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Making 10% of Light Speed Beam Propulsion More Practical With Neutral Beams

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New and revolutionary propulsion systems are needed to undertake challenging long-distance missions, such as to the Kuiper belt, Oort cloud and nearby stellar systems. We propose an innovative beamed propulsion architecture that would enable interstellar missions to Proxima Centauri b at nearly 10% the speed of light. This architecture dramatically increases the distance over which the spacecraft is accelerated while simultaneously reducing the beam diameter at the transmitter and probe from 10’s of kilometers to less than 10 meters. These attributes translate into a more compact system and increased payload mass compared with laser propulsion alone, enabling science missions that are not currently feasible.

The key innovation of our propulsion concept is the unique coupling of a neutral particle beam with a laser beam, a scheme which nearly eliminates thermal expansion and diffraction during beam propagation through space. In Phase I, the physical basis of this concept and the unique features of this unexplored mode of propulsion were investigated. Through this effort, the underlying physical foundation has been verified and the governing equations have been derived from first principles. Exploration of numerical approaches to high fidelity modeling has also been initiated. Using a mission design tool developed in Phase I, a strong scaling of payload mass with velocity was discovered, leading to the finding that a 60-year mission at 7.5% the speed of light provides a payload mass of order 1 kg. A survey of the technical literature revealed that the basic elements of the combined beam propulsion system currently exist or are near-term extrapolations of present capabilities. The Phase I results, having validated the physical basis and motivating application of this new propulsion technology, advanced its technology readiness from TRL 0 to TRL 2.

The primary research objectives of the Phase II study are:
A) to analyze the feasibility and design of momentum transfer mechanisms to generate thrust for the spacecraft,
B) to understand the dynamical behavior of the combined beam system through computational modeling, and
C) to develop experimental capability for a high mass flow rate and low divergence neutral beam source. New computational tools are needed to address the stability of self-guiding, and will proceed through improving and coupling the current 2D axisymmetric beam propagation models developed in Phase I.

A fully-coupled solution framework running on high performance computing resources will allow investigation of self-guided propagation, including the effects of laser heating and radial instabilities and their dependence on non-dimensional system parameters. The development of the necessary atomic beam source will be addressed through a specialized laboratory facility for cold neutral beam experiments based on laser Doppler cooling of a supersonic alkali vapor jet. This cold atomic beam will be injected into an extended path, ultra-high vacuum system to study its properties during propagation. Diagnostic measurements of the beam thrust, density profile and temperature at stations along the propagation path will provide fundamental data for characterizing the system performance and validating numerical models. Through the above combined theoretical, modeling and experimental efforts, we expect to advance self-guided beamed propulsion from TRL 2 to TRL 3+ by the end of the Phase II research program.

12 thoughts on “Making 10% of Light Speed Beam Propulsion More Practical With Neutral Beams”

  1. I think the issue is more likely to be evaporation from the beam boundaries. This isn’t going to make the combined beam 100% stable over an unlimited distance. It’s just going to enhance stability.

    Rather like fusion research, actual testing will be necessary, even if it works in simulations.

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  2. Sounds very handwavey to me. The sort of conceptual problem I see is that it sounds unstable, because it’s open loop. If for any reason the particle beam becomes unfocused and density variations drift to the outside, the light beam becomes unfocused, it will stop “diverting sideways thermal motion” (I don’t know what that means, but let’s just take it at face value), the particle beam will become less collimated, drift to the outsides, and unfocus the later, etc etc. Deviations from optimality will compound because the two effects are both positively and negatively coupled. Small perturbations away from optimality would exponentially increase in instability.

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  3. No, I think you’re missing the point. By itself, the light would spread. By itself, the particle beam would spread. Together, they don’t spread, because each focuses the other. It’s synergistic.

    The closest analogy would be to a solid object moving through space, and not spreading, because the EM interactions between the particles of the object tie it together.

    The light is focused by the density gradient of the particle beam, the beam is focused by the dielectric effects from the light.

