ISBN 978-3-95450-115-1 - http://accelconf.web.cern.ch/AccelConf/IPAC2012/papers/moppc007.pdfInternational audienceBased on what has been learned with the ﬁrst high β* = 90 m operation in 2011, we describe the potential and practical scenarios for reaching very high β* in the LHC in 2012 and beyond. Very high β* optics require dedicated running time and conditions in the LHC. We describe a plan which is optimized to maximize the physics potential in a minimum of dedicated running time

Burkhardt, Hans

Cavalier, S.

Macpherson, A.

Puzo, P.

HAL-IN2P3

PLANS FOR HIGH BETA OPTICS IN THE LHCH. Burkhardt, A. Macpherson, CERN, Geneva, SwitzerlandS. Cavalier, P. Puzo, LAL,Universite Paris Sud 11´AbstractBased on what has been learned with the β∗ = 90mcommissioning and operation in 2011, we describe the po-tential and practical scenarios for reaching very high β∗in the LHC in 2012 and beyond. Very high β∗ optics re-quire dedicated running time and conditions in the LHC.We describe a planning which is optimized to maximizethe physics potential in a minimum of dedicated runningtime.INTRODUCTIONThe top priority for 2012 LHC operation is to maximizethe integrated luminosity. The current LHC schedule fore-sees about 129 days of standard proton running, 22 days formachine development and about 8 days for special runningwhich is operation at high β∗ as discussed here and lumi-nosity normalization using separation scans in a couple ofspecial physics ﬁlls. This leaves relatively little beam timefor new optics developments, which will therefore have tobe well targeted to allow to reach realistic goals in a smallnumber of dedicated ﬁlls.0 100 200 300 400 5000.10.20.30.40.5squeeze, inj. -> physics90 mde-squeeze to 90 m      ΔQ =  0.2811 m90 m to 500 m    ΔQ =  0.07β∗[m]Φ/2π   from insertionFigure 1: Tune contribution from the insertion±26m fromthe interaction region as relevant for the LHC.One major challenge in the de-squeeze from the standardinjection optics to high-β is the signiﬁcant change in phaseadvance which requires external tune compensation [1, 2].A low-β insertion with β∗  , where  is the distancebetween the interaction point (IP) and the centre of the ﬁrstquadrupole, contributes with a phase advance of π and tuneof 0.5. For very high β∗   instead, the phase advanceand tune contribution of the drift space between the ﬁrstquadrupoles drops to zero. This is illustrated in Fig.1. Wesee that the local tune change from the free space betweenthe low-β quadrupoles for the squeeze from 11m to 0.55mas relevant for the standard physics optics is approximately+0.1. This can be compensated internally in the insertion(by a rematch in the insertion between Q13 left and right ofthe interaction point) such that the overall LHC optics doesnot change during the normal squeeze to low β∗. The tunereduction in the de-squeeze is much larger and requires ex-ternal tune compensation which modiﬁes the optics of theentire LHC.The experience with the 90m optics has been very posi-tive in 2011. We succeeded to reach 90m by de-squeezingfrom the standard 11m injection and ramp optics in a sin-gle ﬁll [3] using the main quadrupoles of all LHC arcs forthe external tune compensation and to commission the new90m optics with parallel separation bumps to allow to ad-just for collisions in the second ﬁll [4, 5].STRATEGY FOR 2012The de-squeeze to β∗ = 90m will be extended to higherβ∗. This allows to maintain the 90m for further operationas requested by TOTEM and to minimize the extra beam-time required for the commissioning of the higher-β∗ op-tics.From 90m to 500mThe extension to higher β∗ is done using sufﬁcientlysmall steps of approximately 15% in β∗ (105m, 120m,140m) for a smooth transition in the de-squeeze. The phaseadvance between the IP and the relevant roman pots (at220m from IP5 for TOTEM) is kept at π/2 in y for all stepsabove 90m, while the phase advance in x is not constraint.The matching