McGuire aims to generate the extreme magnetic fields necessary for plasma confinement with superconductors. Currently, the highest temperature at which we've observed superconductivity capabilities (in specially engineered materials) is around -135C. If he wants to drive turbines with heat generated from the fusion core, he'll require at least a 200C temperature difference across a material that needs to have extreme magnetic permeability. This material is pictured in orange, residing between the confinement vessel's walls, and the superconducting electromagnets.
It always boils down to materials science.