The name Proxima will always have resonance with interstellar theorists given that our nearest target — and one with a potentially life-bearing planet at that — is Proxima Centauri. Thus an acronym with the same pronunciation is bound to catch the attention. PROCSIMA stands for Photon-paRticle Optically Coupled Soliton Interstellar Mission Accelerator, one of 25 early-stage technology proposals selected for Phase I funding by the NASA Innovative Advanced Concepts (NIAC) office. A number of Phase II proposals selected for funding was also announced.
These awards are always fascinating to watch because they’re chosen from a host of bleeding edge ideas, helping us keep a finger on the pulse of deep space thinking even if many of them end with their Phase I funding, $125,000 over nine months to produce an initial definition and analysis. Should the results be encouraging, Phase II funding becomes a possibility, ramping the money up to $500,000 over two years to encourage further development.
The 2018 Phase I competition involved over 230 proposals and just 25 winners, a tough selection process that resulted in a number of interesting proposals. NIAC works by fostering ideas from a wide range of scientists working outside NASA’s umbrella, as Jim Reuter, acting associate administrator of NASA’s Space Technology Mission Directorate, notes:
“The NIAC program gives NASA the opportunity to explore visionary ideas that could transform future NASA missions by creating radically better or entirely new concepts while engaging America’s innovators and entrepreneurs as partners in the journey. The concepts can then be evaluated for potential inclusion into our early stage technology portfolio.”
Creating a Tight Beam
We’ll have plenty to work with over the next few days, but I’ll start with PROCSIMA, which comes from Chris Limbach (Texas A&M Engineering Experiment Station), and points to the possibility of solving a tricky problem in beamed propulsion. Specifically, if you’re using a laser beam to push a sail, how can you reduce the spread of the beam, keeping it collimated so that it will disperse as little as possible with distance? A perfectly collimated beam seems impossible because of diffraction, thus limiting the length of time our sail can remain under acceleration.
Image: Texas A&M’s Christopher Limbach. Credit: Texas A&M.
Particle beams, which actually offer more momentum per unit energy than laser beams, likewise tend to diverge, although as we’ve seen in earlier articles, the nature of the divergence is problematic (see the contrasting views of Jim Benford and Geoff Landis on the matter, as in Beaming to a Magnetic Sail). Particle beams might turn out to be just the ticket for fast in-system transportation as far out as the Oort Cloud, while being limited because of beam spread when it comes to interstellar applications. That makes divergence an issue for both types of beam.
But I should quote Geoff Landis (NASA GRC) first, because he thinks the neutral particle beam problem can be surmounted. Landis works with mercury in his example:
[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.
Benford sees the divergence problem as fundamental. Charged beams would interact, spiraling around each other to produce transverse motion that creates beam divergence. Neutral particle beams would seem to be the ticket if Landis is right, but Benford sees three problems. Let me quote him (from Sails Driven by Diverging Neutral Particle Beams; a JBIS paper on these matters has been accepted for publication but is not yet available):
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.)
Reducing the first two causes of beam divergence, Benford believes, is theoretically possible, but he sees the last source of divergence as unavoidable, nor does he accept Gerald Nordley’s idea of reducing neutral particle beam divergence through laser cooling. And he finds Geoff Landis’ idea of having neutral atoms in the particle beam condense (see Landis citation below) to be unlikely to succeed. Are our beaming strategies hopelessly compromised by all this?
PROCSIMA tries to get around the problem by combining a neutral particle beam and a laser beam, a technique that, according to Chris Limbach, could prevent spread and diffraction in both kinds of beam. Let me quote him from the NIAC description:
The elimination of both diffraction and thermal spreading is achieved by tailoring the mutual interaction of the laser and particle beams so that (1) refractive index variations produced by the particle beam generate a waveguide effect (thereby eliminating laser diffraction) and (2) the particle beam is trapped in regions of high electric field strength near the center of the laser beam. By exploiting these phenomena simultaneously, we can produce a combined beam that propagates with a constant spatial profile, also known as a soliton.
Image: Graphic depiction of PROCSIMA: Diffractionless Beamed Propulsion for Breakthrough Interstellar Missions. Credit: C. Limbach.
