Intensity limitations due to space charge for bunch compression in synchrotrons

Abstract

Bunch compression achieved via a fast bunch rotation in longitudinal phase space is a well-accepted scheme to generate short, intense ion bunches for various applications. During bunch compression, coherent beam instabilities and incoherent single particle resonances can occur because of increasing space charge, resulting in an important limitation for the bunch intensity. We present an analysis of the relevant space charge driven beam instability and resonance phenomena during bunch compression. A coupled longitudinal-transverse envelope approach is compared with particle-in-cell (PIC) simulations. Two distinct cases of crossing are discussed and applied to the GSI SIS-18 heavy-ion synchrotron. It is shown that during bunch compression, the 90°condition of phase advance is associated with a fourth order single particle resonance and the 120°condition with the recently discovered dispersion-induced instability. The agreement between the envelope and PIC results indicates that the stop band is defined by the 120°d ispersion instability, which should be avoided during bunch compression.

Beam quality and beam loss predictions with space charge for SIS100

“…The space charge tune shifts are generally much smaller at top energy. Only during a very short (several turns) period the space charge tune shift of the fast rotated bunch reaches up to −0.7 [3]. The bunch compression in SIS100 is not covered in this report.…”

“…Besides charge stripping, space charge induced resonance crossing is expected to be a main beam loss mechanism for intense heavy ion beams in SIS100 at injection energy. For proton and light ion beams space charge is expected to be the main intensity limitation at injection energy and for bunch compression [3] at top-energy. The six SIS100 arcs are optimised for the collimation of stripping losses in heavy-ion beams, with low dispersion, straight sections in between, for the rf cavities, injection/extraction and also laser cooling of selected ion beams [4].…”

Simulation Study of the Space Charge Limit in Heavy-ion Synchrotrons

“…The main beam loss mechanism expected for intense heavy ion beams in SIS100 at injection energy -besides charge stripping of such intermediate charge state ions -is space charge induced resonance crossing. For proton and light ion beams space charge is expected to be the main intensity limitation at injection energy and for bunch compression [2] at top-energy. In the case of (intermediate charge state) heavy ions, the tolerance for space charge induced beam loss is determined by the effectiveness of the vacuum system to stabilise the residual gas pressure against desorption due to the lost particles.…”