Instability observations Week 190618 290618 S ANTPOV X
Instability observations Week 19/06/18 – 29/06/18 S. ANTPOV, X. BUFFAT, N. MOUNET, B. SALVANT ON BEHALF OF LHC TRANSVERSE INSTABILITY MONITORING TEAM 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 1
Probably, something bad has happened… 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 2
In order to efficiently follow-up on instabilities as they occur we need a point of contact in Operation Notify lhc-transverse-instability-monitoring when a problem occurs We need to know: • What happened? • When it happened? • Is it still critical? • Fill number? • Link to logbook? 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 3
What happened? A lot of fills! Most have no useful data 6819 - TS 6820: No beam 6821: First beam after TS. 6822: Loss maps at Inj. 6823: No Beam 6824: Loss maps 6825: Dump, no beam 6826: Loss maps at Ramp 6827: Q’ Meas. 6828: “Normal" fill, Q change 6829: IP optimization 6830: No beam 6831: No beam 6832: Dump at Inj. 6833: Loss maps 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 6834: Loss maps and asyn. dump 6835: Dump on orbit probem 6836: No beam 6837: 90 m beta* fill 6839: Only inj. , no beam 6840: Orbit problems 6841: No beam 6842: loss maps at Inj. 6843: Physics with 3 bunches 6844: Loss maps after Adj. 6845: Loss maps FT 6846: Inj. stopped due to SPS 6847: 140 bunches physics fill 4
Summary of instabilities Decrease of octupole current – Fills 6821, 6826, 6828 Change of tunes, decrease of tune separation – Fill 6821 Bunch length shortening (possibly RF problem) – Fill 6826 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 5
Octupole current decrease 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 6
Fill 6821 ØTwo clear instabilities after the end of the squeeze (especially strong for the second): BBQ �� * IP 1/5 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 7
Fill 6821: Instability correlated with decerease of octupole current oct. current BBQ 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 8
Mode 1 from the Headtail monitor: 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 9
A similar instability in Fill 6826 B 2 right after beginning of Adjust Time in the cycle (min) No intensity loss! B 2 H 120 110 100 (No ADT data plot for HB 2) Adjust begins 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 10
Fill 6826: Seems to start right after the Octupole current is reduced to 300 A B 2 H Oct. lowered to 300 A Time in the cycle (min) 120 110 100 • Consistent with the observations in Fills 6821, 6828 by N. Mounet • Recommend increasing the octupole current to 376 A Adjust begins 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 11
Fill 6828: Most instabilities occur at relatively low octupole current (300 A) oct. current BBQ But not the first one… maybe related to a very small tune separation H/V (coupling)? 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 12
Tune separation (coupling) 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 13
An instabilty in Fill 6821 seems to be related to the reduction of the separation between H and V tunes 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 14
Change of tunes 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 15
Bunch length decrease 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 16
Fill 6826: Instability in Adjust Seems to be related to bunch shortening According to George, the RF cavities were not all correctly phased on Beam 1 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 17
Leads to intensity loss and emittance blow-up Observed on both beams ~50% LOSS OF INTENSITY 6/26/2018 OVER 100% EMITTANCE BLOW-UP LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 18
We Recommend Ø Correction of coupling / chromaticity to +15 Ø One should never use standard collimation settings for Vd. M scans Ø Beams are offset which makes them closer to the jaws Ø Octupoles & ADT as in Physics operations as soon as single-bunch intensity is nominal. Moreover, in physics the long range provide an additional 50 to 100 A so one should rather even increase the octupole current when those are not present. Ø Increasing ADT gain seemed indeed to have cured the first instability (still, this is half the gain of physics operation? ): Ø It would be useful for us to be able to see what the ADT gain is in Timber 6/26/2018 LHC INSTABILITY OBSERVATIONS: WEEK OF 19/06/18 19
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