    Now, would there be some leakage? I don’t know, maybe. But together they’re going to remain intact a lot longer together than apart.

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  4. The beam will spread. Not by much, but it will spread. Any apperture constrains the location of the particle when it goes through the apperture. Heisenberg’s uncertainty principle constrains the minimum amount of diffraction that must occur when going through the apperture.

    For 500 nm green light and a 1 meter apperture, the angle to the first minima in the typical airy disk diffraction pattern of a circular hole is 0.6 microradians. E.g. 10 000 km out the central spot size is 13 meters, 100 000 km out it is 120 meters, 1 million km out it is 1.2 km and so on.

    The particles have much smaller wavelengths, and so they will spread much less from diffraction. But when the photon beam is no longer confining them, will they spread quickly anyway from thermal motion?

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  5. Another nice thing about this concept is that because of the particles having a lot more momentum than pure light, you can get some sort of efficient power/momentum transfer even at local (orbital to interplanetary) speeds.
    Which means it’s useful long before any interstellar probe gets built.
    Which both

    1. Is useful. A 1 MW system can send probes zipping to Mars/asteroids etc. You could build one right now and get applications right away.
    2. By the time we can actually start on an interplanetary probe, we would have years/decades of experience with the systems and need only scale up. Or even just combine some 10s of the existing systems for the time it takes to do a launch.
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  6. Well, yes. Hundreds of megawatts for days, instead of gigawatts for minutes, basically. OTOH, it’s suitable for accelerating sizeable probes, rather than hypothetical interstellar chip probes.

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  8. I imagine this concept would require pretty massive infrastructure in space.

    The particle accelerators, the laser itself, which won’t be something for the faint of heart.

    And it would need to be committed to a single missions for a while, keeping the stream of particles and laser focused continuously in a single trajectory for this to work.

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  9. “The laser diverts any sideways thermal motion of the particles to push the particles back to the center of the beam. So the particle beam remains focused. (I can’t remember how this works)”

    Dielectric forces. Neutral dipole particles are attracted towards higher EM densities, because they line up with the local field, and the side in the higher intensity is attracted very slightly more than the side in the lower intensity is repelled.

    Nice thing about this concept is that, because the light IS moving a lot faster than the particles, various instabilities get suppressed, and aiming jitter will tend to be averaged out over some period of time.

    That last is pretty important: No point in having a beam remain focused over a long distance if it’s waving around so that your probe can’t stay in that focus.

    I think this concept is MUCH more practical than StarShot’s pure laser approach. But it absolutely has to be originating in space, due to the particle beam component.

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  11. We’ve seen this tech mentioned before, and the author came into the forum to answer questions, so Brian has not gone into the tech detail.

    As I understand/remember the concept, we have 3 components:

    1. A particle beam of negative particle, accelerated to near light speed (standard off-the-shelf particle accelerator)
    2. A particle beam of positive particle, accelerated to near light speed (standard off-the-shelf particle accelerator)
    3. A laser of megawatt scale, which is just about off-the-shelf these days, and becoming more so with military projects underway.

    Negative and positive particle beams are combined to give a neutral particle beam, so there is no net electrostatic force causing the beam to spread. But it still won’t stay together long (certainly not over space travel distances) because you get thermal motion dispersing what is really just a very high speed stream of gas.

    BUT you also have the laser combined with the beam.

    Now the laser interacts with the gas atoms to have 2 effects:

    1. The laser diverts any sideways thermal motion of the particles to push the particles back to the center of the beam. So the particle beam remains focused. (I can’t remember how this works)
    2. The particle beam is denser at the center and less dense towards the outside, this acts as a (weak) lens to refocus the laser back to the centre of the beam.

    So the laser keeps the particles focused at the center of the beam, the particles keep the laser focused at the center of the beam.

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  12. “…unique coupling of a neutral particle beam with a laser beam…”
    How does this coupling happen? Quantum mechanical effect? Is the light attracted by a mass of the particle or something?! Probably more like “or something.”

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