the “artifical sun” is an extremely thin stream of plasma, and only exists for as long as you can keep it from touching anything - as soon as it does it grounds itself out, loses magnetic containment and dissipates harmlessly.

What you see in spiderman is (presumably, I have never seen it) some infinitely expanding ball of plasma that threatens to escape containment and explode the city. In reality, one of the largest unsolved problems in fusion research is how to even introduce additional fuel without instantly collapsing the magnetic field and, once again, letting the plasma dissipate harmlessly.