Acceleration and Storage of a Dense Single Bunch in the CERN SPS

Impedance issues in the CERN SPS

“…Already in the first two years of operation the impedance of two higher order modes, one transverse at 460 MHz and the other longitudinal at 628 MHz [1] in the main RF cavities, were identified as causing instability and intensity limitation and had to be passively damped. When preparing for SppS, bunch intensities were limited when crossing transition by both a longitudinal negative mass instability and a transverse head-tail instability to 2 × 10 10 protons [2]. This limitation was removed by raising the injection energy to E = 26 GeV, above transition, γ tr = 23.…”

Methods of observing the microwave instability above and below transition

“…In Fig. 1 one can clearly see the high frequency signal growing after transition crossing in the SPS [13]. With 2 ns long bunches signals were seen only above 1.2 GHz.…”

“…However it seems that strong µw signals were observed just after transition crossing in accelerators both with capacitive (space charge) [4] and inductive type [13] of low frequency impedance, suggesting the possibility of another explanation. In fact in all cases in real accelerators measurements are unavoidably done in the presence of resistive impedances.…”

“…In proton machines µw instability is often observed at transition crossing [4,13,14,15]. In this special case the azimuthal motion in the phase space is practically frozen and particles are moving along momentum axes under the influence of external (RF) and induced voltages.…”

“…In this special case the azimuthal motion in the phase space is practically frozen and particles are moving along momentum axes under the influence of external (RF) and induced voltages. It is noticed that strong microwave signals appear just above transition (AT) [4,13,15] which can be interpreted as a difference in bunch stability above and below transition (BT). In Fig.…”