LHC Emittance Growth During the LHC Ramp The TRUE Story M. Kuhn, G. Arduini, V. Kain, A. Langner, Y. Papaphilippou, M. Schaumann, R. Tomas 1 28/01/2014
Emittance Growth During the LHC Ramp The TRUE Story
Feb 22, 2016
Emittance Growth During the LHC Ramp The TRUE Story. M. Kuhn, G. Arduini, V. Kain, A. Langner, Y. Papaphilippou, M. Schaumann, R. Tomas. Motivation: Emittance Blow-up 2012. Overall average emittance blow-up through the LHC cycle: - PowerPoint PPT Presentation
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LHC
28/01/2014
Emittance Growth During the LHC Ramp
The TRUE Story
M. Kuhn, G. Arduini, V. Kain, A. Langner, Y. Papaphilippou,
M. Schaumann, R. Tomas
1
LHC
28/01/2014
o Overall average emittance blow-up through the LHC cycle:− ~ 0.5 – 0.8 mm from injection to start of collision (convoluted e)
• Similar for ATLAS luminosity
Motivation: Emittance Blow-up 2012
2
After TS3: Q20 optics in SPS and spare wire scanner system in LHC
Convoluted e:• Collision values
from CMS bunch luminosity (nominal b*)
• Injection values from LHC wire scanners (average of first 144 bunch batch), b from beta beat meas.
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Introductiono 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
3
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What’s New: LHC Beta Fct. Measurements
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.
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Beta Functions through LHC Ramp
o Results obtained with new algorithm− Measurements performed in October 2012 (MD3)− Beta functions during the LHC ramp at location of the wire
scanners:
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Many thanks to A. Langer and R. Tomas!
Note: large relative errors in B2H
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Comparison of Beta Functions1. Interpolation of k-modulation values from injection to flattop2. AC dipole measurements during the ramp + interpolation
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Wire Scanner Measurementso Comparison of emittances with different beta values
− K-modulation interpolation vs. AC dipole measurements− Example: Fill 3217, B1H (other planes look similar)
Total growth through ramp reduced with new optics in rampBut non-physical growth and shrinking still there! 7
Beta beat values K-modulation values
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Where do the shrinking emittances come from?
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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
Emittance vs. Beta Function – B1
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Emittance vs. Beta Function – B2o Monotonic growth of beta function at wire scanner (same for
B2V) no shrinkage
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Résumé – Non-Physical Emittance Evolution
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
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IBS Simulations (1)o Use input parameters from wire scans at the start of the rampo Simulate emittance blow-up due to IBS with MADX
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Fill 3217, batch 1 (6 bunches)
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o Beam 2: relative emittance growth during the ramp fits very well with IBS simulations
IBS Simulations (2)
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IBS Simulations (3)o If it is ONLY IBS…why is it same growth for different initial eo Fill 3217, all bunches (2 x 6):
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Bunch lengths and bunch intensities similar for both batches, but different initial emittances
Almost same growth in IBS simulation
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IBS Simulations (4)o Fill 3217, all bunches, relative emittance growth
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Smaller initial emittance (B2H batch 1) gives slightly larger growth ~ 5 % instead of ~ 4 %
BUT NOT MUCH DIFFERENCE!
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Résumé - IBS and LHC Rampo 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 % ( mm) growth through the ramp
• Again dependent on initial beam parameters• Prediction for physics fills before TS3: ~ 3 % (mm)
o The what is the simulated IBS emittance growth through the LHC cycle compared to measurements?− For test Fill 3217− For physics fills
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LHC
Emittance through 2012 LHC Cycle
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Fill 3217 (Oct. 2012, after octupole polarity switch), large growth during squeeze!
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Example IBS during the Cycle – B2H
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
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IBS simulations and measurements for B2H very compatible!
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IBS during the LHC Cycleo Estimates of mean horizontal emittance growth:
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)
o Total average growth of convoluted e through the LHC cycle− For Fill 3217: 0.29 mm − For physics fills: ~ 0.5 mm – 0.8 mm
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Mean time [s]
Fill 3217 simulated
Fill 3217 measured
Mean time [s]
physics fill simulated
Injection 590 6 %, 0.09 mm 8 %, 0.12 mm ~ 900 5 – 10 % mm
Ramp 770 4 %, 0.06 mm 3 %, 0.04 mm 770 3 – 5 % mm
Flattop – start coll.
1500 5 %, 0.08 mm 9 % , 0.15 mm 1800 3 – 5 %
TOTAL 2860 (47 min)
15 %, 0.23 mm 21 %, 0.31 mm 3470 (58 min)
~ 10 – 20 %
mm
Why this large difference?
Additional meas. growth from 50 Hz noise
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First puzzle: discrepancy wire scanner – ATLAS/CMS luminosity and LHCb SMOG measurements
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ATLAS/CMS vs. Wire Scannero 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)
o Wire scan results much smaller than ATLAS/CMS results!− Similar for other test fills measured in 2012
21
Wire scanner
ATLAS CMS
Emittance at injection [mm] 1.48 ± 0.06Emittance at collision [mm] 1.77 ± 0.06 2.36 ± 0.35 2.63 ± 0.38Emittance growth [mm] 0.29 ± 0.12 0.88 ± 0.41 1.15 ± 0.44Relative growth 20 % 59 % 77 %
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o Measurements during 30 min in stable beams with beam gas interactions in LHCb (SMOG) and wire scanners
SMOG vs. Wire Scanner
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LHCb e calculated with nominal b* = 3 m, WS e calculated with b from AC dipole meas., average e of 6 bunches per batch
Discrepancy up to 1 mm, but no systematic difference between wire scanner and LHCb emittances!
