Plasma is the 4th state of matter involving the splitting apart of overheated atoms. It is found in stars, Lightning, And other sources of energy. Plasma could be used for peaceful research purposes like digging down into the mantle or powering satellites sent to outer planets but weapons. . . I don't think we really need that.
Let's not stop the military because we failed to stop them from making nukes (when we wanted them to make nukes to scare the USSR) so what if we need to make plasma weapons to scare of nuke empires like North Korea or Iran? I believe it is fine to have plasma weapons stored and confined and not used unless absolutely necessary.
Even if it was outlawed they would be trying to make it in a lab. I would rather the military be working on that technology because eventually it will end up in the public sector.
So much of our stuff came from the militarily trying to make stuff to kill people but eventually it will help the world.