A powerful and dangerous source of energy. It is also just called Plasma Energy
While Nuclear Fusion generates plasma as part of its process, with the Plasma Core, the source of energy is a large (relatively speaking) sphere of plasma, mimicking a star and held under control by magnetic forces.
However, a plasma core is like a time bomb, as powerful as it is, it is high density plasma held together under force. Any failures in its containment field could result in either a beam of energy, or even a nuclear blast. The fact that setting up and maintaining the field takes a great deal of power has held Plasma Core energy as a purely theoretical source of power.