Relativity effects will start to bite. The proton's going to stop being interested in your magnetic field[1], and you're approaching the velocity of the exhaust

Relativity effects will start to bite. The proton's going to stop being interested in your magnetic field[1], and you're approaching the velocity of the exhaust mass of the fusion reaction. [1] The momentum vector just completely flattens almost any other physical characteristic. I'm not sure there's even enough time for nuclear fusion to take place.