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Assuming that humanity can build as large as a fusion reactor needed to sustain fusion for as long as needed, would it even be half feasible to vent out the spare plasma (kind of like how a feasible Tokamak fusion engine would release some plasma to produce thrust) to create a plasma shield, or would a more exotic fuel like antimatter-matter reaction be required to create any sort of a useful plasma shield to deflect possible railgun slugs in space?

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    $\begingroup$ Is a plasma shield a thing? What does it defend against? Charged particles? $\endgroup$
    – Willk
    Feb 17, 2021 at 23:52
  • $\begingroup$ @Tantalus'touch. Edited with more details $\endgroup$ Feb 18, 2021 at 0:25
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    $\begingroup$ No fusion reactor plasma in space will stop, deflect, divert or even be noticed by Railgun Slugs. Density is many magnitudes too low, incoming energies way too high. It might work against charged particle beams, and possibly might fry electronics of incoming smart missiles. $\endgroup$
    – PcMan
    Feb 18, 2021 at 0:34
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    $\begingroup$ Plasma is just superheated gas; it's absurdly hot but there's very little of it. Unless the "plasma shield" is several miles thick and at something at least approaching atmospheric pressure, there won't even be enough exposure time for the slug to even get warm before it strikes the ship. $\endgroup$ Feb 18, 2021 at 3:50
  • $\begingroup$ Do you mean Corona Mass Ejection but absurdly scale down? Reflect projectiles I don't think so but you can surely vapourise them hee hee ;D $\endgroup$
    – user6760
    Feb 18, 2021 at 6:45

2 Answers 2

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Fusion reactors (well at least Tokomak design reactors) operate at a relatively low pressure - to the order of 2-3 atmospheres. And they have plasma volumes to the order of 100-1000 cubic meters (the latter enough to produce ~500 MW continuously). So there is just not a lot of plasma in one.

You could propose an upscaled reactor to produce 500 GW - this would be larger and/or would operate at much higher pressure - but even that would only contain ~10^6 cubic meters of atmospheric pressure gas (and why would even a giant space-bound reactor need that much power?).

The plasma is very hot, but its temperature would be of little/no advantage in shielding against fast moving solid slugs. In fact, it would be a disadvantage as the hot gas would dissipate into the vacuum of space faster than would a normal cold gas. And a gas is, in general, not a very efficient way to stop/slow/shield-from a projectile unless you have to the order of many km of gas to do so.

Your (top end volume) million cubic meters of gas has a volume not much larger than a big hot air balloon ~100m in diameter - so unless you can imagine said balloon stopping the projectile dead, your plasma shield is doomed to failure.

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  • $\begingroup$ I was all set to talk about PV=nRT, then I did some searching. You are correct. 2 atmo only achieved in 2016. news.mit.edu/2016/…. $\endgroup$
    – puppetsock
    Feb 18, 2021 at 3:09
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    $\begingroup$ Is the mW (milliwatt) a typo and should it be MW (megawatt)? There's a 9 orders of magnitude difference there. $\endgroup$
    – AI0867
    Feb 18, 2021 at 9:01
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    $\begingroup$ I know current fusion reactors don't produce much power, but half a watt seems really low. $\endgroup$
    – user253751
    Feb 18, 2021 at 9:42
  • $\begingroup$ Thumbs missed the Caps key :). MW is correct. $\endgroup$
    – Penguino
    Feb 21, 2021 at 20:40
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Even if it were entirely practical (and it probably isn't, see Penguino's answer for some reasons why) it wouldn't be very useful as a shield against projectiles.

The incoming matter in a railgun slug is travelling pretty fast, to the point where it probably won't be able to notice the difference between, say, a cloud of cold dust and a cloud of hot plasma. There won't really be much time for thermal effects to disrupt the slug, unless the cloud of plasma is both dense and pretty big, which is probably impossile to arrange.

What you have, then, is a tissue-thin shield of warmed-over gas around your ship which is less effective than a minimal Whipple shield.

Take home message: if you're defending against projectiles, don't bother with this plan.

That's not to say that plasma shields are useless, of course. They have applications as radiation shields (useful for spaceflight in general, not just military purposes) and as laser and radar absorbing layers, but for defensive applications what you'll probably be wanting are cold plasmas which you'd generate on demand using power from your fusion reactor, but without actually venting it. Cooler plasmas are much easier to work with than the fusion-temperature stuff, so they can be more reasonably confined and shaped so as to be more like shields than explosions.

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  • $\begingroup$ Thank you for your response! Cold Plasma sounds interesting, is there any other videos/papers/articles about how effective it would be and how much power would be required? $\endgroup$ Feb 19, 2021 at 0:56
  • $\begingroup$ @itisyeetimetoday its a broad field, there's lots to read and i don't have anything particularly good to hand. Using it as a shield against weaponry is strictly scifi. Assume that the tech level is high, and the power requirements are higher. $\endgroup$ Feb 19, 2021 at 8:55

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