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I've recently read some news about progress in building plasma reactors. The technology is basically that plasma is held by magnet field because otherwise, it would melt everything it touches. It has temperatures of millions of degrees celsius, i.e. hotter than the sun.

Now in sci-fi novels they often use fusion drives in order to move a spaceship. So if such an engine were to be constructed, it would probably again use some kind of a magnetic field in order to contain plasma.

What would however happen, if such a spacecraft were to be attacked? Let's say the engine hull is damaged (there is literally a big hole in it) and the magnet field is not working. It's vacuum out there. What would happen? Will there be a huge explosion? Would the plasma ball just be left behind (assuming the spaceship is still moving)? Would it melt the spaceship immediately?

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Well, a "Fusion Drive" as we understand it today would generally be a toroidal magnectially confined stream of plasma that is hot enough to undergo fusion. The destruction of one of these drives would likely be much less catastrophic than you would imagine.

Diagram of the Tokamak reactor

Plasma Dispersion

At its core, a fusion reactor has superheated plasma that is millions of degrees Kelvin, and compressed by the powerful magnets around it. However, this likely wouldn't create as big of an explosion as say, a fuel powered engine of the same size. Why? Because in a traditional explosion of an engine (say when a plane crashes), what you're actually seeing burn is the fuel that was was stored to be used in the engine. For a fusion reactor, that fuel is potentially only a few kilograms of deuterium and tritium at most for a scaled up fusion reactor.

At a few million kelvin, the venting of the internal plasma would still be significant, something on the order of 10-100 lightning strikes going off at once. But this is nothing compared to the catastrophic explosion you'd get from a traditional fuel explosion that you'd need to power the same ship.

Magnetic Explosion

The larger danger you'd face in space from a catastrophic fusion reactor failure is the magnetic explosion that will happen as the toroidal array of magnets comes out of alignment. I can't find any exact numbers on how fast the parts might accelerate away from each other, but the power behind the magnets on fusion engines are immense. Some designs even use the magnetic compression itself to heath the plasma to the required millions of Kelvin to begin the reaction. If any misalignment were to happen due to damage, the forces holding the plasma inside would rip the engine apart, sending the electromagnetic parts flying away from each other in every direction. In space, this kind of shrapnel would be far more deadly than some hot gas being vented.

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    $\begingroup$ A typical fusion reactor will contain fuel on the order of a few tens of grams of fuel in plasma form at any given time. Based upon description of ITER operating with about 50g of fuel. $\endgroup$ Mar 9, 2022 at 20:07
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    $\begingroup$ @GaultDrakkor absolutely, but the ITER only produces ~200 MW of power, I was imagining something much more substantial for a futuristic starship. Additionally, I was referring to the fuel bank, not just the fuel being used at any given moment. $\endgroup$
    – Skyler
    Mar 9, 2022 at 20:14
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    $\begingroup$ Not really clear when previous few sentences were about fuel in engine or lines. How much fuel is plasma in chamber is important to the question where general fuel stores are not. $\endgroup$ Mar 9, 2022 at 20:30
  • $\begingroup$ @GaultDrakkor "In line to be used in the engine" does not mean only what is in the fuel lines, but an expression for fuel that will eventually be used (as in 'queued up'), but I can see the cause of your confusion, so I'll change the wording. $\endgroup$
    – Skyler
    Mar 9, 2022 at 20:34
  • $\begingroup$ Fusion fuel cannot fuse without immense pressure. It won't fuse in the fuel tank unless you set off a nuclear bomb next to it. If it's a hydrogen isotope, you could set fire to it and get a little explosion, but anything less than a nuke would only release a miniscule fraction of the amount of energy it would have produced in the reactor. $\endgroup$
    – Robyn
    Mar 9, 2022 at 23:44
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It depends, but depending on conditions it will either explode or simply evaporate - probably the latter.

Plasma can have temperatures of millions of degrees, but it is important to note that it isn't hot because it's plasma, rather it is plasma because it's hot. The magnetic field isn't just containing the plasma, it is compressing it as well, which is why it is hot. Take away the magnetic field and the plasma will rapidly expand and cool into gas; how much damage it does in the process depends on how tightly it was being compressed and how much of it there was.

