What's the best design for a receiving end of a Plasma Sail/Catch.

That is, (ignoring the specific means of how for now), a broadcaster in solar system x sends out a soup of macro-scale particles at a non-negligible speed(a fraction of C), of nonpolarized(my bad. I do want the beam to be coherent after all) "dust" at spaceship x. The magnetic catch's job is to first, allow the dust to(assuming some basic design) hit the catch itself and so transfer the magnetic dust's momentum, or by catching its momentum by some other means and so transfer it to x. The primary goal of this is to accelerate said ship up to 0.5 C.

So, What would the best design for such a catch be for the said dust, assuming a few things

-The dust remains something of a coherent beam, it expands, but not to the point at which it would be impractical.

-Room temperature superconductors are present, and are available for such a construct, though lacking in material strength.

-A Shadow shied/Whipple shield/just high-speed space dust shield is preset at the bow(If you want it) of the Ship and covers a large enough cross-section for the plasma catch to not be torn apart by high-velocity space dust.

-You don't have to worry about resources regards the construction of the catch.

P.S. It was late at night when I initially wrote, and I'm still mildly exhausted, so please don't get mad at me if I do or say something stupid(Such as putting on the wrong tag).

  • $\begingroup$ P.S. Apologies to Sprout with the late response $\endgroup$ Nov 2, 2021 at 5:03
  • $\begingroup$ All good, haha :D $\endgroup$
    – sprout
    Nov 2, 2021 at 5:06
  • $\begingroup$ this one en.wikipedia.org/wiki/… $\endgroup$
    – MolbOrg
    Nov 2, 2021 at 9:10
  • 1
    $\begingroup$ Please make sure you read the description of the hard science tag as there are specific things you need to do when you're using it as it has extra rules attached. You may choose to either flag your question as described in the tag wiki, or remove the tag if you've decided it is too restrictive. $\endgroup$ Nov 2, 2021 at 20:30
  • $\begingroup$ @Starfish prime. My apologies, it was late at night and I was incredibly exhausted. $\endgroup$ Nov 3, 2021 at 17:48

1 Answer 1


So, you've tagged this one [hard science] which makes it rather hard to answer... especially given definitions like "soup of charged particles at a non-negligible speed(a fraction of C), of polarized(nonpolarized, my bad. I do want the beam to be coherent after all) magnetic dust". I'll spare you a joke about coherency, at least.


The dust remains something of a coherent beam, it expands, but not to the point at which it would be impractical.

Discussions of focussing a beam of pretty much anything onto a plausibly sized starship over interstellar differences are a bit outside of the scope of this question, but you'd do well to consider that "magnetic dust" sounds suspiciously like handwavery to me.

You'd do well to read up on the subject. There's a paper that's well worth reading: Mass beam propulsion: an overview which looks at this specific problem and mentions a few possible solutions. Magnetic dust doesn't appear in any of them, FWIW, but maybe you have a more recent source for that sort of thing?

FWIW, my personal favorite interstellar propulsion proposal is Jordin Kare's Sailbeam which used "smart" projectiles capable of basic aiming at the target ship.

The primary goal of this is to accelerate said ship up to 0.5 C.

This makes things straightforward, though not simple. There's too much energy in any particle travelling at that speed to try and soak it up by letting it collide with the spacecraft. You've got to deflect it some other way, and the usual approach is to use a big magnetic field.

The incoming mass stream is generally assumed to be reduced to ionised gas by some means... self destruction for larger projectiles, collision with a much smaller mass of gas or ionisation via lasers for smaller projectiles. The dust being merely magnetic probably isn't going to cut it here, but this is drifting outside of the scope of the question so I won't go into further detail.

From the Nordley and Crowl paper, here's a diagram of a "dual loop" magnetic reflector:

Dual loop magnetic mirror diagram

Note two important things: a stream of plasma shooting forwards through the reflector, and the obvious stream shooting backwards. This has some implications:

  1. Your ship probably wants to be donut like, to allow undeflected plasma to shoot forwards without the need to shield against all of it. This forward stream of plasma isn't wasted, but helps deflect the interstellar medium in front of you, reducing damage and radiation received by the ship during its journey.
  2. Your propulsion beam probably wants to be pulsed, or the deflected plasma of particles will interfere with incoming mass. This will reduce the amount of mass hitting the magnetic reflector, hence reducing thrust and efficiency. Fire a blob of propellant, then wait a bit for it to be deflected and for it to dissipate into harmlessness.

