[MHGinTN]

But, but, but ... there will be two protons, each moving at 99.9% the speed of light smashing head-on into each other in a magnetic confinement chamber! That's almost twice the speed of light inertia, er, um, I mean ... oh, nevermind.

[Telepathic Intruder]

I’ve heard similar arguments, actually. The magnetic confinement chamber is more like a choke, it narrows the beam rather than holds it captive. At 99.99% the speed of light there is really no way to prevent the resultant sub-atomic particles from flying away again at the same speed in random directions.