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A novel low b * scheme S. Fartoukh (CERN, BE/ABP)

A novel low b * scheme S. Fartoukh (CERN, BE/ABP). Performance goal of the HL-LHC Layout & optics limitations of the existing LHC An “Achromatic Telescopic Squeezing” (ATS) scheme to overcome the limits An overview of the optics challenges and mitigation measures

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A novel low b * scheme S. Fartoukh (CERN, BE/ABP)

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  1. A novel low b* scheme S. Fartoukh (CERN, BE/ABP) • Performance goal of the HL-LHC • Layout & optics limitations of the existing LHC • An “Achromatic Telescopic Squeezing” (ATS) scheme to overcome the limits • An overview of the optics challenges and mitigation measures  Conclusions and outlook Main References: Optics challenges and solution for the Phase I Upgrade, Chamonix 2010 & SLHC-PR38 Breaching the Phase I optics limitations for the HL-LHC, Chamonix 2011 & SLHC-PR53 OMCM Workshop, CERN, 20-22 June, 2011

  2.  Integrated luminosity: ~250 fb-1 /year, i.e. about 1 fb-1 per fill.  Running luminosity: Sustained to 5E34 cm-2s-1 with leveling during 3-5 h + decay of a few hours: Performance goal for the HL-LHC Technique for leveling not yet decided: Crab-cavity, X-angle, b*,… Illustration from E. Todesco  Concept of “Virtual” luminosity: Need more than 5E34, typically ~1.E35 cm-2s-1 “stored”, even if not usable due to limitations on the machine side or on the detector side.  The “effective target” is a luminosity increased by a factor of 10 w.r.t. nominal. OMCM Workshop, CERN, 20-22 June, 2011

  3. Luminosity vs. b* in the Xing plane (with hour-glass effect) for different values of b* in the other plane: nominal emittance and bunch length, ultimate intensity, no crab-cavity Example of flat optics: b* =30 cm in the crossing-plane b* = sz =7.5 cm in the other plane Qc = 10s in the plane of biggest b*  Peak lumi ~5.6 1034cm -2s – 1 0.16cm “Equivalent” round optics: b* =15 cm in both planes Qc = 10s  Peak lumi ~3.5 1034cm -2s – 1 0.40cm 1.0cm 7.5cm 16cm 40cm 100cm Case of round beam optics (saturation due to Xing angle) • The “virtual” performance of the two optics becomes equivalent with crab-cavity (~8-9E34), • In all cases the two options requires to push the beam parameters beyond ultimate andb* a factor 4 to 8 below nominal (55 cm), which cannot be only given by Nb3Sn triplets (25% improvement w.r.t. NbTi). OMCM Workshop, CERN, 20-22 June, 2011

  4. Main layout and optics constraints Triplet Separation dipoles D1/2 Matching section Q4/5/6 Layout constraints Optics constraints • More or less fixed L* ~ 20 m (detector) • Constant length of the FFS (matching section) LFFS ~250 m from the IP till the arcs (Q7) • Beam fully separated, Dsep=194 mm, in the arcs 1) Strength of the IR quadrupoles (specifications for the min./max. gradient). 2) Aperture of the IR magnets, not only triplet (IT) but also D1, and matching section (D2, Q4, Q5). • Correction of the chromaticaberrations within the strength of the arc sextupoles (chroma Q’, Q’’,.., but also off-momentum b-beating, spurious H and V dispersion induced by the crossing angle in IR1/5): • All these constraints can be quantified by the maximum allowed peak beta functionbmax reached in the triplet (IT), • This bmax limit coming from the ring then gives theoptimum IT aperture  (bmax)½, then the IT gradient & layout for a given technology (NbTi or Nb3Sn) and then b*min  1/bmax/G½. L*=23 m LFFS=268m OMCM Workshop, CERN, 20-22 June, 2011

  5. 1) IR quadrupole strength at high bmax (low b*) • Neglecting the aperture and chromatic related constraints, the existing insertions IR1/5 are squeezable down to b*=15 cm, i.e.bmax~16 km. Flexibility given by the largecofocal length of the existing IT, P=(b/a)Q3-exit ~1km (independent of b*) bQ4 ~ 1.5 km bQ4~ 6 km Nominal collision optics with bmax=4.5 km  b*=55 cm (@ 205 T/m in the triplet) Can be pushed up to bmax=16 km  b*=15 cm (@ 205 T/m in the triplet) • Below b*=15 cm, gradient limitations are observed in the matching section quadrupoles (Q5/Q6  0 T/m) and in the dispersion suppressor (Q7 200 T/m). • Re-optimizing the matching section layout will not drastically improve the situation. OMCM Workshop, CERN, 20-22 June, 2011

  6. 2) Aperture constraints vs.triplet cofocal length P=(b/a)@Q3exit (example given for a 120 mm-135 T/m triplet, bmax~12 km  b*=25cm, and nominal matching section) A difficult optimization game! The game is over for bmax > 13 km at nominal matching section aperture TAN Q5 D2/Q4 TAS-IT-D1 TAN • MS aperture restored (except at the TAN) but quad. gradient at the limit in the MS • (Q4/5/60) and DS (Q7~200T/m)  DS and MS gradients well within limits but Aperture bottle-neck in the TAN-D2-Q4-Q5 (12km bmax is a bit too much for 120 mm coil_ID) OMCM Workshop, CERN, 20-22 June, 2011