An interesting concept because it draws from recent work in high-energy lasers as well as high-energy neutral particle beams, producing a hybrid notion that seems worth exploring. In his precis on PROCSIMA, Limbach says he believes this beamed propulsion architecture would increase the probe acceleration distance by a factor of ~10,000, allowing us to send a 1 kg payload to Proxima Centauri at 10 percent of lightspeed, making for a 42-year mission.
We watch laser developments with interest particularly with regard to Breakthrough Starshot, which assumes a similar high-energy laser capability in the 50 GW range. Starshot is an investigation into nano-scale payloads carried by small beamed sails to nearby stars. Can we tap neutral particle beam technology to achieve increased delta-V, solving the diffraction problem at the same time? As Limbach points out, such technologies are a hot topic within the nuclear fusion community, which looks at heating magnetically confined fusion plasmas.
Expect more on this concept in short order. The Landis paper is “Interstellar flight by particle beam,” Acta Astronautica 55 (2004), 931-934. I’ll have a full citation on the Benford paper as soon as it is published.
Is there a more detailed proposal for this? Will they do lab work, for example? Will they construct a minimal prototype?
More soon, including answers to questions like these. I’ll get the next post on PROCSIMA out as soon as I can, but it shouldn’t be long.
Thanks, Paul. No problem, take your time.
I don’t get this proposal ….???
PROCSIMA is an attempt to create a sail probe-pushing laser beam that would not diverge (spread out) as it traveled farther from the laser beam projector (this beam divergence greatly limits the distance–the “acceleration run”–over which a laser can impart useful amounts of energy [acceleration] to lightsail-equipped interstellar probes), and:
Our current technologies require that such a lightsail starprobe–if it is to reach velocities of 10% – 20% of c, the speed of light–must meet both of two requirements:
[1] The probe must be ^very^ lightweight (just a handful of grams in mass, or even less [perhaps no more than 1 gram, not including the lightsail]), which would be very difficult to achieve. The probe would have to endure crushing acceleration forces, yet all of its systems–instruments, camera, power supply, communications laser, laser photon thrusters, integral structural substrate, etc.–must “fit” within that extremely low “mass budget.” Also:
[2] The laser that pushes the lightsail probe up to such tremendous velocities would have to emit tens of terawatts of power, in order to accelerate the probe up to 0.1 – 0.2 c in the limited “acceleration run” distance, before the laser beam’s divergence–which would make the beam much wider than the lightsail (so that most of the beam would miss it)–would make its photon thrust on the sail too weak to be effective. Making this problem even harder to solve is the fact that no *single* laser of the required output power could be built using today’s technology. An array of many–perhaps thousands or even tens of thousands–much smaller lasers’ combined beams would have to be used, but combining so many small beams to form a single, collimated, multi-deca-terawatt laser “thrust beam” would be very difficult, but:
The PROCSIMA concept, if it could be made to work, would reduce the above-described difficulties. Its “composite beam” (a combination of a laser beam and a neutral particle beam) would eliminate divergence (spreading) of the laser beam. This would provide a lightsail probe “acceleration run” distance of indefinite–and potentially, in theory–*infinite* length. This would enable a much less powerful laser “thrust beam” to be used, and it could also “take its own sweet time” in accelerating the lightsail probe up to a significant fraction of the speed of light (since the “acceleration run” distance could be as long as desired). As well:
This would also have a “*positive* knock-on effect” on the probe’s design, because it could be more massive than the very tiny, midge-like Breakthrough Starshot probe design (the PROCSIMA folks are speaking in terms of 1-kilogram [2.2-pound] starprobes). Because a PROCSIMA “composite beam”-pushed lightsail probe could be accelerated at a much lower rate of acceleration than the Breakthrough Starshot probes, the PROCSIMA probe need not be designed to endure bullet-like (actually, much greater than this–and for much longer–for the Breakthrough Starshot probes) acceleration. Plus:
Hopefully, the PROCSIMA “beam scheme” will work as its advocates hope, for it would make *fast* (involving trip times shorter than a human lifetime), laser-pushed lightsail “exoplanetary system fly-through” interstellar probes less challenging to develop and fly. If such probes were dispatched in “streams,” they could examine such systems’ stars and planets in detail, despite their quick views–they would be like tens, hundreds, or perhaps even thousands (if the probes could be mass-produced cheaply) of sequentially-flying ‘still-photo’ cameras (ditto for the probes’ instruments’ views).
