AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo 1 Final results of the E-Cloud Instability MDs at the SPS (26 and 55 GeV/c) G. Rumolo, in SPSU-WG (18/12/2007) G. Arduini, E. Benedetto, R. Calaga, E. Métral, G. Papotti, B. Salvant, E. Shaposhnikova Acknowledgments: T. Bohl, W. Höfle, F. Roncarolo, R. Tomás, 2007 MDs -autumn sessions: – , first attempt – , ECI at 55 GeV/c !!! Summary of the observations

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo 2 Vertical chromaticity was lowered at the measurement points, till the beam becomes unstable. Look for Q‘ threshold for instability Measurements were done with the damper on and off Measurements were done with different batch distributions MDs at 26 and 55 GeV/c Dedicated SPS supercycle for MDs 3 LHC batches of 72 bunches at nominal intensity Flat bottom ~11 GeV Ramp ~2 s Intermediate flat top ~6 GeV ~ 5% losses at the beginning of ramp Beam dump Measurement GeV Measurement GeV Flat top ~1 GeV

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo 3 Observations: The electron cloud monitor showed an e-cloud signal growing along the cycle. As expected, the signal was more intense with 2 or 3 batches (see next slide) Q‘ could be set to a slightly negative at 26 GeV/c, provided that the damper was on. With the damper off, the beam would become unstable at about Q‘~0. At 55 GeV/c Q‘~ 4 is the observed threshold for instability (damper on) Measurements with a different batch distribution (3 batches uniformly distributed around the ring) seemed to significantly stabilize the beam at 55 GeV/c The instability always starts from the tail of the batch(es) However, the instability evolution along the batch(es) seems to point to coupled- bunch both at 26 and 55 GeV/c, even if a variety of modes is present, with probably some single bunch component MDs at 26 and 55 GeV/c: cycle

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo 4 Signal from the e-cloud monitor with one (left) or two (right) batches in the SPS Even if the flat bottom ends at ~11 s, the e-cloud is observed to appear at ~5 s because by that time the uncaptured beam has smeared all over the machine and traps the electrons (E. Shaposhnikova) That was proved by cleaning the gap and observing no e-cloud signal at the flat bottom (G. Arduini) MDs at 26 and 55 GeV/c: Build up

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo MDs at 26 GeV/c: instability

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo MDs at 26 GeV/c: instability Spectra of the bunch by bunch motion: The individual spectra of the bunch by bunch time traces show an instability all over the bunch train for the third batch The 2D spectrum has one single peak at the tune and low mode number (resistive wall ?)

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo MDs at 26 GeV/c: instability Some times a more complex structure is visible both in the spectra of the single time traces and in the 2D Fourier transform. Still, the dominating contribution seems to come from the low mode number coupled bunch mode

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo MDs at 55 GeV/c: instability

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo MDs at 55 GeV/c: instability Also the Fourier analysis did not show any significant difference with what aboserved at 26 GeV/c. Again, the instability is dominated by a low number couploed-bunch mode.

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo MDs at 55 GeV/c: Cycle Dedicated SPS supercycle for MDs 4 LHC batches of 72 bunches at nominal intensity Flat bottom ~11 GeV Ramp ~2 s Intermediate flat top ~6 GeV Beam dump Measurement GeV Flat top ~1 GeV Controlled amittance blow up Measurements only at 55 GeV/c, with and without transverse emittance blow up (done by excitation with the transverse damper)

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo MDs at 55 GeV/c: Emittances w/o blow up with blow up Profiles taken with the ionization profile monitor (thanks to E. Métral and J. Koopman)  Using the transverse damper the beam sizes are increased by 2 and 1.4 in the horizontal and vertical plane, respectively.

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo MDs at 55 GeV/c: Build up Signal from the e-cloud monitor with 4 batches in the SPS with (right) or without (left) transverse emittance blow up With emittance blow up the two stripes become more dense (x 2), move inwards and a third stripe appears.

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo MDs at 55 GeV/c: instability Without emittance blow up:  An instability caused beam loss at the tail of the fourth batch, when vertical chromaticity was lowered to 0.05 toward the end of the intermediate plateau at 55 GeV/c

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo MDs at 55 GeV/c: instability

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo MDs at 55 GeV/c: instability The transverse emittance blow up can suppress this instability!!  The black trace is the BCT signal from a cycle w/o emittance blow up, the red trace from a cycle w blow up

AB-ABP/LHC Injector Synchrotrons Section CERN, Giovanni Rumolo 16 Summary of the observations Measurements on the were not conclusive: –Electron cloud was observed along the cycle with up to three batches injected into the SPS. The signal was stronger at higher energy, mainly induced by untrapped beam for one batch only. –The observed instability appeared to be coupled bunch on a low mode number (perhaps resistive wall) and at 55 GeV/c Measurements on the allowed us to draw several conclusions –An instability is observed at 55 GeV/c when lowering chromaticity to 0.05 and it causes beam loss at the tail of the fourth batch –Bunch by bunch centroid signal clearly reveals a single bunch instability affecting the few last bunches of the last train –Transverse emittance blow up can suppress this instability, which proves: The observed instability is induced by electron cloud, because it is was found to have a strong direct dependence on the beam transverse size The scaling law with energy as found with HEADTAIL simulations, because the decrease of the threshold with energy was explained as mainly due to the beam becoming thinner at higher energies.