was ﬁrst done for beam 1 in IP5 and thenfor beam2. The start values for the strength in beam2 aretaken from beam1 after exchanging left and right sides. Fi-nally, a second version of these ﬁles was produced for IP1,by re-matching for a phase advance of π/2 in the verticalplane to the roman pot at 240m, as done last year for theIP1 version of the 90m optics [6]. The other optics param-eters are all very similar for IP1 and IP5.Ratio constraintThe interaction region quadrupoles use a single returncable, which restricts the current ratio for the main inser-tion quadrupoles (from Q10 left to Q10 right) to 0.5 <beam1/beam2 < 2. This is generally ﬁne for thebeam1/beam2 and left/right (anti)-symmetric lower-β op-tics, but instead a major restriction for the high β-opticsProceedings of IPAC2012, New Orleans, Louisiana, USA MOPPC00701 Circular and Linear CollidersA01 Hadron CollidersISBN 978-3-95450-115-1133 Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)12.7 13.0 13.3 13.6 13.9500mb1 4TeV  3.75mum beta*=500m MAD-X 5.00.15  12/04/12 15.17.170.0200.400.600.800.1000.1200.1400.1600.1800.2000.-3.-2.-1.0.01.2.3.? x ? y Dx Dys [km]Dx,y[m] ?x , ?y [m]12.7 13.0 13.3 13.6 13.9500mb2 4TeV  3.75mum beta*=500m MAD-X 5.00.15  12/04/12 15.17.170.0200.400.600.800.1000.1200.1400.1600.1800.2000.-3.-2.-1.0.01.2.3.? x ? y Dx Dys [km]Dx,y[m] ?x , ?y [m]Figure 2: 500m optics in IP5. Up for Beam1 and down forBeam2.which have to respect single sided phase advance con-straints to the roman pots. The strength ratios in the 90moptics as used in 2011 and planned to be re-used in 2012are listed in Table 1. The beam1 / beam2 ratios are alreadyclose to the limits for Q7 and Q8.Table 1: Beam 1 / beam 2 current ratios at 90mQuadrupole left rightQ4 0.971 1.11Q5 1.04 0.961Q6 1.05 0.939Q7 1.58 0.525Q8 1.33 0.572Q9 1.03 0.964Q10 0.95 1.054The 500m optics proposed for this year is shown inFig. 2.Going well beyond 90m required many matching itera-tions to respect the ratio constraint and required to acceptsome compromises : the continuity during the de-squeeze(i.e. quadrupoles moving only up or down during the de-squeeze) could not be fully assured and the difference inhorizontal phase advances between beam 1 and beam 2 in-creased. The current ratios at 500m are listed in Table 2.Table 2: Beam 1 / beam 2 current ratios at 500mQuadrupole left rightQ4 1.10 1.07Q5 0.997 1.01Q6 0.974 1.03Q7 1.86 0.563Q8 1.56 0.641Q9 1.12 0.879Q10 1.09 0.914Table 3 shows main optics parameters at 90 and 500m− the β functions at the IP and the roman pots, the effectivelength Ly =√β∗y βy,RP sin(μy,RP) (equivalent to the off-diagonal element R34 of the transport matrix) between theIP and the roman pot, and the tune reduction compared tothe standard optics.Table 3: Optics parameters at 90m and 500m in IP5 forthe roman pot at 220m from the IP, and the tune reductioncompared to the standard optics. Beam sizes are given fora beam energy of 4 TeV and the design emittance N =3.75μm.Optics 90m 500mBeam 1 2 1 2β∗x,y , m 90 90 500 500σ∗x,y , mm 0.281 0.281 0.663 0.663βx,RP, m 313.4 313.4 508.7 507.3βy,RP, m 769.5 769.5 152.8 152.9Ly , m 263.2 263.2 276.4 276.5μx,RP/2π 0.500 0.500 0.487 0.487μy,RP/2π 0.250 0.250 0.250 0.250ΔQx 0.2219 0.2203 0.2251 0.2175ΔQy 0.0546 0.0528 0.2292 0.2290ApertureFig. 3 shows the aperture in terms of n1 (number of σ in-cluding tolerances) around the interaction region. At 90m,the aperture in the triplet region was not an issue and infact larger than in the LHC arcs [2]. At 500m, the aperturebottleneck moves to the TAS region, but remains still largerthan the usual n1 = 7 speciﬁcation shown as green lines.Files have been matched up to β∗ = 1000m for studypurposes. At 1000m, we have n1,min = 6.3which is belowspeciﬁcation. In addition, it will be difﬁcult to maintainMOPPC007 Proceedings of IPAC2012, New Orleans, Louisiana, USAISBN 978-3-95450-115-1134Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)01 