A beam that does not disperse through space and time might also make a good instrument for METI. Just how powerful can these beams get, how much data could they carry, and how far could they reach to dispense useful information to any recipients?
A million mercury atoms must mass many orders less than 10-16 g. I’m guessing a Greek symbol failed to print in the text I am viewing.
An error at this end, inserting a bit of bogus code. Sorry…
The originator of the suspected omitted hyphen (“-“) in the guidance program of Mariner 1’s Atlas-Agena B launch vehicle would probably have gladly traded places with you (your error didn’t “splash” a multi-million-dollar Venus flyby probe, America’s first). :-)
Good point! I feel better now.
Could this possibly solve the problem of shielding against interstellar dust and gas, by punching a hole through the interstellar medium, that the ship or probe would travel through?
OK, crazy idea here: If you take the combined beam, and put it through something similar to a traveling wave accelerator, you could transfer energy from the photonic component to the particle component. As the particle component is traveling slower, and is more massive, this ought to result in a reaction force that would slow the spacecraft.
Anyway, looking forward to reading the paper.
Brett, are you referring to a TWT (Traveling-Wave Tube [the broadband RF amplifier that is the heart of satellite and space probe radio systems])–or something similar to it? Also:
I can’t answer your question, but your ISM (interstellar medium [gas and dust]) shielding idea–and your idea about using the “composite [laser light and neutral particles] beam” as a reservoir of energy for producing braking thrust for the probes–both sound plausible to me, and I think both possibilities should be looked into. The PROCSIMA concept also suggests exciting possibilities, including the following:
If the beam divergence can be eliminated as they hope, interstellar probes with unlimited range, and (theoretically) with *no* top speed (getting closer and closer to c without actually reaching it), could be possible, and:
Even if beam divergence couldn’t be totally eliminated, but “only” greatly eliminated (such that the divergence at the distance of, say, Alpha Centauri, or Altair, was unimportant, but became significant at greater distances), this would revolutionize interstellar travel *and* communication via laser. Laser-pushed lightsail spacecraft (starprobes and starships) intended for journeys of such distances could be accelerated by Earth-, Moon-, or solar orbit-based lasers (or laser arrays) at more leisurely rates of acceleration without worries about “running out of sufficient laser thrust.” Similarly, laser communication–both to and from the interstellar spacecraft–could be achieved using small and lightweight laser optics/sensor/transmitting laser units aboard the spacecraft–yet they would have robust (with tight beams, capable of high data transfer rates), high-quality communications link circuits.
“Brett, are you referring to a TWT (Traveling-Wave Tube [the broadband RF amplifier that is the heart of satellite and space probe radio systems])–or something similar to it? ”
Somewhat: The same exact principle is used in some particle accelerators. If the group velocity of the EM wave is arranged to be slower than the charged particles, energy flows from the particles to the wave. If it’s the EM wave that’s traveling faster, it flows from the wave to the particles.
The difficulty is that it’s tough to influence group velocity without apparatus that’s comparable in size to the wavelength of the EM wave.
But, still, you’re supplying mass, and supplying energy. In principle the opportunity is there to use the combined beam to slow down.
Ah–thank you for explaining and clarifying that. If it could be made to work (even “just” keeping a laser beam tightly-focused for a significantly greater distance than a “purely photonic” laser beam), the PROCSIMA composite laser/neutral particle beam could power a laser-photovoltaic array-powered ion drive starprobe, which could use a smaller-than-otherwise photovoltaic array (and the composite beam projector might be able to be of smaller aperture as well).
I am curious how the concept is supposed to work. I think I intuitively understand how the laser is meant to stay contained. It could work the way a fiber optic cable works – a central core which is slightly less transparent than the surrounding glass. This refracts any stray photons back toward the core. The centerline of this particle beam would be slightly less transparent than the surrounding vacuum of space. So, that could be a mechanism by which the laser refracts back toward the centerline.
But I don’t understand what forces are supposed to bring stray particle beam particles back toward the centerline. If my intuition about the laser is accurate, then I’d intuitively expect there to be a small outward force on the particles caused by them bending the photons back inward.
