Lifetimes in Stable Beams revisited

1. Lifetimes in Stable Beams revisited Mike Lamont, Olli Johnson* Observations. No model. No bunch-by-bunch. *Oxford University

2. 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)

3. 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…

4. Single beam lifetime

5. 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

6. Lifetime Analysis at High Intensity Colliders Applied to the LHC B. Salvachua, R.W. Aßmann, R. Bruce, F. Burkart, S. Redaelli, G. Valentino, D. Wollmann IPAC 2013 In this paper, we calculate the calibration factor for the BLM downstream of the 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 other locations (such as IR3) will not be taken into account for this analysis. “Notice that the intensity lost due to luminosity burn-off is not subtracted.”

7. 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

8. Typical loss breakdown

10. Single beam lifetimes Good agreement giving some confidence that the approach is valid • Fit to BCT data – sliding 10 minute window • Lifetime from loss contribution from IR7 and luminosity

11. Cross-check 1/2 Note same factor used for both B1 and B2 – optimization possible Where’s the difference coming from?

12. IR3 losses Starts to become reasonably significant for beam 1 in later fills Given a IR3 calibration factor (!) could easily be taken in account

13. Cross check 2/2 • Remarkable – no tweaking! • Note low IR3 losses both beams

14. 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)

15. Loss breakdown Before OCP After OCP

16. Single beam lifetime breakdown

17. Losses versus intensity Further optimization of octupoles and chromaticity might have been possible.But wait for the first hour to pass! 2710: peak lumi 6.76e33 3192: peak lumi 6.66e33

18. 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

19. 2011- lifetimes

20. 2011 - losses • 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….

21. Luminosity

22. Luminosity lifetime • Rolling 20 minute window

23. First 8 hours – global fit Note that first hour

24. 2011 – first hour’s always been with us.

25. Luminosity lifetime

26. Luminous region (ATLAS)

27. Emittance from luminous region Naively calculate corresponding emittances • Standard picture: • Emittancessimilar at t=0 • Steeper increase in horizontal – flattening • Almost linear in vertical

29. 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

30. Luminous region emittance From local and global fit: Emittance growth rate: ~0.15 micron/hour Initial lifetime around 16 hours

31. Luminous region emittance Using only the global fit: Initial emittance growth rate: ~0.15 micron/hour Initial lifetime around 17 hours

32. Comparison of inferred and LR

33. 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

34. IBS - reminder Courtesy Sekazi K. Mtingwa • 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

35. Luminosity lifetime breakdown Dominated by losses rather than emittance growth

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

37. 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