Update on possibilities to reach *=40cm (work in progress) R. - PowerPoint PPT Presentation

Update on possibilities to reach β *=40cm (work in progress) R. Bruce, S. Redaelli Acknowledgement: R. de Maria, S. Fartoukh, M. Giovannozzi, M. Huhtinen R. Bruce, 2015.01.19 R. Bruce, 2015.01.19 1 1

Introduction • LMC 3/9/2014: Decision to start relaxed in 2015 with β *=80cm, nominal optics • Later in the year, when machine behaviour is better known, push performance • Predicted (optimistic) limit (Evian 2014) : β *=40cm – Based on optimistic assumptions: Can go to tighter collimator settings, decrease emittance and beam-beam separation, use full theoretical gain from BPM buttons in collimators • RLIUP 2013: β *=40cm declared as target in Run II • J. Wenninger in Chamonix 2014: discussion of different studies needed to conclude on whether β *=40cm is within reach R. Bruce, 2015.01.19 2

Triplet aperture at 40cm Chamonix 2014 • Assuming the startup beam-beam separation of 11 σ is kept, the estimated aperture is ~9.5 σ at β *=40cm (205 µrad) • A priori very challenging for protection R. Bruce, 2015.01.19 3

Calculation of collimation margins • Significant reduction of collimation margins is a key element in reaching β *=40cm – In 2012: Triplet aperture was ~4.5 σ behind IR7 TCSGs – With 6.5 TeV, mm kept settings, 11 σ BB, 40 cm: aperture 1.5 σ behind TCSGs inIR7 – With 2 σ retraction settings: 40 cm aperture is 2 σ behind TCSGs inIR7 • Protection against asynchronous beam dumps is driving the large margins IR6 -> TCT -> triplet. – Remember: IR7 hierarchy also larger than nominal to keep down impedance R. Bruce, 2015.01.19 4

Calculation of collimator settings • In Run I, used simplified model to calculate collimator settings and resulting β * – Assumes 90 deg phase advance from dump kicker MKD to any TCT or bottleneck – in Run 1, we knew that real phase advance was better (hidden margin). Pessimistic! • Room for improvement in Run I model – If we have a good phase advance, we might be unnecessarily pessimistic • Better method: account for the real phase advance and base the margin IR6->TCT on the estimated impacts during fast failure and TCT damage limit – Ongoing work since long time: see earlier studies in e.g. MPP review 2013 R. Bruce, 2015.01.19 5

TCT impacts during asynch. dump • Effect of phase advance shown previously ( CWG 13/6/2014, LMC) • Impacts during single-module prefire (worst type of asynchronous beam dump) simulated using SixTrack and summed over all bunches, scan over TCT setting • Knowing range of machine errors and TCT damage limit, we could β *=55cm, use the simulations to decide necessary margin TCDQ -> TCT 2 σ retraction CWG 13/6/2014 R. Bruce, 2015.01.19 6

Phase advance in squeeze • With nominal optics, phase advance between MKD and TCTs is changing in the squeeze • Phase advance close to 90 deg and 270 deg give highest risk R. Bruce, 2015.01.19 7

Phase advance at β *=40cm • Significant changes below β *=55cm • At β *=40cm – Improved phase advance – further away from odd multiples of 90 deg – in IR1 B1 (previously most critical case for nominal optics), IR1 B2 and IR5 B1 – Worse phase advance at IR5 B2 – Worst phase advance at β *=40cm is about 60 deg away from 90 or 270. • At β *=55cm, worst case is about 35 deg away from 90 • β *=40cm with nominal optics seems, from the phase advance, better for asynch. dump protection of TCTs/triplet in terms of phase advance than higher β * values R. Bruce, 2015.01.19 8

TCT impacts at β *=40cm • Repeating SixTrack simulation of asynchronous dump (single module pre-fire) with nominal optics and β *=40cm R. Bruce, 2015.01.19 9

TCT impacts at 40cm • With 40cm, nominal phase: – Losses at IR1 are factor 20 below plastic deformation limit even with TCTs inside IR7 secondary collimators – Losses from secondary halo at IR5 B2 are very similar to 55cm • Expect that damage limit from secondary halo is higher than for primary halo (ongoing study BE/ABP, EN/STI, EN/MME) – Much larger impact parameters – see talk E. Quaranta in ColUSM 19/9/2014 • Anyway, impacts from secondary halo in IR5 are in this range independent of TCT setting – Have to deal with them even if there are no errors in the machine. Increasing margins not likely to help R. Bruce, 2015.01.19 10

