Venting Fusion reactor plasma to create a plasma shield?

Question

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?

Answer 1

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.

Answer 2

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.