Updates on the LHC Wire Scanner Application --- Transverse Emittance Blow-up during the LHC Ramp
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Transcript of Updates on the LHC Wire Scanner Application --- Transverse Emittance Blow-up during the LHC Ramp
LHC
11/12/2013
Updates on the LHC Wire Scanner Application
---Transverse Emittance Blow-up
during the LHC Ramp
Verena Kain, Maria Kuhn
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OutlinePART 1o Updates on the LHC wire scanner application
− Requirements from last meeting− First version of the new GUI− The new philosophy
PART 2o Emittance blow-up during the LHC ramp
− Transverse emittance measurements in 2012− Possible sources of emittance blow-up during the LHC ramp
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PART 1: Updates on the LHC Wire
Scanner Application
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o Requirements from last meetingo First version of the new GUIo The new philosophy
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Requirements from Last Meeting (1)
o Create LSA parameter for nominal settings− Load to frontend with sequencer− Monitor on application
o Injection sequencer sets circulating + next injected bunch pattern for each new injection
o Automatically use optics for resident beam process− Display used optics name− Use measured optics if available in DB for beam process
o Can launch scans for all 4 scanners in one go− Application looks after scheduling Framework already provides
possibility to work with several scanners
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Requirements from Last Meeting (2)
o Average emittance of all bunches for log booko Emittance and integral of absolute of residuals per bunch as bar
graph
o Automatic logging of emittance results after each scan− Also logging optics ID
o Can retrieve profiles from logging and refit …fitting is not included yetWorking on data sources
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Requirements from Last Meeting (3)
o Launch Multiple Scans− Display emittance/beam sigma versus time (also subscribe to
energy and b*, optics ID) − Display mean versus time− Or…average over multiple scans
…framework foresees thisalready
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GUI – Mockup…..from Last Meeting
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First Version of the New GUIChoose your acquisition mode: logging/hardware; single/multi
Result for one or more scanners…scroll pane
Bunch-bunch resultProfile for one bunch
Configure scanners
Configure multi acquisition (# scans per time unit)
Configure logging access
Default values for everything
in/out
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The New Philosophyo Application works with objects WireScanner
− Status− Beam− Plane− …− Scans
o Object Scan− Timestamp− Bunch-by-bunch Profile data in/out, position− Optics ID− Energy
Optics and energy not on application level defined− Increased flexibility for online analysis of several scans e.g.
through the ramp or through the squeeze9
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PART 2: Transverse Emittance Blow-up
during the LHC Ramp
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o Transverse emittance measurements in 2012o Possible sources of emittance blow-up during the LHC ramp
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Emittance Growth during the LHC Cycle
o Total average emittance blow-up during the cycle for 50 ns physics beams in 2012 ~ 0.7 – 1 mm− Calculated from wire scans at injection and peak luminosity
o Transverse emittances are mainly growing during the injection plateau and the ramp− Contribution of the ramp: ~ 20 % blow-up depending on the beam
and the plane
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Sources of emittance growth during injection are intra beam scattering (IBS) and noise
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The LHC Ramp – A Closer Looko Energy ramp from 450 GeV to 4 TeV in 2012 (13 min)o CAVEAT: Measurements during the ramp only possible with wire
scanners only low intensity fills! measurement precision not verified (see later)o Closer look at beam 2 horizontal (Fill 3217)
− b functions from k-modulation (linear interpolation between injection and flattop values)
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Blow-up ~ 20 % (0.3 mm)
Similar for other MD fills
Absolute blow-up is independent of initial emittance!
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Puzzling Resultso It seems the emittance evolution during the ramp is different
for all planes. Why do we sometimes see decreasing emittances?
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Emittance blow-up during the ramp:Larger in the horizontal plane (15 – 30 %) than in the vertical plane (~ 5 %).
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Wire Scanners during the Rampo Have to change voltage and filter settings of wire scanners
during the ramp− This influences the measured beam sizes possibly due to
photomultiplier saturation [1]− Resulting in large uncertainties up to ~ 0.5 mm at injection and
~ 0.8 mm at 4 TeV
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Measurement at 450 GeV
But still cannot explain different emittance evolution for different planes because all wire scanners settings are changed similarly
Leads to additional error on beam size!
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2012 Test Cycles – Beam 1 Horizontal
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Emittance growth stops at around 2.5 TeV
– shrinking emittances are puzzling
12 bunches/batch
12 bunches/batch
6 bunches/batch 6 bunches/batch
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2012 Test Cycles – Beam 2 Horizontal
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Steeper slope of emittance growth during ramp starts only at around 1 TeV – before about the same growth as during injection
plateau (IBS).
