Cost. Building huge magnets and a huge vacuum vessel is simply expensive. Fusion scales very well with two parameters: size and magnetic field strength. Size

Cost. Building huge magnets and a huge vacuum vessel is simply expensive. Fusion scales very well with two parameters: size and magnetic field strength. Size works great, but it's just too expensive. Even if you could justify the costs in theory, ITER is demonstration that the political will to invest that much capital with such a long payoff time just isn't there. Magnetic field strength is the new hope of the fusion industry: 'Cheap' high temperature superconductors have been commercialized and could make smaller fusion designs practical and economical.