Perhaps a stupid question, but why does the plasma need to stay confined for long periods of time? If you can devise a process that performs a full power-generating cycle, then no matter how long (or short) that cycle takes, you will get net power out of it.
1. Ions in the plasma need to interact with each other for enough fusion reactions to occur. For this to happen, the plasma needs to be somewhat dense and the temperature needs to be very high. If the plasma escapes, the temperature and densities fall off rapidly, and the nuclear fusion rate drops quickly. (Nuclear fusion has an incredibly steep temperature dependence --- the triple alpha process, for instance, goes as T^28 or something like that).
2. When the plasma escapes, it interacts with whatever is containing it and can destroy it very quickly.
But I'm an astrophysicist, not a nuclear physicist, so I could very well be wrong. Plasma confinement is much easier in stars. Just let the gravity do the work for you. :)
Quite right. The figure of merit for a fusion plasma is called the 'triple product' since it's the temperature, density and confinement time multiplied together.
The problem in general with pulsed systems is that they take a lot of energy to get them from idle to active, and the research machines have miniscule efficiency. The claim is often that "we'll scale this up and it'll be more efficient" but there are always unknown unknowns. Nature is a cruel mistress, especially in the fusion business.
I forgot to address this in my earlier comment, but you don't worry about the plasma escaping -- it is very delicate, generally orders of magnitude less dense than air. Rather, keeping it alive is the difficult part. If the plasma hits the wall of a fusion device in an uncontrolled manner, it will dump all its energy into it. If the plasma picks up too many impurities, it will radiate energy away in the form of bremsstrahlung. It is hard to keep the ions hot enough to fuse while there are many phenomena conspiring against you.
Interesting question. I don't have the answer but it could be because it's costly to get the whole thing running. Once it's up, you want to keep it up to avoid all sorts of problems/costs. And that's not considering how long it takes to generate said power from the plasma.
Might be like an MRI machine. You could quench the magnet every other month, but at what cost and benefit?
