The limiting factor is the strength of magnetic fields you can achieve. The more momentum the particles have, the stronger a field you need to bend them into a circular orbit. The attainbale strength is determined by material properties and there is nothing to suggest that we can get orders of magnitude improvement here. The improvements have been quite linear for decades, see the figure here:
And you eventually run into rather fundamental problems of tearing your atoms apart with the magnetic field you create. But I am not up to speed on how close we actually are to these limits.
https://www.nature.com/articles/d41586-019-01869-1
And you eventually run into rather fundamental problems of tearing your atoms apart with the magnetic field you create. But I am not up to speed on how close we actually are to these limits.