Anyway, regardless of the way this system works, it suffers from a problem I see with other particle beam concepts – how do you test and scale it? The way photons basically don’t interact with each other makes it straightforward to develop a laser on a smaller scale and scale it up with confidence. The way incoming photons from light years away cruise straight toward our telescopes gives us confidence that outward photons will also cruise just as straight. But particle beams? Particles interact with each other, and with interplanetary effects, in ways which make it hard to predict scaling. Just because a low power short range particle beam works nicely, that doesn’t tell us how a high power long range particle beam will work. When we try to crowd more particles into the beam, its behavior will change in ways that aren’t relevant to lasers. If we try to constructively overlap multiple particle beams it…okay it just doesn’t work that way. If we try to constructively overlap multiple laser beams, we can predict what will happen with confidence.
For these reasons, I’m not really a fan of particle beam schemes. I’m more comfortable with things like laser sails and sail-beam (which uses a bunch of laser sail drones for superior range).
“ If my intuition about the laser is accurate, then I’d intuitively expect there to be a small outward force on the particles caused by them bending the photons back inward.”
Indeed, this is an important point. Momentum IS conserved. But a diverging beam has zero net radial momentum about some axis. If the proposal can use radial momentum on one side of the beam to cancel out radial momentum on the other side, it can still work. But there needs to be some dissipative effect present to suppress instabilities. Would the beam naturally cool by evaporation?
And, of course, the beam would have to be extraordinarily transparent to the electromagnetic component, because any absorbed photons would be re-emitted in random directions, resulting in both the particle and photon being lost from the beam.
I’m really looking forward to this paper.
I wonder, Isaac, if the same basic idea that you described in relation to the PROCSIMA laser/neutral particles “composite beam” (the fiber optic cable “model”) might also be able to work purely with photons? Such a “multi-strand ‘cable’ of laser beams” might be “wound” using laser beams of different energy intensities (and/or different wavelengths–say, various ultraviolet through X-ray wavelengths), such that over great distances, a remaining portion of the beam would impart the desired amount of photon thrust.
At the relevant energy densities, photons don’t cause any difference in the refractive index of space. Basically, photons just sail through each other like the others aren’t even there. As such, there’s no way for some photons to cause other photons to refract back toward the beam (or any other direction).
They have wave properties though and can influence each other.
I’m not completely sure if that is the case. Tests of NVIS (Near-Vertical Incidence Skywave) radio communication (where signals are transmitted nearly straight up to be reflected back down by the ionosphere, providing location-untraceable communication over an area several hundred kilometers wide; it was discovered during World War II and has been utilized ever since) indicate that directly overhead, there is a small “hole” in the “coverage dome,” and:
This “hole” results because the upgoing and downcoming signals can’t occupy the same location at the same time (at the same but opposed phase, if memory serves), and they mutually cancel each other out. At other “upgoing/downcoming signal intersection angles” (again, if I recall correctly), even mutual strengthening can occur (to the received, reflected NVIS signal); since the laser beams would all be travelling in the same direction, maybe mutual cancellation like that in the NVIS overhead “hole” wouldn’t occur? But I can see how the different environments might make a big difference; NVIS radio signal propagation takes place in the thin plasma of the ionosphere, while the laser “sail thrust beams” will travel through the near-perfect vacuum of outer space.
Now this is a very exciting idea if possible as the payload is like a 1U CubeSat and likely to offer much more reasonable instruments than a 1 gram computer chip.
I hope we get more details on the beam and the possible probe configurations. If the particle beam is neutral, what is being proposed to absorb/reflect the momentum?
Alex Tolley wrote, in part, “If the particle beam is neutral, what is being proposed to absorb/reflect the momentum?”
That’s a good and interesting question. One possibility might be simple “direct, impact ‘impartation’ of energy” (elastic collision, like levitating an object with a high-velocity stream of air or water, directed against it from below). Also:
Even a beam of electrically-neutral atoms might be able to be manipulated electromagnetically. (Un-ionized water and carbon dioxide molecules–and probably many other compounds’ molecules as well–orient themselves [they rotate to point in certain directions, that is] in magnetic fields.) Or:
Something like an “ion engine in reverse,” perhaps powered by the laser component of the “composite propulsion beam,” might be used. Instead of ionizing on-board propellant and then ejecting it electrostatically at high velocity, the “reverse ion engine” would ionize the neutral atoms of the incoming “composite beam” electrostatically, by means of one or more charged grids or grid/wire combinations. (They might be physically similar to the large, charged ISM [interstellar medium] braking grids [or grid/wire systems] that Greg Matloff and Eugene Mallove covered in “The Starflight Handbook.”) The second of two, spaced grids (or perhaps a single grid, if it was a grid/wire system) could be charged in such a way as to repel the atoms of the beam, once they were ionized by the first grid (or by the wire), imparting thrust to the vehicle from the beam.
