[jonjacky]

Particles accelerating in a cyclotron at sufficiently high energy reach relativistic speeds. You have to account for their relativistic mass increase to get the cyclotron to work. Figuring this out was a big issue in cyclotron design in the 1930s. The remedy is to strengthen the magnetic field near the outer edge of the cyclotron where particles move fastest, by adding coils there to carry more current. I don't recall what the energy is where this becomes necessary - it is certainly needed at tens of MeV.

Plutonium was first synthesized in a cyclotron by Lawrence's group at Berkeley. I don't know what energy they used so I don't know if they needed the extra coils, but they did know of the effect and must have considered it.

Also, U235 was separated at Oak Ridge using machines called Calutrons invented by Lawrence that might have encountered the same problem -- at least they must have considered it.