There's a further issue here, which is that large and powerful magnetic fields in space induce drag on the spacecraft by interacting with the interstellar medium. This is what ultimately killed the Bussard Ramjet, but does give us the possibility of using a magsail as a sort of interstellar parachute for breaking from relativistic speeds.

To ensure drag forces do not exceed thrust, Kare proposed an larger additional coil with an opposite dipole to to the main thrust-generating reflector coil(s):

  • Drag is dominated by low-field region far from loop
  • 2nd larger loop with opposite dipole cancels field

and from the main paper

the the fields do not cancel accurately until well outside the radius of the outer loop

This cancellation loop could have ten times or more the radius of the reflector coils and a much lower current. He doesn't supply a nice diagram of this, but did offer this excellent example of powerpoint art in the presentation that went with the paper linked above:

Diagram of a sailbeam magnsail, showing drive coil, much larger field-cancelling coil and small payload

The diagram is not to scale. The payload here is a point shown in blue, but given that it is at the bullseye of the target it probably isn't recommended for mass-stream driven spacecraft, rather than merely solar wind-driven ones. Note that the payload and cancellation coil are dragged behind the main drive/reflector coil.

To summarise then, and to paraphrase Elijah: the appearance and work of your spacecraft will be as a donut within a donut, possibly within a donut. A very large screening coil to reduce magnetic drag on the interstellar medium whilst the reflectors are operating, and one or two much smaller but higher-current coils making up the reflector. The payload will be distributed around the drive coils, allowing stray propulsion matter to shoot through the middle without needing to be armoured against. The drive coils will still need cooling and shielding, and the payload may need shielding inside a superconducting shell whilst the reflector is operating, depending on whether or not it is something that is unduly affected by strong magnetic fields.

  • $\begingroup$ "which is that large and powerful magnetic fields in space induce drag on the spacecraft by interacting with the interstellar medium. This is what ultimately killed the Bussard Ramjet" - No. arxiv.org/pdf/0710.0295.pdf "Equation of motion of an interstellar Bussard ramjet with radiation and mass losses" -- cite "An ideal ramjet could reach a velocity very close to the speed of light c. If some energy extracted from the interstellar medium is lost in form of thermal radiation, the ramjet speed is limited to a value below c" - so it is quite capable, and not the drag is the problem. $\endgroup$
    – MolbOrg
    Nov 3, 2021 at 9:11
  • $\begingroup$ The paper isn't perfect, and I would argue with the authors, as loss does not matter, but it is enough to decline your statement. Also, there was a better concept of mag sail, but I lost the link to it, their key difference was the absence of those excessive huge coils in physical form, en.wikipedia.org/wiki/… - "mini-magnetospheric plasma propulsion (M2P2), currents that run through the plasma will augment and partially replace the currents in the coil." - something along that line $\endgroup$
    – MolbOrg
    Nov 3, 2021 at 9:20
  • $\begingroup$ @MolbOrg I am dubious that such an idealised ramjet could ever be constructed. As far as I can tell, they've also significantly simplified the drag problems (of which there are more than one). An interesting paper then, but I can't see it reanimating the dead horse of bussard ramjets. As regards physical magnetic coils rather than m2p2 systems, there are non-trivial problems of mass loss from the m2p2 trapped plasma cloud that limit the system's Isp, but they are amenable to some handwaving away, I'll admit. $\endgroup$ Nov 3, 2021 at 10:16
  • $\begingroup$ (I also suspect that having two nested m2p2 fields to reduce drag might be an incredibly hard problem to solve, and maybe impossible) $\endgroup$ Nov 3, 2021 at 10:17
  • $\begingroup$ " I am dubious that such an idealized ramjet could ever be constructed" - sure, it a valid concern, and is part of why I'm not fully satisfied by that paper I linked, as it does not matter if it is idealistic or not, but it has no relationship to your statement as it is right now and explaining why it is a different topic. The main obvious problem is fusion itself, as Hydrogen isn't ideal fuel, far from it. I could build some defense for BJ-case, but it is not what my comment was about, which is about your statement which isn't exactly correct, I guess. $\endgroup$
    – MolbOrg
    Nov 3, 2021 at 10:27

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