A Response to Comments on ‘Sails Driven by Diverging Neutral Particle Beams’

by Paul Gilster on August 27, 2014

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

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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.

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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.

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{ 15 comments… read them below or add one }

Adam August 27, 2014 at 16:58

Hi Jim
You’re right. After making that hopeful comment I went chasing the latest experimental and simulation work. Some 2014 Japanese research on the scaling behaviour of magnetic sails indicates that the drop-off for thrust is *worse* than inverse square for large mag-sails, in the solar-wind. The chief dependence is particle number, to which thrust is related by N^1.15, thus in an inverse square particle field, like a diverging particle beam, that’s an 1/R^2.3 drop-off in thrust. Small mag-sails scale at ~1/R^2.

The good side of that is that mag-sail braking into a star-system is much easier to do, so it’s not all bad news.

James M Essig August 27, 2014 at 18:32

Re 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.

The thrusters may conceivably be just as powerful if not more so then the beam. With a solar output of a total of 4 x 10 EXP 26 watts, we have no lack of input energy.

Solar radiation may conceivably be processed by a complex optical system including positive refractive index, negative refractive index, and reflective materials.

Another methods involves converting solar energy to laser beam energy. Here, the systems can be photon pumped, thermally pumped, or electrically pumped lasers.

Thermoelectric generators for solar energy conversion to electrical power are also possible.

The mass of the solar light concentrators may be greatly reduced using inflatable structures or inflation rigidizable structures with UV hardening resins. Patents for some folks where obtained for this technology for space applications.

Alternatively an array of inflatable reflectors may be used. My brother John and I achieved full deployment of reflectors with a maximum Delta P of only 0.1 PSI or less. We have patented material for lots of reflector types including faceted or continuously tuned non-parabolic systems.

With a self hardening resin, any of the concave pressurized reflectors we disclosed can be produced in space.

Tapping a mere 1 millionth of the solar output would yield 4 x 10 EXP 20 watts and over 10 EXP 12 newtons starting out for light-thrust. Particle beams whether charged or neutral would be most efficient well below light speed. A region of contained plasma or neutral gas located in back of the craft could easily interact with the neutrons to provide not only direct thrust, but also, charged collision products which can then be electrodynamically handled.

Charged particle beam fall-back onto a magsail craft can be shielded by thick shielding of reduced mass provided the chargon fallback can be focused. The thermal energy generated may be converted to electrical power by thermo-electric systems or turbo-electric systems which may include large cooling radiators that back reflect thermal radiation to provide additional thrust.

Ideally, the backward falling chargons would simply be directed toward the center of a toroidal spacecraft so as not to irradiate the payload. Chargon buildup may be depressurized in continuous or batch mode to provide thrust.

Neutralized chargon build-up can be used as a source of nuclear fusion fuel for a fusion rocket mechanism.

Michael M August 28, 2014 at 6:00

@James Benford

Thanks for taking the time to answer and provide more information about a viable and serious contender for space expansionism.

‘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,’

‘James Benford

but the thrusters must have the same thrust as the Beamer in order to prevent some serious motion.’

The beamer would be much more massive and longer than the probe, we could resolve the momentum reaction issue by having other probes going in the opppoite direction say for a solar focus mission i.e. comms/observations of the target star. The transverse reaction could be controlled by ion thrusters easily.

‘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.’

Although B falls off quickly the magnetic moment is still there and the particles will still be influenced by the field and there is no requirement for one large field, a number of fields forming a flat surface could be used instead.

‘Alex Tolley correctly points out that deflecting high-energy particles produces synchrotron radiation, which will require some shielding of the payload.’

The synchrotron radiation although it’s rate is high the penetrative power is low due to its frequency and may aid us for propulsion by been reflected off of a mesh or even aiding finding the position of the craft in space. Further the synchrotron radiation is in a narrow cone perpendicular to the magnetic field lines so it will be away from the craft except when the charged particles pass the craft perpendicularly which would be very narrow and depleted in energy.

‘And using multiple smaller beams doesn’t reduce divergence.’

Multiple adjustable beams spaced a distance apart that follow the craft would reduce the divergence firstly by reducing the starting repulsion force ‘which is to the square’ of one tightly packed beam and secondly as the beams start to interact as they meet there would be a tendency to force the particles in the axial direction as well.

‘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.’

As you approach the speed of light and ultra-relativistic particles do, the magnetic field component tends to cancels the charged force pushing them apart.

‘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:’

Cooling the charged particles with a laser before acceleration has a significant positive effect on the transverse velocity.

‘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.’

As the neutral atom cuts the magnetic flux a voltage is induced between the electron and positive charge of the nucleus to V=B.l.v -so a very fast neutral particle could be stripped of its electrons in a weaker field.

‘Consequently, very much higher fields are necessary for deflection. That would mean either much more current or much larger diameter in the magsail.’

Not that much of a problem, conductors can be very malleable to form a coil.

‘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.’

In the moving frame of reference everything is slowed done by ‘gamma’, so any forces that contribute to divergence are slowed down as well. And finally any reduction in divergence helps but at the expense of using more energy.

Adam August 28, 2014 at 6:13

Hi James Essig,
Thermoelectric or thermoelectronic? Thermoelectronic converters use thermionic emission to create current from concentrated heat, using a magnetic field to eliminate the space charge issue that plagues purely thermionic energy conversion. They have the potential to convert raw heat to electricity at ~40% efficiency.

