LHC Em Emit ittance tance Gr Growth owth Dur uring ing the he LH LHC Ramp mp The he TRUE E Story ory M. Kuhn, G. Arduini, V. Kain, A. Langner, Y. Papaphilippou, M. Schaumann, R. Tomas 05/02/2014 1
Mo Motivati ivation: on: Em Emit ittance tance Blo low-up up 2012 012 LHC o Overall average emittance blow-up through the LHC cycle: ~ 0.5 – 0.8 m m from injection to start of collision (convoluted e ) − • Similar for ATLAS luminosity Convoluted e : After TS3: • Collision values Q20 optics from CMS bunch in SPS and luminosity spare wire (nominal b *) scanner system in Injection values • LHC from LHC wire scanners (average of first 144 bunch batch), b from beta beat meas. 05/02/2014 2
Int ntrodu roducti ction on LHC o 2012 available transverse profile monitors through the cycle: ONLY WIRE SCANNERS! − • Could only measure low intensity test fills • Problem with photomultiplier saturation during the ramp o Conclusions from wire scanner measurements: Emittances are mainly growing during injection plateau and ramp − − Sometimes shrinking emittances during the ramp Sometimes large blow-up at the end of squeeze − o Sources of emittance blow-up: Injection: IBS and 50 Hz noise − Ramp: no clue so far − Squeeze: probably single bunch instabilities − 05/02/2014 3
What’s New: LHC C Bet eta a Fct ct. Mea easu surements ements LHC o The beta functions were measured through the ramp in 2012 With turn-by-turn phase advance method at discrete energies − • at 0.45, 1.33, 2.3, 3.0, 3.8, 4.0 TeV for beam 1 • at 0.45, 1.29, 2.01, 2.62, 3.66, 4.0 TeV for beam 2 Large uncertainties because of not optimal phase advance between − the BPMs and problems with the algorithm o Measured beta functions through the ramp could therefore not be used for emittance determination in 2012 − Used linear interpolation between measured injection and flattop values from k-modulation o Now: improvements of the algorithm − re-calculated beta values through the ramp from AC dipole meas. 05/02/2014 4
Beta ta Fu Func ncti tions ons thr hrough ough LH LHC Ramp mp LHC o Results obtained with new algorithm Measurements performed in October 2012 (MD3) − − Beta functions during the LHC ramp at location of the wire scanners: Many thanks to A. Langer and R. Tomas! Note: large relative errors in B2H 05/02/2014 5
Com omparison parison of of Beta ta Fu Func ncti tions ons LHC 1. Interpolation of k-modulation values from injection to flattop 2. AC dipole measurements during the ramp + interpolation 05/02/2014 6
Wi Wire e Scan canner ner Me Measu asurements rements LHC o Comparison of emittances with different beta values K-modulation interpolation vs. AC dipole measurements − − Example: Fill 3217, B1H (other planes look similar) Beta beat values K-modulation values Total growth through ramp reduced with new optics in ramp But non-physical growth and shrinking still there! 05/02/2014 7
LHC Wh Where ere do o the he shri hrinki nking ng emit ittances tances com ome e from? om? 05/02/2014 8
Emi mittance tance vs. . Beta ta Fu Func ncti tion on – B1 B1 LHC o Growing- shrinking emittances due to non-monotonic changes of optics at wire scanners (same for B1H) Not enough beta- measurements to remove all “non - physical” points − 05/02/2014 9
Emi mittance tance vs. . Beta ta Fu Func ncti tion on – B2 B2 LHC o Monotonic growth of beta function at wire scanner (same for B2V) no shrinkage 05/02/2014 10
Résumé sumé – Non Non-Ph Phys ysical ical Emittance tance Evoluti ution on LHC o Most probable reason behind non-physical evolution of emittances during the ramp in 2012 Insufficient knowledge of beta function evolution at wire scanners − during ramp Still not enough beta measurement points to remove all “outliers” in − emittance evolution for B1H and B1V o Next: emittance measurement with new beta functions vs. IBS simulations (MADX) during the ramp Using nominal optics − Measured bunch length through the ramp − − Initial emittance at start of ramp from wire scans CAVEAT: MADX algorithm assumes no coupling − • therefore predicts no growth in the vertical planes 05/02/2014 11
IBS S Sim imul ulati ations ons (1) LHC o Use input parameters from wire scans at the start of the ramp o Simulate emittance blow-up due to IBS with MADX Fill 3217, batch 1 (6 bunches) 05/02/2014 12
