AB-ABP/LHC Injector Synchrotrons Section CERN, 05.07.2007Giovanni Rumolo 1 Final results of the E-Cloud Instability MDs at the SPS (26 and 55 GeV/c) G

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



• 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



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



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



25.09.2007 MDs at 26 GeV/c: instability

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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 ?)




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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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.



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)



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.



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.




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




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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



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.

Documents

AB-ABP/LHC Injector Synchrotrons Section CERN, 05.07.2007Giovanni Rumolo 1 Final results of the E-Cloud Instability MDs at the SPS (26 and 55 GeV/c) G