ProjectStudio August 28, 2014 at 9:48

Some of the modeling for beam deflections for axial and normal current loop orientations relative to incident solar wind (or ionized beam) were surprising:

http://casa.colorado.edu/~danforth/science/magsail/magsail.html

Figure 4d appears to show deflections occurring far behind and outside the current loop of a mag sail with a normal orientation (loop axis normal to incident ions). Potentially this orientation could mitigate synchrotron radiation to some degree – with enough current it would seem to make the loop act “bigger.” Given the accelerations that were considered for an interstellar beam boost, this would not be a life-friendly vessel in any case, and the radiation mitigation or shielding would need to be adequate to protect the probes systems only.

James M Essig August 28, 2014 at 14:33

Adam, good points. I was considering thermoelectric cells but thermoelectronic sounds like be a better mechanism.

Eniac September 1, 2014 at 3:00

While there are many tough issues with the particle beam concept, station-keeping of the generator is not one of them. Simply have them orbit, say, Mercury, in a forced orbit such that recoil is balanced by gravity. Surely these things are heavy enough that a slight offset of the orbit from center will do the trick.

The elephant in the room at this point has to be the mag-sail itself. In all of this discussion there is no mention of how much current would actually be needed to create a field sufficiently strong and large to stop the beam. And, consequently, what the mass of the superconducting loop would be.

Other critical issues: Would the power density of the beam be such that it would heat the loop and stop superconductivity? I think so. Would the beam be dense enough to tear the field right off the loop and carry it away, leaving the craft behind? Yes, I think moving plasma does wreak havoc on fields that way.

Lastly, there are conflicting statements and no concrete numbers on the question of whether a neutral beam would be re-ionized simply by encountering the weak, macroscopic field of the mag-sail. My feeling is no, not by quite a few orders of magnitude.

Eniac September 1, 2014 at 3:13

@Michael: Ultrarelativistic beams need a much deeper and stronger field at the sail, because they don’t bend easily. As Jim has said, this does not scale well. At all.

Eniac September 1, 2014 at 5:28

James Benford:

… 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.

Ionizing atoms requires extremely strong fields, on the order of 1 V/Angstrom, or 10^10 V/m. Could you please substantiate the claim that these relativistically induced fields are “sufficient to ionize the atom”?

Ole Burde September 2, 2014 at 12:05

”And using multiple smaller beams doesn’t reduce divergence. ”
Why is that self evident ? If the divergence is due to the reppeling of same-charge particles , then each low intensity beam will have a much bigger distance between the particles , and so less repelling force ? If the multible beams start out seperated by a great disance ,like the earth orbit diameter , it might be possibele to keep them apart for a long distance .
I’me probably wrong , but it would be nice to know why !

Michael September 3, 2014 at 2:30

@Ole Burde September 2, 2014 at 12:05

‘If the divergence is due to the reppeling of same-charge particles , then each low intensity beam will have a much bigger distance between the particles , and so less repelling force ? If the multible beams start out seperated by a great disance ,like the earth orbit diameter , it might be possibele to keep them apart for a long distance .
I’me probably wrong , but it would be nice to know why !’

As I mentioned earlier in the post if the large beam is spread to multiple beams the forces between them are much smaller as the distance between them is larger. i.e. double the distance and you quarter the force of repulsion. Increase the speed of the particles in each beam and you have a greater distance between them which is again less diverengence force. So we need multiple ultra high velocity beams with low starting temperature ionised atoms and finally an effective way of catching them at the craft.

Ole Burde September 4, 2014 at 16:34

@Michael , that was exactly what I thought from the beginning, exept that you expres it a lot better . James Benford seem to have a very different view , but he didnt explain why . It seems that he found it self-evident …. Would it be necesary to seperate the component-beams by a fysical distance or could it be done somehow utilizing the earths moovement in space ?

Eniac September 5, 2014 at 21:27

I think Michael’s argument is correct, but it leaves open the question of how to aim two or more beams such that they intersect precisely at the targeted craft. Aim has to be accurate to within whatever spread you are trying to achieve, nano-radians, as I understand, for interstellar applications. And even the weakest and most relativistic charged beams will still undergo Coulomb expansion, just less of it. I don’t think nano-radians are even remotely in sight, here.

Eniac September 5, 2014 at 21:30

Add Lorentz bending in solar or interstellar magnetic fields, and things are starting to become really hopeless, for charged beams.

Michael September 10, 2014 at 10:14

Sorry I missed these ones.

@Ole Burde September 4, 2014 at 16:34 .

@Eniac September 5, 2014 at 21:30

‘Would it be necesary to seperate the component-beams by a fysical distance or could it be done somehow utilizing the earths moovement in space ?’

Even centimetres makes a big difference, the multiple beams could all be in the same main beam system, joined.

‘but it leaves open the question of how to aim two or more beams such that they intersect precisely at the targeted craft. Aim has to be accurate to within whatever spread you are trying to achieve, nano-radians, as I understand, for interstellar applications. And even the weakest and most relativistic charged beams will still undergo Coulomb expansion, just less of it. I don’t think nano-radians are even remotely in sight, here.’

I believe nano-radians are within reach, we can get about 0.2 micro radians at the moment, now if we had a dedicated system I am confident we can approach it closely.

‘Add Lorentz bending in solar or interstellar magnetic fields, and things are starting to become really hopeless, for charged beams.’

The velocity of the particles is so high they will have little effect.

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