IBS S Sim imul ulati ations ons (2) LHC o Beam 2: relative emittance growth during the ramp fits very well with IBS simulations 05/02/2014 13
IBS S Sim imul ulati ations ons (3) LHC If it is ONLY IBS…why is it same growth for different initial e o o Fill 3217, all bunches (2 x 6): Bunch lengths and bunch intensities similar for both batches, but different initial emittances Almost same growth in IBS simulation 05/02/2014 14
IBS S Sim imul ulati ations ons (4) LHC o Fill 3217, all bunches, relative emittance growth Smaller initial emittance (B2H batch 1) gives slightly larger growth ~ 5 % instead of ~ 4 % BUT NOT MUCH DIFFERENCE! 05/02/2014 15
Résumé sumé - IBS IBS and nd LH LHC Ramp mp LHC o Emittance growth in the horizontal plane during ramp probably only from IBS For test fills ~ 3 - 5 % depending on initial beam parameters − o First guess for physics fills during ramp: Small would predict ~ 5 % ( ≤ 𝟏. 𝟐 m m) growth through the ramp − • Again dependent on initial beam parameters Prediction for physics fills before TS3: ~ 3 % ( ≤ 𝟏. 𝟏𝟔 m m) • o The what is the simulated IBS emittance growth through the LHC cycle compared to measurements? For test Fill 3217 − For physics fills − 05/02/2014 16
Em Emit ittance tance thr hrough ough 2012 012 LH LHC Cycle cle LHC Fill 3217 (Oct. 2012, after octupole polarity switch), large growth during squeeze! 05/02/2014 17
Ex Exampl mple e IBS S dur uring ing the he Cycle cle – B2H LHC o Monotonic optics changes for B2H during the LHC cycle Therefore smooth emittance growth − o Full IBS simulation during the entire cycle compared to wire scanner measurements IBS simulations and measurements for B2H very compatible! 05/02/2014 18
IBS S dur uring ing the he LH LHC Cycle cle LHC o Estimates of mean horizontal emittance growth: Mean Fill 3217 simula lated ted Fill 3217 measu sured ed Mean physics ics fill time [s] [s] time [s] [s] simul ulated ted 6 %, 0.09 m m 8 %, 0.12 m m Injection 590 ~ 900 5 – 10 % ≤ 𝟏. 𝟑 m m Additional meas. growth from 50 Hz noise 4 %, 0.06 m m 3 %, 0.04 m m Ramp 770 770 3 – 5 % ≤ 𝟏. 𝟐 m m Flattop – 1500 5 %, 0.08 m m 9 % , 0.15 m m 1800 3 – 5 % ≤ 𝟏. 𝟐 m m start coll. 15 %, 0.23 m m 21 %, 0.31 m m TOTAL 2860 3470 ~ 10 – 20 % ≤ 𝟏. 𝟓 m m (47 min) (58 min) o IBS Simulations agree well with wire scanner measurements! Growth at flattop larger than expected! − But also some growth in the vertical plane (coupling for this fill) − Total average growth of convoluted e through the LHC cycle o For Fill 3217: 0.29 m m − Why this large difference? For physics fills: ~ 0.5 m m – 0.8 m m − 05/02/2014 19
LHC CAN WE WE TRUST ST WI WIRE RE SC SCANNER NNER ME MEASUREMENTS??? SUREMENTS??? Fi First t puz uzzle le: : dis iscrep crepancy ancy wire ire sca canne nner – ATLAS/C LAS/CMS lu lumi mino nosity ity and nd LH LHCb SMO MOG G me measure asurements ments 05/02/2014 20
ATL TLAS/ AS/CMS MS vs vs. . Wi Wire re Scanner anner LHC o Low intensity test fill in 2012 (Fill 3217): Injection values measured with wire scanners − • Beta function from AC dipole measurement − Collision values measured with wire scanners and obtained from ATLAS and CMS luminosity Average value of 6 colliding bunches (batch 2) − Wire scanner ner ATLAS AS CMS CMS Emittance at injection [ m m] 1.48 ± 0.06 Emittance at collision [ m m] 1.77 ± 0.06 2.36 ± 0.35 2.63 ± 0.38 Emittance growth [ m m] 0.29 ± 0.12 0.88 ± 0.41 1.15 ± 0.44 Relative growth 20 % 59 % 77 % o Wire scan results much smaller than ATLAS/CMS results! − Similar for other test fills measured in 2012 05/02/2014 21
ATL TLAS/ AS/CMS MS vs vs. . Wi Wire re Scanner anner LHC o Low intensity test fill in 2012 (Fill 3217): Injection values measured with wire scanners WITH THOUT OUT CORE E FIT − • Beta function from AC dipole measurement − Collision values measured with wire scanners and obtained from ATLAS and CMS luminosity Average value of 6 colliding bunches (batch 2) − Wire scanner ner ATLAS AS CMS CMS Emittance at injection [ m m] 1.58 ± 0.06 Emittance at collision [ m m] 1.84 ± 0.06 2.36 ± 0.35 2.63 ± 0.38 Emittance growth [ m m] 0.25 ± 0.12 0.78 ± 0.41 1.05 ± 0.44 Relative growth 16 % 49 % 66 % o Wire scan results much smaller than ATLAS/CMS results! − Similar for other test fills measured in 2012 05/02/2014 22
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