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. :)
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.
2. If the plasma escapes, I would say you could dump it into a large tank (like a reservoir of water), and extract energy from it.
Might be like an MRI machine. You could quench the magnet every other month, but at what cost and benefit?