I would like to argue that prototypes have been what's called for. We don't even have a clear idea of what a commercial fusion reactor would look like. The cooling and tritium production issues, the structural materials, and the lifetimes required of the super cooling and superconducting systems, are massive problems that haven't really been solved yet.<p>It's possible that replacing D-D with D-T would have yielded break-even for a few seconds, (I've heard that claim before too) but I'm very skeptical; increasing the internal heat generation is sure to change the plasma conditions: for example, there would be a lot of adiabatic expansion, which, even restrained against the tokamak fields, is liable to have a variety of unstable modes.