Margins at β *=40cm • At β *=40cm, nominal optics, the TCT impacts during asynchronous dump are more than 1 order of magnitude below mildest form of damage even if inside IR7 TCSGs (2 σ inside TCDQ) • TCT settings could be decoupled from the protection against asynchronous dumps – Different approach required: step away from the 90 deg assumption – Not meaningful to base the calculation of margins on this scenario R. Bruce, 2015.01.19 11

Caveats • Result is sensitive to phase errors – Taking the worst case out of 1000 random imperfect optics (phase advance 33 deg -> 45 deg , dynamic error), TCT losses increase by factor ~25 – Extremely unlikely case – Should probably not be a concern, but to be verified with beam • With “good” phase advance to TCT/triplet, we have bad phase advance to experiments – On paper much larger aperture margin there. – However: In IR5 B2, we have ~90 deg phase advance to the experiment already at higher β * – SixTrack results: no losses in detectors even in extremely pessimistic conditions (backup slides) => should not be a concern R. Bruce, 2015.01.19 12

Limits on TCT setting • If we don’t have constraints from asynchronous dumps, what limits the TCT setting, aperture and reach in β *? • All other loss scenarios (to our knowledge!) slow and less serious – Triplet BLMs should dump the beam (and possibly other interlocks). – No risk of damage • We could put a tighter and less pessimistic margin TCT – aperture – If too tight: increased number of beam dumps. Can step back in β * • Main other loss source that could influence TCT setting: cleaning R. Bruce, 2015.01.19 13

Cleaning constraints • TCT setting constrained – Largest allowed setting given by triplet protection from cleaning losses – Smallest allowed setting given by background constraints and cleaning hierarchy • Using SixTrack to simulate the halo cleaning performance • Scan over TCT setting, keeping all other collimators constant – With 40cm optics – 6.5 TeV – Collimator settings: 2 σ retraction – Simulating B1 and B2 – Simulating horizontal and vertical halo R. Bruce, 2015.01.19 14

Minimum TCT setting? • Studying TCT losses vs setting – What losses are acceptable? 7 TeV LHC design configuration Run 1 configuration R. Bruce, 2015.01.19 15

Minimum TCT setting • Higher TCT losses => Higher background What is tolerable? – Limit not fully clear – Nominal LHC configuration studied in detail in the past – hopefully OK • If we should not surpass this level, the limit is at 8.8 σ – Discussion M. Huhtinen: Even a factor 10 increase compared to Run 1 could possibly be tolerated, but experimental verification needed • Scaling Run 1 result with the energy ratio, the limit is at 8.5 σ – Observed background will also depend on the beam lifetime – many uncertainties – Open question if we can go further R. Bruce, 2015.01.19 16

Maximum TCT setting? • Integrated triplet losses vs TCT setting – Triplet aperture artificially reduced to expected machine aperture R. Bruce, 2015.01.19 17

Maximum TCT setting • No significant losses observed if TCT opening is smaller than triplet aperture (9.5 σ) => Try to avoid exposing the triplet – To be noted: no imperfections included • Below 9 σ , at most 1 macro-particle per simulations lost in triplet • Cleaning margin TCT-triplet less strict than for asynch dumps – Do not risk serious machine damage if violated – Protected by interlocks: TCT BPMs and triplet BLMs • First estimate on TCT setting: 8.8 σ – probably OK for both background and triplet protection – Corresponding to about 0.7 σ margin TCT – triplet – Possibly to be followed up by imperfection studies and MD tests R. Bruce, 2015.01.19 18

Preliminary collimator settings at 40cm, 6.5 TeV • mm-kept settings: probably too tight (but not fully excluded) • 2 σ retraction: seems to provide a working hierarchy • TCT setting and aperture must be further studied and verified in MDs before confirmation Setting (2 σ retr) Collimator Possibly to be put at 8.3 σ – under TCP IR7 5.5 discussion with LBDS (reduces secondary halo leakage during TCSG IR7 7.5 asynch dumps). See backup slide TCSG IR6 8.3 TCDQ IR6 8.8 Assumes 11 σ beam-beam TCT IR1/5 (preliminary) 8.8 separation with ε =3.75 µm . Gives Aperture 9.5 205 µrad half crossing angle [ σ with ε =3.5 μ m] R. Bruce, 2015.01.19 19