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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LHC

11/12/2013

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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LHC

11/12/2013

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

LHC

11/12/2013

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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LHC

11/12/2013

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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LHC

11/12/2013

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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LHC

11/12/2013

GUI – Mockup…..from Last Meeting

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LHC

11/12/2013

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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LHC

11/12/2013

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

LHC

11/12/2013

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

LHC

11/12/2013

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

LHC

11/12/2013

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!

LHC

11/12/2013

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 %).

LHC

11/12/2013

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!

LHC

11/12/2013

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

LHC

11/12/2013

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).

LHC

11/12/2013

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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LHC

11/12/2013

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

LHC

11/12/2013

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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LHC

11/12/2013

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.

LHC

11/12/2013

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!

LHC

11/12/2013

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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LHC

11/12/2013

Back-up

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LHC

11/12/2013

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

LHC

11/12/2013

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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LHC

11/12/2013

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]

LHC

11/12/2013

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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LHC

11/12/2013

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

LHC

11/12/2013

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

LHC

11/12/2013

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

LHC

11/12/2013

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)

LHC

11/12/2013

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

LHC

11/12/2013

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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LHC

11/12/2013

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.

LHC

11/12/2013

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.

LHC

11/12/2013

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!

LHC

11/12/2013

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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LHC

11/12/2013

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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LHC

11/12/2013

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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