Emittance Growth During the LHC Ramp The TRUE Story

Emittance Growth During the LHC RampThe TRUE Story M. Kuhn, G. Arduini, V. Kain, A. Langner, Y. Papaphilippou, M. Schaumann, R. Tomas 28/01/2014

Motivation: Emittance Blow-up 2012 • 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 • 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. After TS3: Q20 optics in SPS and spare wire scanner system in LHC 28/01/2014

Introduction 28/01/2014 • 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 • 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 • Sources of emittance blow-up: • Injection: IBS and 50 Hz noise • Ramp: no clue so far • Squeeze: probably single bunch instabilities

What’s New: LHC Beta Fct. Measurements 28/01/2014 • 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 • 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 • Now: improvements of the algorithm • re-calculated beta values through the ramp from AC dipole meas.

Beta Functions through LHC Ramp Many thanks to A. Langer and R. Tomas! • Note: large relative errors in B2H 28/01/2014 • Results obtained with new algorithm • Measurements performed in October 2012 (MD3) • Beta functions during the LHC ramp at location of the wire scanners:

Comparison of Beta Functions 28/01/2014 Interpolation of k-modulation values from injection to flattop AC dipole measurements during the ramp + interpolation

Wire Scanner Measurements Beta beat values K-modulation values 28/01/2014 • 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 ramp But non-physical growth and shrinking still there!

Where do the shrinking emittances come from? 28/01/2014

Emittance vs. Beta Function – B1 28/01/2014 • 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 – B2 28/01/2014 Monotonic growth of beta function at wire scanner (same for B2V) no shrinkage

Résumé – Non-Physical Emittance Evolution 28/01/2014 • 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 • 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

IBS Simulations (1) Fill 3217, batch 1 (6 bunches) 28/01/2014 Use input parameters from wire scans at the start of the ramp Simulate emittance blow-up due to IBS with MADX

IBS Simulations (2) 28/01/2014 Beam 2: relative emittance growth during the ramp fits very well with IBS simulations

IBS Simulations (3) Bunch lengths and bunch intensities similar for both batches, but different initial emittances Almost same growth in IBS simulation 28/01/2014 If it is ONLY IBS…why is it same growth for different initial e Fill 3217, all bunches (2 x 6):

IBS Simulations (4) Smaller initial emittance (B2H batch 1) gives slightly larger growth ~ 5 % instead of ~ 4 % BUT NOT MUCH DIFFERENCE! 28/01/2014 Fill 3217, all bunches, relative emittance growth

Résumé - IBS and LHC Ramp 28/01/2014 • Emittance growth in the horizontal plane during ramp probably only from IBS • For test fills ~ 3 - 5 % depending on initial beam parameters • 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) • The what is the simulated IBS emittance growth through the LHC cycle compared to measurements? • For test Fill 3217 • For physics fills

Emittance through 2012 LHC Cycle Fill 3217 (Oct. 2012, after octupole polarity switch), large growth during squeeze!

Example IBS during the Cycle – B2H IBS simulations and measurements for B2H very compatible! 28/01/2014 • Monotonic optics changes for B2H during the LHC cycle • Therefore smooth emittance growth • Full IBS simulation during the entire cycle compared to wire scanner measurements

IBS during the LHC Cycle Additional meas. growth from 50 Hz noise  Why this large difference? 28/01/2014 • Estimates of mean horizontal emittance growth: • 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 ethrough the LHC cycle • For Fill 3217: 0.29 mm • For physics fills: ~ 0.5 mm – 0.8 mm

First puzzle: discrepancy wire scanner – ATLAS/CMS luminosity and LHCb SMOG measurements 28/01/2014

ATLAS/CMS vs. Wire Scanner 28/01/2014 • 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 scan results much smaller than ATLAS/CMS results! • Similar for other test fills measured in 2012

SMOG vs. Wire Scanner Discrepancy up to 1 mm, but no systematic difference between wire scannerand LHCbemittances! LHCbe calculated with nominal b* = 3 m, WS ecalculated with b from AC dipole meas., average e of 6 bunches per batch 28/01/2014 Measurements during 30 min in stable beamswith beam gas interactions in LHCb (SMOG) and wire scanners

Summary & Conclusion 28/01/2014 • 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 • 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 scanners • 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 • 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

Outlook – Beams Post LS1 Thank you for your attention! 28/01/2014 • 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 1011ppb • 1.25 ns bunch length • 20 min ramp to 6.5 TeV • assuming injection and flattop plateau length are same as in 2012 • 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

Additional Slides 28/01/2014

Possible Sources of Blow-Up During Ramp 50 Hz noise and IBS cause emittance growth at the injection plateau 28/01/2014 Non Gaussian beam profiles Beam intensity losses Bunch length and longitudinal emittances instabilities Tune and beam lifetime BBQ amplitudes Transverse damper gain Dispersion Snapback Coupling – could cause emittance growth in the vertical planes IBS Noise Optics Chromaticity …

For Completeness: Optics Correction Knob vs. Beta Function Evolution 28/01/2014 Effect of optics correction knob on beta functions during the ramp not obvious

Wire Scanner Measurements – B1V Beta beat values K-modulation values 28/01/2014  Total growth through ramp reduced with new optics in ramp But non-physical growth and shrinking still there!

Wire Scanner Measurements – B2H Beta beat values K-modulation values 28/01/2014  Total growth through ramp MUCH reduced with new optics in ramp (For completeness: continuing growth in B2H during flattop correlates with large amplitudes in BBQ for that particular fill)

Wire Scanner Measurements – B2V Beta beat values K-modulation values 28/01/2014  Total growth through ramp reduced with new optics in ramp

Emittance vs. Beta Function – B1H 28/01/2014 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

Emittance vs. Beta Function – B2V 28/01/2014 Monotonic growth of beta function at wire scanner no shrinkage

Fill 2687 with Ramp Betas 28/01/2014

Beam Parameters 2012 Test Cycles 28/01/2014

Fill 2687 IBS Simulations 28/01/2014 Fill 2687, batch 2, 12 bunches

Fill 2722 IBS Simulations 28/01/2014 Fill 2722, batch 1, 12 bunches

Fill 3014 IBS Simulations 28/01/2014 Fill 3014, batch 1, 6bunches

Fill 3217 ATLAS/CMS Luminosity Fill 3217 6 bunches (batch 2) colliding. Black line indicates peak luminosity taken for emittance calculation. 28/01/2014 Rather large discrepancy between ATLAS and CMS emittance values from luminosity Closer look at specific luminosity reveals different results for ATLAS and CMS

MD3 Fill 3160 – LHC Cycle 28/01/2014

SMOG vs. Wire Scanner (1) Measurements in the vertical plane agree best. No systematic difference between WS and LHCb emittances visible. 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 28/01/2014 Measurements during 30 min in stable beams: batch 1 and 2

SMOG vs. Wire Scanner (2) 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 horizontal plane agree best. 28/01/2014 Measurements during 30 min in stable beams: batch 3 and 4

Fill 3160 SMOG Data Analysis 28/01/2014 • 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.

Emittance Growth During the LHC Ramp The TRUE Story