  7. 3) Chromatic aberrations: a zoology of effects to sort out(illustration given for the 15 cm b* optics of slide 5, with a full crossing angle of 580 mrad, V in IR1 and H in IR5)  No specific IP1-IP5 phasing: off-momentum b-beat, Q’’, spurious dispersion. Montague function IP5 IP1 IP1 IP5 Tune vs. d H & V dispersion (with X-angle) • Phasing IP1 and IP5 by p/2 only partly solves the problem: off-momentum b-beat in half of the ring, • huge Q’’’, still spurious dispersion. Montague function IP1 IP5 IP1 IP5 Tune vs. d H & V dispersion (with X-angle) OMCM Workshop, CERN, 20-22 June, 2011

  8.  A complicated strategy set up for the former upgrade project (Phase I): active correction anticipating the chromatic kicks induced by the IT by a series of small kicks given upstream by the sextupoles Sextupole gradients (beam1) vs. b* 550A Transition @ b*=1.5 m - 550A • Two implications: • An overall new optics is required to fulfill specific betatron phasing conditions. • Only half the sextupole families participate effectively to the correction of the triplet •  bmax 11 km (instead of bmax 17 km for the correction of Q’ only). b*=30 cm • A by-product: • The new phasing configuration allows the correction of the spurious dispersion by modest H or V orbit • bumps(a few mms) in the arcs surrounding the low-b IR’s. OMCM Workshop, CERN, 20-22 June, 2011

  9.  Illustration for the 120 mm-120 T/m Phase I triplet : squeezed optics with b*=30 cm (bmax~11 km) Bucket: d = 0.36×10-3 Min. momentum window: d = 10-3 Momentum collimator: d = 1.5×10-3 IP3 IP5 IP1 IP7 IP1 • Off-momentum b-beating envelop after • correction(W=100  Db(d)/b(0) =10% @ d=10-3) • Vanishing in the collimation IR’s • Vanishing in the new IT of IR1 & IR5 • Two sectors of sextupoles needed per IT! • Betatron tunes vs. energy • Almost linear up to d=1.5 10-3 (with Q’ matched to 2 units) OMCM Workshop, CERN, 20-22 June, 2011

  10. The Achromatic Telescopic Squeezing (ATS) scheme  Most of the ingredients are already there to blow-up the b’s in the arcs at 7 TeV! 1) Huge aperture margin in the arcs at 7 TeV : ~ factor 16 margin to increase the beta’s in the arcs. 2) About 150 quadrupole knobsmoderately used (IR2/IR8) or not used at all (IR4/IR6) in pp collision @7 TeV. OMCM Workshop, CERN, 20-22 June, 2011

  11. Injection optics: b* = 14 m in IR1 and IR5 • p/2 FODO cells in sectors 81/12 and 45/56. • 4 sectors (23/34/67/78) “free” of any constraints. •  New integer tunes 62/60 (instead of 64/59). Nominal barc (180m) in sectors 81/12/45/56 OMCM Workshop, CERN, 20-22 June, 2011

  12. Pre-squeezed optics: b* = 60 cm in IR1 and IR5: “1111” Similar to the nominal LHC - “Standard” squeeze acting on IR1 and IR5 with p/2 left/right phase advances for the low-b IRs - “Up and down” sextupole powering scheme … till reaching the max current in 50% of the sextupoles of s45/12/45/56. Nominal barc (180m) in sectors 81/12/45/56 OMCM Workshop, CERN, 20-22 June, 2011

  13. Squeezed optics (round): b* = 15 cm in IR1 and IR5: “4444” HL-LHC with round optics (preferable with crabs) Continuation of the squeeze acting only on IR2/8 for squeezing IR1, and IR4/6 for squeezing IR5. barc increased by a factor of 4 in s45/56/81/12 OMCM Workshop, CERN, 20-22 June, 2011

  14. Squeezed optics (flat): b*x/y = 7.5/30 cm alternated in IR1 and IR5: “8228” HL-LHC with flat optics (back-up w/o crabs) barc increased by a factor of 2 or 8 in s45/56/81/12 depending on the b* aspect ratio in IP1 and IP5 OMCM Workshop, CERN, 20-22 June, 2011

  15. …Then a series of fundamental chromatic properties (examples for “8228” optics) 1) Chromatic correction using only one sector of sextupoles per IT IP7 IP1 IP3 IP5 IP7 Montague functions (W=1000  Db/b=100% at d=0.001) Tune vs. dp (+/- 0.0015 window) 2) Correction of the spurious dispersion induced by the X-angles in IR1 and IR5 Dispersion of only 50cm in the IT thanks to ±2.5 mm orbit bumps induced in sectors 81/12/45/56 Closed orbit with X-scheme in IR8/IR1/IR2 and IR5 OMCM Workshop, CERN, 20-22 June, 2011 H and V dispersion

