of the energy propagates in the axial direction, there are always transverse motions caused by the means of generation of beams. For example, most beams are created in a diode and the self-fields in that diode produce some transverse energy. Therefore one cannot simply assume that there is a divergence-less beam.

What I will deal with here is how small that transverse energy can be made to be. The reason this is important for the application is that the beam must propagate over the large distances, to accelerate the probe to 0.3 AU or 45,000,000 km. That requires that the beam divergence be very small. In the original paper on the subject by Dana Andrews (2), the beam divergence is simply stated to be 3 nanoradians. This very small divergence was simply assumed, because without it the beam will spread much too far and so the beam energy will not be coupled to the magsail. (Note that at 0.3 AU, this divergence results in a 270 m beam cross-section, about the size of the magsail capture area.)

Just what are a microradian and nanoradian? A beam from Earth to the moon with microradian divergence would hit the moon with a spot size of about 400 m. For a nanoradian it would be a very small 0.4 m, which is about 15 inches.

One method of getting a neutral particle beam might be to generate separate ion and electron beams and combine them. But two nearby charged beams would need to be propagated on magnetic field lines or they would simply explode due to the electrostatic force. If they are propagating parallel to each other along magnetic field lines, they will interact through their currents as well as their charges. The two beams will experience a JxB force, which causes them to spiral about each other. This produces substantial transverse motion before they merge. This example shows why the intense fields of particle beams create beam divergence no matter how carefully one can design them. But what about divergence of neutral particle beams?

Image: A beamed sail mission as visualized by the artist Adrian Mann.

Neutral Beam Divergence

The divergence angle of a neutral beam is determined by three factors. First, the acceleration process can give the ions a slight transverse motion as well as propelling them forward. Second, focusing magnets bend low-energy ions more than high-energy ions, so slight differences in energy among the accelerated ions lead to divergence (unless compensated by more complicated bending systems).

Third, and quite fundamentally, the divergence angle introduced by stripping electrons from a beam of negative hydrogen or tritium ions to produce a neutral beam gives the atom a sideways motion. (To produce a neutral hydrogen beam, negative hydrogen atoms with an extra electron are accelerated; the extra electron is removed as the beam emerges from the accelerator.)

Although the first two causes of divergence can in principle be reduced, the last source of divergence is unavoidable.

In calculations I will submit to JBIS, the divergence angle introduced by stripping electrons from a beam of negative ions to produce a neutral beam, giving the resulting atom a sideways motion, produces a fundamental divergence. It’s a product of two ratios, both of them small: a ratio of particle masses (?10^-3) and a ratio of neutralization energy to beam particle energy (?10^-7 for interstellar missions). Divergence is small, typically 10 microradians, but far larger than the nanoradians assumed by Andrews and Mole. Furthermore, the divergence is equal to the square root of the two ratios, making it insensitive to changes in ion mass and ionization energy.

In Alan Mole’s example, the beam velocity is highest at the end of acceleration, 0.2 c, twice the ship final velocity. Particle energy for neutral hydrogen is 18.8 MeV. The energy imparted to the electron to drive it out of the beam, resulting in a neutral, is 0.7 eV for hydrogen. Evaluation of Eq. 3 gives beam divergence of 4.5 microradians.

This agrees with experimental data from the Strategic Defense Initiative, SDI. The observed divergence of a 100 MeV neutral beam as 3.6 microradians; for a Triton beam (atomic weight 3), 2 microradians.

The beam size at the end of acceleration will be 411 km. Alan Mole’s magnetic hoop is 270 m in diameter. Therefore the ratio of the area of the beam to the area of the sail is 2.3 10⁶. Only a small fraction of the beam impinges on the spacecraft. To reduce the beam divergence, one could use heavier particles but no nucleus is heavy enough to reduce the beam spot size to the sail diameter.

Laser Cooling of Divergence?

Gerry Nordley has suggested that neutral particle divergence could be reduced by use of laser cooling. This method uses lasers that produce narrowband photons to selectively reduce the transverse velocity component of an atom, so must be precisely tunable. It is typically used in low temperature molecular trapping experiments. The lasers would inject transversely to the beam to reduce divergence. This cooling apparatus would be located right after the beam is cleaned up as it comes out of the injector. They would have substantial powers in order to neutralize the beam as it comes past at a fraction of the speed of light. Consequently, the coupling between the laser beam and the neutral beam is extraordinarily poor, about 10^-5 of the laser beam. This highly inefficient means of limiting divergence is impractical.

Fast Interplanetary Sailing

Beam divergence limits the possibilities for acceleration to interstellar speeds, but fast interplanetary missions look credible using the neutral beam/magsail concept. That enables fast transit to the planets.

