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Lifetimes in Stable Beams revisited

Mike Lamont, Olli Johnson*

*Oxford University

Observations. No model. No bunch-by-bunch.

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Input data• For all SB fill > 8 hours:– Extract (Java API) from logging database:• TCP BLM data B1,B2,IR3,IR7 • DBCT data B1,B2• ATLAS, CMS and LHCb luminosity• BSRT where available

– Extract (Python) ATLAS luminous region data from LPC AFS repository

– Data cleaning, fits etc. in Mathematica

Will show plots from a variety of specific fills – analysis performed for all fills over 8 hours (~95 fills)

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Method• Single beam– Rolling fit single beam lifetime from BCT – Reconstruct losses/lifetimes to collimation– Reconstruct losses/lifetimes to luminosity

• Luminosity– Rolling fit luminosity lifetime– Global fit luminosity lifetime (1 & 8 hours)– Calculate emittance growth from luminous region– Calculate effective emittance growth from luminosity– Calculate lifetime components

Start playing…

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SINGLE BEAM LIFETIME

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Stable beams – where do the protons go?

• Luminosity– Inelastic - experiments– Diffractive – down-stream of IP, momentum cleaning– Elastic - nearly all scattered particles will stay well inside

the beam (3σ) – gentle emittance growth• Betatron cleaning• Momentum cleaning• Scattering on Residual Gas– (multiple) Coulomb, proton-nucleus (inelastic, elastic..) –

local losses, gentle emittance growth

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In this paper, we calculate the calibration factor for the BLM downstream ofthe primary collimators in IR7 (TCP.A) that can measure horizontal, vertical and skew primary losses in IR7 and use the BLM running sum of 1.3 s, therefore losses in otherlocations (such as IR3) will not be taken into account for this analysis.

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

“Notice that the intensity lost due to luminosity burn-off is not subtracted.”

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Calculate loss rates• Luminosity – calculate losses based on “visible” cross-

section at 4 TeV (74.9 mbarn) for 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

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Typical loss breakdown

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Single beam lifetimes

• Fit to BCT data – sliding 10 minute window• Lifetime from loss contribution from IR7 and luminosity

Good agreement giving some confidence that the approach is valid

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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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IR3 losses

Starts to become reasonably significant for beam 1 in later fillsGiven a IR3 calibration factor (!) could easily be taken in account

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Cross check 2/2

• Remarkable – no tweaking!• Note low IR3 losses both beams

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OCP• 7th August 2012:– Flip of octupole polarity plus increase in current– Significant increase of Q’H and Q’V at end ramp

and through squeeze– Octupoles and chromaticity reduced in collision

beam process and then trimmed down further in SB.

• Opened way for increase in bunch population (and peak luminosity)

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Loss breakdown

Before OCP

After OCP

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Single beam lifetime breakdown

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Losses versus intensity

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

2710: peak lumi 6.76e333192: peak lumi 6.66e33

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Lifetimes versus intensity

Cost?Average luminosity lifetime over 8 hours: 12.4 hours before OPC, 10.5 hours after… of the order 10 pb-1 per fill

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2011- lifetimes

2011 - losses

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• Not as good agreement with BCT – relative losses in IR3 considerably higher (looser collimator settings of course)

• Trade off beta*/collimator settings against losses and luminosity lifetime? And other problems….

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LUMINOSITY

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Luminosity lifetime

• Rolling 20 minute window

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First 8 hours – global fit

Note that first hour

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2011 – first hour’s always been with us.

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Luminosity lifetime

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Luminous region (ATLAS)

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Emittance from luminous region

• Standard picture: • Emittances similar at t=0• Steeper increase in horizontal – flattening• Almost linear in vertical

Naively calculate corresponding emittances

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Inferred emittance from luminosity

Given L etc. calculate through a fill.

• Implicit assumption here is that all luminosity reduction besides the loss of particles from the beam is due to emittance growth.

• This likely true in the first approximation but other mechanisms are present.• These include: the effect of increasing bunch length/beam size on the geometric

reduction factor and the hour glass effect, • The effect of orbit drifts on beam separation at the interaction point are also present.• Also implicit: b1 = b2, x = y

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Luminous region emittance

From local and global fit:Emittance growth rate: ~0.15 micron/hourInitial lifetime around 16 hours

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Luminous region emittance

Using only the global fit:Initial emittance growth rate: ~0.15 micron/hourInitial lifetime around 17 hours

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Comparison of inferred and LR

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Emittance growth• Emittance growth mechanisms include :– Elastic scattering at the interaction points,– Elastic scattering from residual gas,– Intra-beam scattering,– Non-linear resonances,– Incoherent beam-beam,– Electron cloud,– Noise, for example, power supplies, phase and

amplitude noise in the RF system, ground motion,– Long range beam-beam

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IBS - reminder• Dispersion

– Arises from the change in longitudinal momentum in a collision at a location of nonzero dispersion.

– Leads to an effective change in the transverse coordinates of the colliding particles with respect to off-momentum orbits.

– The larger the dispersion, the faster the IBS emittance growth from this effect.• Betatron Coupling

– If the vertical dispersion is negligible, but there is some H-V coupling, we expect the horizontal IBS emittance growth to feed directly into the vertical plane.

– In the case that the vertical emittance growth is given entirely by betatron coupling

450 GeV May 2012 MS/ML

CourtesySekazi K. Mtingwa

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Luminosity lifetime breakdown

Dominated by losses rather than emittance growth

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The first hour• High losses and fast emittance growth – beam-beam driven • Low luminosity lifetime as a result.• “Beam-beam tolerance” thanks to a remarkably efficient

collimation system

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

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Conclusions• Loss rates and lifetimes through SB reconstructed• More beam on collimators than to luminosity!• OPC caused lifetime degradation– presumably recoverable by appropriate optimization, but

avoid the first hour• Possible input to beta*/collimator setting choice• Luminosity lifetime– Besides luminosity burn – significant component to losses

on collimators – And initially emittance growth