  16. Optics challenges and mitigation measures • Linear optics distortion due to the arc magnets during the squeeze • - Good local closed orbit correction neededin the arcs and reliable CO feed-back. • - Arc by arc accurate measurement and correction of b2 and a2 (dedicated corrector knobs • available in the LHC, and optimized random b2/a2 seen by the beam thanks to magnet sorting). • Dynamic aperture reduction (big b’s in the arcs combined with strong sextupoles and MB, MQ, MQM field imperfections). … Assuming no field imperfection in the new IT/D1/D2/Q4(courtesy of R.D. Maria) … But a lot of room for improvement (with further developments needed): - Pushing the pre-squeezed b* to reduce the b’s in the arcs by up to 50% at constant b* in physics (with more sextupole current 550A 600A, additional sextupole at Q10, stronger Nb3Sn triplet). - Smaller emittance (3.75  2.5 mm as worked out by the LIU project). OMCM Workshop, CERN, 20-22 June, 2011

  17. Usual concerns related to the triplet at low b*(not ATS specific) • Inner triplet (IT) stability • Tolerance to mechanical vibrations 1/√b*: dx*<s*/10 requires micron to sub-micron stabilitylevel(peak to peak) for b*=157.5 cm. • Tolerance to PC jitter1/b*: dQ ~ 10-4 requires 1ppm to 0.5 ppm level(r.m.s.) for b*=157.5 cm. • IT field quality (w/o forgetting new D1) … not yet studied in details but certainly not more than a fraction of units for the low order multipoles at 2/3rd of the coil aperture • Certainly manageable for round optics at least for large aperture NbTi quadrupoles rescaled from the existing MQX’s… (see E.Todesco et al. LHCPR1010). • Will be more tricky for equivalent flat optics (bmax doubled in one plane, halved in the other plane, at constant IT aperture and therefore constant field quality). • A battery of non-linear correctors may be needed (a fortiori for Nb3Sn triplet). OMCM Workshop, CERN, 20-22 June, 2011

  18. The ATS concept offers a powerful and flexible machinery to reach very low b*while correcting the chromatic aberrations induced. • A series of LHC MDs have already started this year to validate it. • Conceptually and operationally speaking, the actual insertions IR1 and IR5 are replaced by two 7 km long low-b insertions containing • the low-b IRs proper (optically passive below a certain b*), • two LHC sectors (chromatic correction sections) , • two “auxiliary” insertions (powerful matching section). • The existing LHC seems ready to generate, swallow and take advantage of the big b-beating bumps induced in the arcs during the squeeze of IR1 and IR5 thanks to • its conception (aperture available in the arcs and “optics margins” of IR8/2/4/6 at 7 TeV, existing sextupole scheme, sectorization of the a2/b2 corrector knobs, …). • the optimized MB & MQ field quality as seen by the beam (very small systematic even multipoles but b6 of the MQs at 7TeV, and random low order multipoles minimized by magnet sorting at installation), Summary OMCM Workshop, CERN, 20-22 June, 2011

  19. Reserve OMCM Workshop, CERN, 20-22 June, 2011

  20. Performance vs. b* w/o crab-cavity • Round beam optics (b*x=b*y)  The luminosity saturates without crab-cavity: • Flat beam optics (b*x≠b*y) • The lumi is optimalfixing b* in the crossing plane to • The lumi still increase when decreasingb*in the other plane • (and then saturates due to the hour-glass effect at very smallb*). OMCM Workshop, CERN, 20-22 June, 2011

  21. Aperture requirementassuming Nb-Ti IT(nominal emittance, 7.5/30 cm or 30/7.5 cm flat optics, 13s full X-angle, spurious H&V dispersion corrected via orbit bumps in the arcs) NewD1 ~135-140 mm b.s ID  ≥160 mm coil_ID New Q5 (“MQYL” type)  70 mm coil_ID A priori enough aperture at Q6 and beyond (n1 ≥ 10) New Q4 (“MQYY” type) 70 mm b.s ID  ≥85 mm coil_ID NewD2 77 mm b.s ID  ≥92 mm coil_ID NewTAN 33.5-37.5 mm elliptical chamber NewTAS 60 mm ID NewIT 125 mm b.s. ID  ≥150 mm coil_ID • The above requirements are also compatible with a 15/15 cm b*round optics and could be relaxed by ~10% (but for the TAS) with Nb3Sn triplet . OMCM Workshop, CERN, 20-22 June, 2011

  22. Why does it work?.. Zoom in from IP4 to IP5 for the flat optics (beam1) • Dmy (Q11 IP ) = 1.25×p + ey • withey ~ 1/2 tan-1 (bmin/bmax) • (barc× b*)V cst Dmy p between the 12strong SD sextupoles Dmx p between the 9 strong SF’s one missing at Q10 to complete 5 p-pairs Dmx(Q14 IP ) = 1.25×p + ex withex ~ - 1/2 tan-1 (bmin/bmax)  (barc× b*)H  cst Equipping Q10 (MQML) with an MS becomes highly desirable for high barc. OMCM Workshop, CERN, 20-22 June, 2011

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