Circular and Linear CollidersA01 Hadron Colliderss [m]-100 -80 -60 -40 -20 0 20 40 60 80 100n1010203040IP5s [m]-100 -80 -60 -40 -20 0 20 40 60 80 100n1010203040IP5Figure 3: Aperture at 4 TeV, for the design emittance N =3.75μm at β∗ = 500m around IP5 including ±2mm par-allel separation. Top is for beam 1 and bottom for beam2.sufﬁcient parallel separation during the de-squeeze beyonda β∗ of 500m.Given the limited beam time and many constraints, anambitious but not too unrealistic goal for 2012 is to aim fora β∗ = 500m for high-β∗ for physics operation in 2012.If things go extremely well and beam time would be avail-able, studies could be extended towards 1000m.HOW TO GET TO THE COULOMBINTERFERENCE REGION ?Getting to the Coulomb interference region, that is downto a momentum transfer of −t ≈ 6.5 × 10−4GeV2 isknown to be extremely challenging at LHC energies [7].With −t ≈= p2 θ2, where p is the beam momentumand θ the scattering angle, this corresponds to measur-ing down to θC =√6.5× 10−4GeV/p or 3.6μrad at7 TeV. For comparison, the Coulomb region was reachedat θC = 120μrad at the Spp¯S collider[8].Measuring these small angles requires to minimize thebeam divergence and to bring the roman pots very close tothe beam. The beam divergence at the interaction point isσ′ =√/β∗, where  is the geometric emittance, relatedto the normalized emittance N by the Lorentz factors β, γaccording to  = βγN .Numerical values are given in Table 4 for a normalizedemittance of N = 2μrad. The Coulomb region would bereached an angle which is about 7 times the beam diver-gence.Table 4: Scattering angle θC required to reach the Coulombregion and beam divergence for N = 2μradp β∗ σ′ θC θC/σ′TeV m μrad μrad4 TeV 500 0.969 6.37 6.67 TeV 1000 0.518 3.64 7.0The roman pots of TOTEM and ALFA start to be sensi-tive after ∼ 0.5mm and would have to be moved to lessthen 5σ to reach the Coulomb interference region.SUMMARYBased on the successful commissioning and operation at90m in 2011, we developed sets of optics ﬁles for commis-sioning and operation in 2012 which extend to higher β∗.As an already ambitious but not too unrealistic goal, thatshould be reachable within few shifts of dedicated beamtime, we aim for a target β∗ = 500m, to be reached bysimultaneous de-squeeze in both IP1 and IP5. If things goextremely well and more beam time would be available,studies could be extended this year in the LHC towardsβ∗ = 1000m.REFERENCES[1] H. Burkhardt and S. White, “High-beta Optics for the LHC”,LHC Project Note 431.[2] H. Burkhardt, “High-beta optics”,CERN-Proceedings-2011-001, p. 109.[3] H. Burkhardt et al., “Un-squeeze to 90 m”,CERN-ATS-Note-2011-032 MD.[4] S. Cavalier, P. Puzo, and H. Burkhardt, “90 m OpticsCommissioning”, Proc. IPAC 2011.[5] H.Burkhardt et al., “90m optics studies and operation in theLHC”, Proc. IPAC 2012 MOPPC006.[6] S. Cavalier, H. Burkhardt, et al., “90 m Beta* optics forATLAS/ALFA”, Proc. IPAC 2011.[7] ATLAS collaboration, “ATLAS Dectectors for Measurementof Elastic Scattering and Luminosity”, ATLAS-TDR-018 ;CERN-LHCC-2008-004, 2008.[8] UA4 Collab., D.Bernard et al., PLB 198 (1987) 583.Proceedings of IPAC2012, New Orleans, Louisiana, USA MOPPC00701 Circular and Linear CollidersA01 Hadron CollidersISBN 978-3-95450-115-1135 Copyrightc ○2012byIEEE–ccCreativeCommonsAttribution3.0(CCBY3.0)—ccCreativeCommonsAttribution3.0(CCBY3.0)

Plans for High Beta Optics in the LHC

Based on what has been learned with the β* = 90m commissioning and operation in 2011, we describe the potential and practical scenarios for reaching very high β* in the LHC in 2012 and beyond. Very high β* optics require dedicated running time and conditions in the LHC. We describe a planning which is optimized to maximize the physics potential in a minimum of dedicated running time

Plans for High Beta Optics in the LHC

Abstract

Similar works

Full text

Available Versions

HAL-IN2P3

CERN Document Server