I am glad that these ideas are being investigated. Even if this “composite beam”-pushed sail probe idea doesn’t come to fruition (which I am *not* suggesting will turn out to be the case, at this early stage!), these concepts, even in a “degraded form,” could be useful. For example:
“Looking at this composite beam generator from the other end,” far more effective particle beam, photon, and ion thrusters (‘traditional’ gridded ion engines as well as HETs [Hall Effective Thrusters]) could result from the development of this technology. Multiple-grid ion thrusters having greater exhaust velocities (and possibly with much less–or even no–grid erosion) could be developed using this concept, and:
Even partially-effective beam divergence prevention (where beam divergence would still occur, but at a much lower percentage per AU [Astronomical Unit] of distance than otherwise) would be useful for more effective interplanetary *and* interstellar communications. This would enable smaller optical reflector (or lens system)/detector (and/or laser transmitter) systems to be used, both aboard the spacecraft and on the ground (or in Earth orbit or on the lunar surface); also:
This “composite-beam” communications system might also be capable of operating, at least over interplanetary distances, on two totally separate “channels” (one laser photon-based, one particle-based [the laser beam, at least, could also carry many different frequency channels, as they already can, of course]). The particle beam “channel” might be able to utilize dome sort of antenna (perhaps using sequential pulses of particles, forming a type of code, which an electrostatic or electromagnetic director could focus onto a patch antenna, creating pulsed charges to replicate the code at the receiving end). In addition:
The physics behind LDEs (Long-Delay Echoes) of radio signals appear to involve naturally-formed, electron-cloud wave guides, and also plasma structures in the ionosphere which amplify radio signals, forming the LDEs. The physics of these effects, if rigorously studied and understood, might be of help in learning how to produce the composite-beam, laser/neutral particle soliton waves.
Another possible application of these composite-beam (laser/neutral particle) soliton waves just occurred to me. They might be useful for *finally* achieving the development of controlled thermonuclear fusion reactors…including, perhaps, nuclear pulse and/or “steadily thrusting” fusion engines (either rockets or ramjets, such as catalytic ramjets, RAIR [Ram-Augmented Interstellar Rocket] engines, etc.) for propelling starprobes and starships.
OK, I get it. They are going to launch their own fiber optics in form of the profiled electron (ion) gas and use the light beam fields to stabilize it. Looks like they want to eat the cake and eat it, my guess is what the universe will say “no you are not that smart”, mostly because high-current beams are inherently unstable (and one needs a very high-current beam to make a noticeable refraction) and these instabilities are poorly coupled with the beam, but I would really appreciate looking at the math.
I don’t think the beam actually needs to be ionized to work. True, the refractive index of the beam would be barely distinguishable from ordinary vacuum, but if the photon beam started out with extremely low divergence, you wouldn’t need much refraction to keep it focused. Just as an already low divergence neutral particle beam wouldn’t need much focusing itself.
You do need to suppress instabilities, of course. But the fact that the light and the particles are moving at different speeds should help with that.
Mind, even with the best simulation software, every time they take fusion reactors into another regime, they find new instabilities. This would require real world testing, which could only happen in space. And since you’d need to know what was happening a long way out, really, the test would have to propel an actual probe.
The biggest problem I can see, assuming the physics works, is that while this will keep the beam from diverging, it doesn’t keep it pointed in exactly the same direction. The initial stability of the beam pointing would have to be fantastically good, to control the beam location many astronomical units out.
“This would require real world testing, which could only happen in space.”
What about inside LIGO?
That would–happily–be the perfect “excuse” for testing the composite (laser/neutral particles) beam & lightsail system. Since only in-space tests, conducted out to ever-greater distances, would be able to reveal the system’s actual effective range, one or more relatively inexpensive sail probes–pushed by relatively low-power composite beam systems (which could be based in Earth orbit, on the ISS, or even on the Moon)–could push the sail probes as far as they could be meaningfully accelerated (or as far out as their signals could still be heard). Such engineering test vehicles could, like the Pioneer 0 – 4 lunar probes of 1958 – 59, return boatloads of “free bonus scientific data” on all sorts of celestial bodies and phenomena.
