Where did all the protons go?

Mike LamontLBOC 20th January 2015

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Total cross section

• Elastic: (27.1 +/- 1.4) mb;• Inelastic: (74.7 +/- 1.7) mb.

14 TeV

• Inelastic: – 80.9 ± 1.7 (TOTEM 7 TeV meas.) ± 2.2 (extr. to 14

TeV) mb (c/o of ATLAS/CMS)• Elastic:– TOTEM's latest fit to the elastic cross-section gives

30.2 mb at 14 TeV. • Total cross-section:– 111.1 mb plus/minus errors

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Inelastic includes diffractive

1 proton survives with momentum loss .Some of these will either stay within the beam or get lost in the DS or IR3

Cross-sections for different momentum lost ranges can be evaluated

Assume herein that all get lost locally

Non-diffractive processes: ~60 mb at 7-8 TeV

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Elastic scattering

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Mean t? Straightforward integral

2/

12/

1

1

2

2

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SB

SB

pt

Bt

x

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Elastic scattering mean scattering angle

Slope parameter b ~19.9 GeV^-2√s 8000 GeVrms scattering angle – one plane 39.6 microrad

The elastically scattered protons stay within the beam.

~22.6 microrad at 14 TeV

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Elastic scattering emittance growth

Initial ES growth rate: 1.2%/hour

27.1 mb

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IBS

• Transverse emittance– Effective emittance from luminosity– Assume vertical ~ horizontal (not totally satisfactory…)

• dispersion, coupling…• MD (Maria Kuhn)• collimator diffusion measurements (G. Valentino et al)• luminous region size

• Longitudinal emittance – From bunch length, RF voltage etc.

• Bunch current• Trundle through fill – every hour ask MADX what the IBS growth

rates are

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ES and IBS

Fill 2710 3192

ES 1.2%/hour 1.2%/hour

IBS 3.6%/hour 2.8%/hour

Initial growth rate

5.8%/hour 6.4%/hour

ES&ISB/Total 83% 63%

• Majority of emittance growth explained by ES and IBS

• Additional source(s) post OPC…• But growth rate not hugely different

before and after OPC

ES+IBS

Emittance from lumi

MADX pts

Warning: derived emittance input into IBS calculation and H=V

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Luminous region

0 10 000 20 000 30 000 40 000 50 000

0.02

0.04

0.06

0.08

0.10

0.12

Time sRelativegrow

thrate

Emittance growth rates from luminous region size Fill 2710

H

V

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Emittance growth rate not hugely different before and after OPC

2710

3363

• Would not expect IBS and ES to be OPC dependent…• It’s reproducible and quite moderate• And it is not the major contributor to the luminosity lifetime

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Calculate loss rates

• Luminosity – calculate losses based on inelastic cross-section at 4 TeV– 75 mbarn – ATLAS, CMS, LHCb

• Use “squeeze” calibration factor to establish other losses, principally IR7– while recognizing some losses to IR3 (see below)

• Ignore for the moment:– residual gas; small diffractive component

• Sum loss rates to get overall dN/dt• Calculate lifetimes etc.

Lifetime Analysis at High Intensity Colliders Applied to the LHCB. Salvachua, R.W. Aßmann, R. Bruce, F. Burkart, S. Redaelli, G. Valentino, D. WollmannIPAC 2013

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Loss breakdown

Lumi Coll

B1 60% 40%

B2 53% 47%

Loss breakdown

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Cross-check 1/2

Note same factor used for both B1 and B2 – optimization possible

Where’s the difference coming from?

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Cross check 2/2

• Good agreement• Note low IR3 losses both beams

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Loss breakdown

• Most non-luminosity related losses into IR7 – confirmed by Belen (see below)• Significant % of beam going into collimators both before and after OPC• Situation clearly worse after OPC

Cross checks with delivered luminosityBCT doesn’t lie (much)

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Belen@LMC

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Luminosity lifetime breakdown

Dominated by losses rather than emittance growth

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So…

• Relatively gentle core emittance growth given by IBS and elastic scattering and a bit of something else(s)

• Calibrated proton losses at IR7 plus luminosity losses approximately equal to delta BCT

• Losses in IR7 are not ES• Losses in IR7 are not (core) emittance growth• But we were sticking a fair bit of beam into IR7– worse after OPC, high Q’, higher bunch population,

octupoles..

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Start of fill – a closer look

1.3 s running sum

Adjust and Stable beams – first hour

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Start of fill – a closer look

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Losses versus intensity

Further optimization of octupoles and chromaticity might have been possible.But wait for the first 20 minutes to pass

2710: peak lumi 6.76e333192: peak lumi 6.66e33

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Versus BBTS

“Beam-beam limit phenomenon is observed in degradation of luminosity lifetime and/or beam lifetime in hadron colliders.”

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BCT!

1.5e10 p/s

1.95e9 p/s

8.5e9 p/s

2.8e9 p/s

Reference: 7x1033 x 75x10-27 x 2 = 1.1e9 p/s

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FZ & YP

Dependence of diffusion due to long-range collisions on the beam current:(a) the change of action variance per turn as a function of the bunch population;(b) approximate diffusive aperture as a function of the bunch population;

Nominal LHC – we were at 4 TeV, 50 ns, high bunch population, high HO beam-beam shift… (HL-LHC?)

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For discussion…

• Collide – DA comes down– Anything out in the tails is going to get murdered

• Steady state– A lot of beam is being pushed out into the tails

and onto the collimators– Exacerbated by high chromaticity, high bunch

intensity and possibly octupoles– To get that many particles out there, the diffusive

aperture would have to be pretty small…