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Elias Métral 1
LHC BEAM INSTABILITY OBSERVATION ON 17/10/11 AND POSSIBLE INTERPRETATION
Observations => “Christmas tree” at the end of the squeeze with ~ 1.45E11 p/b
Predictions from our model Conclusion and recommendation
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Elias Métral 2
From LHC 08:30 meeting on TU 18/10/11
“Christmas tree” observed on MO 17/10/11
at 18:58:58
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Elias Métral 3
Signals not relevant anymore according to Ralph Steinhagen, as they do not reveal the max.
anymore (18/10/11)
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Elias Métral 4
18:57:00 18:55:00
Changes of gain from Ralph Steinhagen
B2H
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Elias Métral 5
18:59:00 18:57:00
The rise-time of the instability
cannot be observed due to the saturation
level
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Elias Métral 6
0.300 0.305 0.310 0.315 0.320!80
!70
!60
!50
!40
!30
!20
Qh
Amplitude!dB"
18:57:00 => Black 18:57:09 => Blue
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Elias Métral 7
0.300 0.305 0.310 0.315 0.320!80
!70
!60
!50
!40
!30
!20
Qh
Amplitude!dB"
18:57:00 => Black 18:57:09 => Blue 18:58:36 => Red
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Elias Métral 8
Increasing chromaticity will move
these points up and might lead to loss of
Landau damping
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Elias Métral 9
Would lead to an instability with a rise-time of ~ 1 s in the absence of Landau octupoles
=> Will be much longer when it is at the limit of stability
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Elias Métral 10
As already mentioned for the instability observed at the end of the squeeze when the beta* = 1 m was first tried (see https://lhc-beam-operation-committee.web.cern.ch/lhc-beam-operation-committee/m i n u t e s / M e e t i n g 1 7 - 3 0 _ 0 8 _ 2 0 1 1 /RecentObservationsOfLHCBeamInstabilities.pdf), the observed instability “could be” TCBI of mode |m| = 1 (would also explain the Chrismas tree as observed with SBI |m| = 1 and the very slow rise-time)
In the previous case the settings of the collimators were tighter, but the intensity lower, and it seemed that the BBLR played also a role at that time (but this mechanism mush have been also present, maybe with transverse distribution modified by BBLR as already discussed)
Recommendation: Increase a little bit the octupole strength, to go from a current of - 150 A to - 200 A or - 250 A to have some margin. Furthermore, we should also always try to reduce the chromaticity as much as possible