I’d like to see a plausible transverse pressure analysis to make this credible. Jim’s right on the minimum divergence, too–too large to have long tight beams.
Some great comments here already, and I’m assuming that Professors Lubin and Bae, at least, will be all over this as soon as possible. Here’s hoping this is a genuine shot in the arm for StarShot.
One further advantage, which I don’t yet see mentioned, is the likelihood that such a low-divergence system lends itself more readily to the Q-amplification scheme of Bae, whereby the available motive power of the beam upon the target may be multiplied manyfold by consecutive reflections from both source and target mirrors.
You’re right–Young Bae’s first photonic laser thruster test rig produced palpably significant “recycled photon” thrust which he felt on a small mirror that he was holding in his hand (its orientation with respect to the laser projector wasn’t even close to the precision he thought was needed–yet it pushed on his hand)! Being able to keep such a “recycled” beam tightly-focused should increase the effect’s ability to generate photonic thrust.
Cygnus A: seems nature is way ahead of us!
http://images.slideplayer.com/27/9021853/slides/slide_6.jpg
https://cf.ppt-online.org/files1/slide/v/VycTx0fERL4mohF8JN1iCG5aQW3lI7Yjrd96nA/slide-11.jpg
Darpa Black Hole Projects – Tons of of money in little info out! ;-)
This has all the trappings of President Reagan’s SDI, only 35 years later. This would work well for piercing through the atmosphere, which was the main problem with taking missles out. I’m sure the U.S. military, Russia and China have all the details work out for such weapons and are finally being released in a water down version for civilian use.
Soliton Beams to the end of the Universe!
Soliton beam propagation; space-time behaviour and spectral features
S Maneuf, A Barthelemy and C Froehly
Journal of Optics, Volume 17, Number 3
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Abstract
Single mode laser beams may suffer from unstable breakdown (self-focusing) in materials exhibiting power dependence of their refractive index. This well-known instability appears suddenly above the ‘critical self-focusing power threshold’. It disappears if so-called ‘soliton propagation’ conditions are met: diffraction of two-dimensional field distribution obeying the Schrodinger equation with cubic nonlinearity. Presented experiments effectively demonstrate the stable single mode self-confinement of suitably shaped laser beam by its self-induced refractive index gradient, due to optical Kerr nonlinearity. Field intensity requirements (tens of megawatts per square millimeter) for soliton observation in CS2 led to working with a pulsed mode-locked laser source.
http://iopscience.iop.org/article/10.1088/0150-536X/17/3/004/pdf
The comment about 10^4 x distance for acceleration implies 10^-4 x acceleration, assuming the same terminal velocity. So for the same all-up mass, this means a 10^-4 x power requirement, so instead of 100 GW we would only need 10 MW. Now that’s a real benefit, since the 100 GW beamer was the single most expensive part of StarShot. This surely allows much earlier testing to commence.
I’m curious if a cluster of small 1 kg probe “components” could be simultaneously sent to Proxima Centauri using beamed propulsion (perhaps even riding the same beam?). For example, one probe would contain a CCD imaging device, another probe would contain a transmitter, a third probe would contain memory for storing images, and a fourth probe would contain solar panels used to generate power. Once the separate components reach Proxima, they would be programmed to self-assemble into the complete probe.
Since each probe blocks the beam for the one further out, not really. You could probably launch a sequence of probes with each having a slightly higher velocity that the one before, so they’d converge on each other after being launched.
And a time-multiplexed beam would not solve the problem, since the pointing angle would only be correct for one of the channels. It looks like one beamer per probe with this system. Of course, there’s nothing to prevent a probe splitting up at the destination, but that too sounds like a case of diminishing returns.
The main issue I have with particle beams which I am in favour of is they need to be in space which costs a lot of money. As for divergence it could be reduced with several technics.
1 Use massive collection of atoms like carbon 512 with trapped atoms.
2 Use many beams as the ionic divergence forces are to the square.
3 Have sheets of particles released in concave patterns so laser light pushes them towards each other over time.