That being said, it is worth noting that one of the main principles of modern engineering is that systems should "fail safe" whenever possible. For example, nuclear power plants are constructed in such a way that common malfunctions like power failures will cause the reactor to break, but not explode.

Of course there is only so much you could do to account for all contingencies, especially when violent attacks are involved, but any competently-designed fusion drive will probably be designed to account for the possibility of the magnetic field unexpectedly shutting down and not cause an explosion due to something so trivial.

The plasma would definitely not be left behind; it is moving at the same speed as the rest of the ship and will continue to move at the same speed unless acted on by an outside force.

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    $\begingroup$ I really don't like your description of plasma. Plasma is just ionized matter. Room temperature plasmas are possible. Just because a plasma is contained does not make it hot. The same way as typical cylinder of compressed gas is not hot. The act of compression will heat it though. Certainly an easy way to ionize an atom is to heat it until thermal energy well exceeds the ionization energies $\endgroup$ Mar 9, 2022 at 20:18
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    $\begingroup$ "That being said, it is worth noting that one of the main principles of modern engineering is that systems should "fail safe" whenever possible". Depends if it's a military vessel, those often have ways to bypass safety features. I guess it depends if loss of manoeuvre would be considered a greater risk than whatever damage you incur when the fusion drive loses containment. $\endgroup$
    – flibwib
    Mar 9, 2022 at 22:38
  • $\begingroup$ If it shunts; bad things happen, like electricity creating arc flashes and, ironically: plasma where it shouldn't be. This is what circuit breakers are for, +1. Otherwise it kinda just goes dark after you lose containment. Might not want to be in the same room with it though. $\endgroup$
    – Mazura
    Mar 11, 2022 at 1:01
  • $\begingroup$ "On warships, it is common to install battle short shunts across fuses for essential equipment before entering combat. This disables overcurrent protection at a time when removing power to the equipment is not a safe reaction." (I meant short out or if it doesn't shunt, above) $\endgroup$
    – Mazura
    Mar 11, 2022 at 1:10
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Fusion is by far the safest spaceship energy source with our current knowledge (yes, solar is safer as fusion happens far away, but you don't take the sun with you - if you do you have fusion drive again). It is safe mainly because it is such a pain to get the reaction going - so you won't have D+T fuse by themselves in a storage chamber.

(note - I took some liberties in conversion to have nice numbers, but most numbers are fairly close to the correct ones)

Inside the reactor where the fun stuff happens (well, will hopefully happen eventually), you have something like 10g of matter (1) at very high temperature and pressure that is producing energy. D-T fusion gives about 100GWh/kg, so energy content of that little ball of plasma is 1GWh, which is about 1T of TNT. Sounds like a huge explosion, but when the plasma hits magnets, it won't do much fusion - it will simply blast a nice hole in the walls and magnets before expanding and cooling to something non-problematic. Magnets will pop too, but for ITER 10kT of magnets are planned to have only 15MWh of magnetic energy in them - only 15kg of TNT.

That's it, nothing meaningful as far as an explosion goes. Deuterium and tritium (or whatever else the ship is using) won't undergo further fusion without magnets working (and if they did work, it wouldn't blow up in the first place). So the reaction stops as there is nothing to keep it going, D and T that were in storage scape through the new hole.

However, there is a potential problem - the ship might have a lot of oxygen stored somewhere for those humanoids (or other lifeforms). And it might have a lot of fuel to travel far. So, you are in some danger of D/T to make ordinary chemical reaction (= burning, or possibly explosion). If we take say 500T of fuel (2) and enough oxygen, this has potential to release as much energy as the first nuclear bombs, ruins a lot of oxygen and makes a swimming pool of heavy water.

(1): ITER will have 1g for few hundred MW. https://www.nature.com/articles/s41567-020-1023-5

An ITER plasma weighs about a gram, and our fuelling is about a gram per second.

(2): This is comparable to oxygen+RP1 in Falcon 9. https://en.wikipedia.org/wiki/Falcon_9#Pricing

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