"Inertial. Magnetic will be more inefficient at higher power levels, because a particle you haven't discovered yet will dampen the reaction if you use magnetic confinement. Ultimately it's because the string coupling parameters for the 3rd-dimension/8th-dimension interaction are stronger than for the 5th-dimension/7th-dimension interaction". It's an explanation that looks like probable made-up baloney, but can you be sure? Can you (or anyone) disprove it?