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Beam and Machine Parameters to Investigate
o Beam profiles: should be Gaussiano Beam intensityo Bunch length and longitudinal emittanceso Tune and beam lifetimeo BBQ amplitudeso Transverse damper gaino Dispersiono Snapbacko Couplingo IBSo Noiseo Optics o Chromaticityo …
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Beam and Machine Parameters to Investigate
o Beam profiles: should be Gaussiano Beam intensityo Bunch length and longitudinal emittanceso 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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The To Do Listo Full IBS simulation through the ramp with MADX and CTE
− To be compared to measurementso Chromaticity measurements in 2012o Re-analysis of 2012 beta functions measurements during the
rampo Influence of optics correction functions through the rampo Effect of transverse damping time
− Pilot beam measurements?o Estimates of 50 Hz noise on emittance growth during the ramp
− Use AC dipole modelo Tune spectra during the rampo Longitudinal blow-upo (Vacuum)o …
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Preliminary IBS Simulation Results
o Simulations through the ramp are performed with MADX o IBS in the horizontal plane not negligible during the ramp
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For B2H and sometimes B1H the emittance growth at the start of the ramp is essentially IBS.
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Post LS1 Goalso Wire scanner calibration
− Including photomultiplier saturation curves
o Pilot beam measurements− To evaluate emittance growth in absence of transverse damper
o Alternative transverse beam size measurement methods− Possibly Beam Gas Vertex detector (BGV)
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Thank you for your attention!
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Literature[1] M. Kuhn, " Emittance Preservation at the LHC, " Master Thesis, University of Hamburg/CERN, Geneva, Switzerland 2013. [2] O. S. Bruening, P. Collier, P. Lebrun, et al., " LHC Design Report - The LHC Main Ring, vol. 1. , " CERN, 2004.[3] R. Steinhagen, "Real-Time Beam Control at the LHC," in Proceedings of
2011 Particle Accelerator Conferences, (New York, New York, USA), pp. 1399-1403, March 2011.
[4] A. Langner, R. Calaga, R. Miyamoto, et al., "Optics Measurement in the LHC Close to the Half Integer Tune Resonance," CERN-ATS-Note-2011-095 TECH, December 2011.
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Back-up
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Transverse Beam Profileso Transverse profiles through the LHC cycle
− Measured with wire scanners− Fitted the core of the profiles at 80 % intensity cut
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𝑓 (𝑥 )=𝑐+𝑘∗𝑥+𝑎∗exp (− (𝑥−𝑏)2
2σ2 )
Transverse profiles through the entire LHC cycle are Gaussian!
Very small error on the beam size from the Gauss fit
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Transverse Beam Profileso Transverse profiles through the LHC cycle
− Measured with wire scanners− Fitted the core of the profiles at 80 % intensity cut
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Beam Intensityo Bunch intensity stays almost constant during ramp
− Has no influence on emittance evolution during the ramp
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Beam intensity measured with Fast Beam Current Transformer (FBCT) [2]
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Longitudinal Emittance during Ramp
o Bunch lengths are kept constant during the LHC ramp with targeted longitudinal blow-up− The blow-up is on during the entire ramp− The longitudinal RF blow-up during the LHC ramp is the same for
both beams • Except for the frequency: slightly higher for beam 2 than for beam 1
o Longitudinal emittance evolution cannot explain blow-up (decrease) in the transverse planes during the ramp
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Tune and Beam Lifetime for Fill 3217
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No visible peaks in tune signal
Lifetimes: no indication for instabilities
Beam 1Beam 2Energy
Beam 1HBeam 1VBeam 2HBeam 2VEnergy
No hint from tune signal or beam lifetime for emittance growth during the ramp
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BBQ Amplitudes for Fill 3217o Beam oscillation amplitudes increase after reducing the
transverse damper gain at the start of the ramp− Measured with the LHC Base Band Tune (BBQ) monitors [3]− Reduction of transverse damper gain for sufficient tune signal
during the ramp
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Large amplitudes at the of ramp for B2H – explanation for continuing blow-up at 4 TeV?
Reduction of transverse damper gain
No large beam oscillation amplitudes during the ramp – cannot explain emittance growth!
