Booster free from spin resonance for future 100-km-scale circular e plus e- colliders

Acceleration of polarized electron (positron) beams in a booster synchrotron may suffer from depolarization due to crossings of many spin depolarization resonances and this could limit its applications. We have studied the structure of spin depolarization resonances of a 100-km-scale booster lattice of the Circular Electron Positron Collider (CEPC). The lattice has eight arc regions with hundreds of FODO cells, interleaved with straight sections, leading to a high periodicity. Our analysis shows the contributions to the strength of intrinsic and imperfection spin resonances add up coherently near the superstrong resonances beyond 120 GeV but mostly cancel out and result in generally weak resonance strengths at lower beam energies. Detailed simulations confirm that beam polarization can be mostly maintained in the fast acceleration to 45.6 and 80 GeV but that severe depolarization may occur at even higher energies. This study suggests the possibility of acceleration of polarized electron (positron) beams to ultrahigh beam energies without the help of Siberian snakes and supports injecting highly polarized beams into the collider rings as an attractive solution for resonant depolarization measurements and longitudinally polarized colliding beam experiments for future 100-km-scale circular e thorn e- colliders.