Given that the beam divergence is fundamentally limited to microradians, I used that constraint to make rough examples of missions. A neutral beam accelerates a sail, after which it coasts to its target, where a similar system decelerates it to its final destination. Typically the accelerator would be in high Earth orbit, perhaps at a Lagrange point. The decelerating system is in a similar location about another planet such as Mars or Saturn.

From the equations of motion, To get a feeling for the quantities, here are the parameters of missions with sail probes with microradian divergence and increasing acceleration, driven by increasingly powerful beams.

The first column shows a fast interplanetary probe, with high interplanetary-scale velocity, acceleration 100 m/sec², 10 gees, which a nonhuman cargo can sustain. Time required to reach this velocity is 27 minutes, at which time the sail has flown to 135,000 km. The power required for the accelerator is 24GW. If the particle energy is 50MeV, well within state-of-the-art, then the required current is 490A. How long would an interplanetary trip take? If we take the average distance to Mars as 1.5 AU, the probe will be there in 8.7 days. Therefore this qualifies as a Mars Fast Track accelerator.

An advanced probe, at 100 gees acceleration, requires 0.78 TW power and the current is 15 kA. It takes only 34 hours to reach Mars. At such speeds the outer solar system is accessible in a matter of weeks. For example, Saturn can be reached by a direct ascent in the time as short as 43 days.

A very advanced probe, an Interstellar Precursor, at 1000 gees acceleration, reaches 0.8% of light speed. It has a power requirement 34 TW and the current is 676 kA. It takes only 8 hours to reach Mars. At such speeds the outer solar system is accessible in a matter of days. For example, Saturn can be reached by a direct ascent in the time as short as a day. The Oort Cloud at 2,000 AU, can be reached in 6 years.

Implications

The rough concepts that have been developed by Andrews, Mole and myself show that neutral beam-driven magnetic sails deserve more attention. But the simple mission scenarios described in the literature to date don’t come to grips with many of the realities. In particular, the efficiency of momentum transfer to the sail should be modeled accurately. Credible concepts for the construction of the sail itself, and especially including the mass of the superconducting hoop, should be assembled. As addressed above, concepts for using laser cooling to reduce divergence are not promising but should be looked into further.

A key missing element is that there is no conceptual design for the beam generator itself. Neutral beam generators thus far have been charged particle beam generators with a last stage for neutralization of the charge. As I have shown, this neutralization process produces a fundamentally limiting divergence.

Neutral particle beam generators so far have been operated in pulsed mode of at most a microsecond with pulse power equipment at high voltage. Going to continuous beams, which would be necessary for the minutes of beam operation that are required as a minimum for useful missions, would require rethinking the construction and operation of the generator. The average power requirement is quite high, and any adequate cost estimate would have to include substantial prime power and pulsed power (voltage multiplication) equipment, a major cost item in the system. It will vastly exceed the cost of the magnetic sails.

The Fast Interplanetary example in table 1 requires 24 GW power for 27 minutes, which is an energy of 11 MW-hours. This is within today’s capability. The Three Gorges dam produces 225 GW, giving 92 TWhr. The other two examples cannot be powered directly off the grid today. So the energy would be stored prior to launch, and such storage, perhaps in superconducting magnets, would be massive.

Furthermore, if it were to be space-based the heavy mass of the high average power required would mean a substantial mass System in orbit. The concept needs economic analysis to see what the cost optimum would actually be. Such analysis would take into account the economies of scale of a large system as well as the cost to launch into space.

We can see that there is in Table 1 an implied development path: a System starts with lower speed, lower mass sails for faster missions in the inner solar system. The neutral beam driver grows as technology improves. Economies of scale lead to faster missions with larger payloads. As interplanetary commerce begins to develop, these factors can be very important to making commerce operate efficiently, counteracting the long transit times between the planets and asteroids. The System evolves.

We’re now talking about matters in the 22nd and 23rd centuries. On this time scale, neutral beam-driven sails can address interstellar precursor missions and interstellar itself from the standpoint of a much more advanced beam divergence technology than we have today.

References

Alan Mole, “One Kilogram Interstellar Colony Mission”, JBIS, 66, pp.381-387, 2013

Dana Andrews, “Cost Considerations for Interstellar Missions”, Acta Astronautica, 34, pp. 357-365, 1994.

Ashton Carter, Directed Energy Missile Defense in Space–A Background Paper, Office of Technology Assessment, OTA-BP-ISC-26, 1984.

G. A. Landis, “Interstellar Flight by Particle Beam”, Acta Astronautica, 55, pp. 931-934, 2004.

G. Nordley, “Jupiter Station Transport By Particle Beam Propulsion”, NASA/OAC, 1994. And http://en.wikipedia.org/wiki/Laser_cooling.