It is not possible.
I don't even need to do much science to reason this. Plasmas radiate their heat very quickly and are very hot. If you're inside a sphere of plasma, you will be in a sphere of gas instead of plasma if you do not continuously supply power to the plasma to keep it from cooling down too much. That means that you will need to put in as much heat as the plasma is radiating, meaning it will continuously radiate that much heat, meaning that you will very soon be cooked because you have pretty much just made the thing you were trying to protect the interior of a fusion reactor.
Plasma is useful for vaporizing projectiles, possibly, but you cannot have a continuous plasma shield. The best you could do is some device to "flash in" some plasma in front of incoming projectiles, but that requires being able to react to projectiles at lightning-speed arbitrarily, and if you can do that, you might as well just shoot a counter-projectile instead of going through the massive amount of work of creating a plasma generator.
What you see in Star Wars is not actual plasma. I would probably identify that as some kind of ultra-low-density fluid, responsive to EM fields as though it had metal ions dissolved into it, that has very high absorption in the spectrum of laser weapons. Then again Star Wars laser weapons are definitely not lasers, so it's probably just an ultra-low-density electromagnetically-interacting magnetorheological fluid or other non-Newtonian that turns virtually-solid upon impact.
That is actually a really cool idea in my opinion for a plasma-alike shield system: mostly see-through, manipulatable with EM devices, and passable at low speeds but impenetrable (to an extent) at high speeds. I could also imagine that on each impact, a small amount of fluid would be dissipated, and if too much dissipates, the large-scale mass of fluid ends up maintaining only a copious connection with the EM field generator that holds it in place, and it would collapse.