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Summary & Conclusiono Measurements through the LHC cycle in 2012 only possible with
wire scanners− Main blow-up occurs during injection and ramp− Sources of emittance growth at injection: IBS and 50 Hz noise
o NOW: new beta function analysis for values through the ramp− Total growth through ramp reduced with new optics in ramp− Growing- shrinking emittances due to non-monotonic changes of
optics at wire scannerso Comparison of wire scans and IBS Simulations:
− Emittance growth in the horizontal plane during ramp probably only from IBS
− But large blow-up during squeeze that cannot be explained with IBS
o PUZZLE: discrepancy between wire scanner and ALTAS/CMS/LHCb emittances at start of collisions− ATLAS/CMS suggest larger total blow-up− LHCb measurements not fully compatible with wire scans 2
3
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Outlook – Beams Post LS1o Emittance growth for high brightness beams post LS1
− With the following beam parameters for IBS simulations:• 1.3 mm injected emittance• Bunch intensity of 1.2 x 1011 ppb• 1.25 ns bunch length
− 20 min ramp to 6.5 TeV• assuming injection and flattop plateau length are same as in 2012
o Estimated emittance blow-up in the horizontal plane from injection to start of collision: ~ 20 % ( 0.3 mm) only from IBS− Similar as in 2012
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Thank you for your attention!
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Additional Slides
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Possible Sources of Blow-Up During Ramp
o Non Gaussian beam profileso Beam intensity losseso Bunch length and longitudinal emittances instabilitieso Tune and beam lifetimeo BBQ amplitudes o Transverse damper gaino Dispersiono Snapback o Coupling – could cause emittance growth in the vertical planeso IBSo Noiseo Optics o Chromaticityo …
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50 Hz noise and IBS cause emittance growth at the injection plateau
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For Completeness: Optics Correction Knob vs. Beta Function Evolution
o Effect of optics correction knob on beta functions during the ramp not obvious
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Wire Scanner Measurements – B1V
Total growth through ramp reduced with new optics in ramp
o But non-physical growth and shrinking still there!
28
Beta beat values K-modulation values
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Wire Scanner Measurements – B2H
Total growth through ramp MUCH reduced with new optics in rampo (For completeness: continuing growth in B2H during flattop
correlates with large amplitudes in BBQ for that particular fill)
29
Beta beat values K-modulation values
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Wire Scanner Measurements – B2V
Total growth through ramp reduced with new optics in ramp
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Beta beat values K-modulation values
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Emittance vs. Beta Function – B1H
Growing- shrinking emittances due to non-monotonic changes of optics at wire scanners. Even more obvious for B1 V, next slide.
− Not enough beta-measurements to remove all “non-physical” points
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Emittance vs. Beta Function – B2V
o Monotonic growth of beta function at wire scanner no shrinkage
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Fill 2687 with Ramp Betas
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Fill 2722 with Ramp Betas
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Fill 3014 with Ramp Betas
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Beam Parameters 2012 Test Cycles
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Fill Beam Bunch length [ns]
Bunch intensity [1011 ppb]
2687(batch 2: 12 bunches)
1 1.2 1.4 1.92 1.732 1.18 1.4 1.68 1.80
2722(batch 1: 12 bunches)
1 1.23 1.35 1.92 1.732 1.27 1.4 1.68 1.92
3014(batch 1: 6 bunches)
1 1.17 1.55 1.92 1.782 1.17 1.6 1.92 1.97
3217(batch 1: 6 bunches)
1 1.2 1.55 1.51 1.422 1.15 1.6 1.52 1.49
3217(batch 2: 6 bunches)
1 1.15 1.55 1.51 1.442 1.15 1.65 1.68 1.65
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Fill 2687 IBS Simulationso Fill 2687, batch 2, 12 bunches
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Fill 2722 IBS Simulationso Fill 2722, batch 1, 12 bunches
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Fill 3014 IBS Simulationso Fill 3014, batch 1, 6 bunches
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Fill 3217 ATLAS/CMS Luminosityo Rather large discrepancy between ATLAS and CMS emittance
values from luminosityo Closer look at specific luminosity reveals different results for
ATLAS and CMS
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Fill 3217 6 bunches (batch 2) colliding.
Black line indicates peak luminosity taken for emittance calculation.
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MD3 Fill 3160 – LHC Cycle
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o Measurements during 30 min in stable beams: batch 1 and 2
SMOG vs. Wire Scanner (1)
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LHCb emittances calculated with nominal b* = 3 m, l WS emittances calculated with b from beta beat meas.Average e of 6 bunches per batch
Measurements in the vertical plane agree best. No systematic difference between WS and LHCb emittances visible.
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o Measurements during 30 min in stable beams: batch 3 and 4
SMOG vs. Wire Scanner (2)
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Measurements in the horizontal plane agree best.
LHCb emittances calculated with nominal b* = 3 m, l WS emittances calculated with b from beta beat meas.Average e of 6 bunches per batch
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Fill 3160 SMOG Data Analysiso Some observations:
− We had 2 hours of SMOG operation.− The true beam width at LHCb varies between 35 and 70 mu,
leading to a systematic uncertainty of 0.7 - 0.8 mu.− The bunches had different intensities leading to a beam-gas rate of
10 to 45 Hz. We can reach about 0.7 micron statistical uncertainty in 5 minutes, but this vary with the bunch intensity.
− The resolution deconvolution is made assuming a simple Gaussian beam which is reasonable for this fill. A double Gaussian analysis might help on some bunches, but most fit chi^2 are close to 1.
− The beam width given is 1 sigma of a Gaussian distribution.− The statistical and systematic uncertainties are provided. It is
therefore possible to average multiple time bins to reduce the statistical error, however, the systematic error should only be averaged and can not be reduced by averaging.
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