Beam 1(H)Beam 1(V)Beam 2(H)Beam 2(V)Energy
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Dispersion
o Dispersion at the wire scanners is in the order of a few mm − Negligible compared to measured beam size− Not the reason for emittance blow-up during the ramp
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𝜺𝒙 ,𝒚=𝜸𝜷𝒙 ,𝒚 (𝝈𝟐−𝑫𝟐(𝒅𝒑𝒑 )
𝟐)
Beam size from dispersion:30 mm
s D450 GeV 600 – 1000 mm 0.1 m 3.0 x 10-4 4 TeV 200 – 400 mm 0.2 – 0.3 m at 7 TeV 1.3 x 10-4
measured with wire scanners in 2012
for nominal LHC optics at location of wire scanners
for 2012 LHC parameters
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Snapbacko Example: Fill 2722o Tune trims during the ramp:
o Snapback cannot explain continuous emittance growth during the ramp
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Beam 1HBeam 1VBeam 2HBeam 2VEnergy
Snapback is over at the very beginning of the ramp (~ 500 GeV)
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o Emittance growth in the vertical plane always smaller than in the horizontal plane
o Acceptable coupling for normal LHC operation ~ 0.002− For this fill beam 1 shows large coupling during the entire ramp− Beam 2 has strongly coupled planes at injection
o Coupling could be an explanation for emittance growth in the vertical planes
Coupling for Fill 3217
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threshold
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Optics Measurementso The beta functions were measured through the ramp in 2012
− With turn-by-turn phase advance method at discrete energies • at 1.3, 2.3, 3.0 TeV for beam 1 and at 1.3, 2.0, 2.6, 3.6 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 − Used linear interpolation between injection and flattop values
o During LS1: improvements of the algorithm− Expect re-calculated beta values through the ramp in December
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Optics Corrections during Rampo Global and local beta beat correction for both beams during the
ramp:
o Is there a visible effect on emittance growth during the ramp?− Caveat: global correction started in 2012 but there is also
measurable emittance growth in 2011
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At 250 s = 1065 GeV during the LHC ramp the global beta beat for both beams is switched off.
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Optics Corrections during Ramp – B1
o In general, emittance growth during the ramp in the horizontal plane stops ~ 2.5 TeV− emittances in the vertical are starting to shrink ~ 1.5 TeV
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Fill 2722: 2 x 12 nominal 50 ns bunches
For some fills emittance growth in both planes of beam 1 is changing at ~ 1 TeV! But it is not clear, whether this evolution is correlated to optics corrections.
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Optics Corrections during Ramp – B2
o In general, the slope of emittance growth of beam 2 horizontal becomes steeper at around 1 TeV during the LHC ramp− Is there a correlation to beta beat?
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Fill 3014: 2 x 6 nominal
50 ns bunches
Emittances in the horizontal plane are growing more and in the vertical plane are shrinking at ~ 1 TeV!
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Summary Part 2o Main unresolved emittance growth issue during the LHC cycle:
source of blow-up during the rampo Emittance growth during the ramp
− Larger in the horizontal plane (15 – 30 %) than in the vertical plane (~ 5 %)
− Causes are unclearo Puzzling results:
− Beam 1 horizontal: growth stops at ~ 2.5 TeV− Beam 2 horizontal: continuously grows through ramp, sometimes
even still on flattop• growth steeper slope after > 1 TeV, before probably essentially IBS
− No common pattern of emittance blow-up during the ramp in the different planes• Sometimes even decreasing emittances (especially in B1H, B1V and
B2V)
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Conclusion Part 2 (1)o Possible sources that can be excluded
− Non Gaussian profiles, intensity losses, longitudinal emittance evolution, tune, beam lifetime, reduced transverse damper gain, dispersion
o Puzzling results:− Independent of initial emittance same absolute emittance growth
• Indication of problem’s origin - noise?• But higher transverse damper gain does not help• And no clear indication from beam oscillation amplitudes
o How trustable are transverse profile monitors (wire scanners)?− Wire scanner photomultiplier saturation during the ramp− But asymmetry cannot be explained by changing wire scanner
settings− Measurements in ATLAS, CMS and LHCb suggest even more
emittance blow-up during the LHC cycle
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Conclusion Part 2 (2)o Speculating growth in vertical plane comes from couplingo IBS contributes to growth during ramp in the horizontal plane
− IBS growth rate decreases only very slowly with energy, negligible only from ~ 2 - 3 TeV (depending on initial emittance)
− BUT: IBS cannot explain growth during ramp - growth rate is larger during ramp than at injection
o Measurement of beta functions through the ramp could help to understand the problems
o Optics corrections through the ramp influence emittance evolution during the ramp: beta beat is switched off at 1065 GeV in the ramp− For beam 2 horizontal that could explain the steeper emittance
growth starting at the same time− All other planes indicate no correlation to beta beat or show
shrinking emittances at that time
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