CERN, 05.07.2007 Giovanni Rumolo 1
AB-ABP/LHC Injector Synchrotrons Section
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:– 25.09.2007, first attempt– 08.11.2007, ECI at 55 GeV/c !!!
• Summary of the observations
CERN, 05.07.2007 Giovanni Rumolo 2
AB-ABP/LHC Injector Synchrotrons Section
• 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
25.09.2007 MDs at 26 and 55 GeV/c
Dedicated SPS supercycle for MDs3 LHC batches of 72 bunches at nominal intensity
Flat bottom ~11 s @26 GeV Ramp ~2 s
Intermediate flat top ~6 s @55 GeV
~ 5% losses at the beginning of ramp
Beam dump
Measurement point @26 GeV
Measurement point @55 GeV
Flat top ~1 s @270 GeV
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AB-ABP/LHC Injector Synchrotrons Section
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.
25.09.2007 MDs at 26 and 55 GeV/c: cycle
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AB-ABP/LHC Injector Synchrotrons Section
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)
25.09.2007 MDs at 26 and 55 GeV/c: Build up
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AB-ABP/LHC Injector Synchrotrons Section
25.09.2007 MDs at 26 GeV/c: instability
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AB-ABP/LHC Injector Synchrotrons Section
25.09.2007 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 ?)
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AB-ABP/LHC Injector Synchrotrons Section
25.09.2007 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
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AB-ABP/LHC Injector Synchrotrons Section
25.09.2007 MDs at 55 GeV/c: instability
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AB-ABP/LHC Injector Synchrotrons Section
25.09.2007 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.
CERN, 05.07.2007 Giovanni Rumolo 10
AB-ABP/LHC Injector Synchrotrons Section
08.11.2007 MDs at 55 GeV/c: Cycle
Dedicated SPS supercycle for MDs
4 LHC batches of 72 bunches at nominal intensity
Flat bottom ~11 s @26 GeV Ramp ~2 s
Intermediate flat top ~6 s @55 GeV
Beam dumpMeasurement point @55 GeV
Flat top ~1 s @270 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)
CERN, 05.07.2007 Giovanni Rumolo 11
AB-ABP/LHC Injector Synchrotrons Section
08.11.2007 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.
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AB-ABP/LHC Injector Synchrotrons Section
08.11.2007 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.
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AB-ABP/LHC Injector Synchrotrons Section
08.11.2007 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
CERN, 05.07.2007 Giovanni Rumolo 14
AB-ABP/LHC Injector Synchrotrons Section
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08.11.2007 MDs at 55 GeV/c: instability
CERN, 05.07.2007 Giovanni Rumolo 15
AB-ABP/LHC Injector Synchrotrons Section
08.11.2007 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
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AB-ABP/LHC Injector Synchrotrons Section
Summary of the observations
• Measurements on the 25.09.2007 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, e-cloud @26GeV/c 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) both @26GeV/c and at 55 GeV/c
• Measurements on the 08.11.2007 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.