HiLumi LHC FP7 High Luminosity Large Hadron Collider Design Study. Deliverable Report. Corrector magnets specifications

Transcription

1 CERN-ACC HiLumi LHC FP7 High Luminosity Large Hadron Collider Design Study Deliverable Report Corrector magnets specifications Fartoukh, S (CERN) et al 28 November 2014 The HiLumi LHC Design Study is included in the High Luminosity LHC project and is partly funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement This work is part of HiLumi LHC Work Package 2: Accelerator Physics & Performance. The electronic version of this HiLumi LHC Publication is available via the HiLumi LHC web site < or on the CERN Document Server at the following URL: < CERN-ACC

2 Grant Agreement No: HILUMI LHC FP7 High Luminosity Large Hadron Collider Design Study Seventh Framework Programme, Capacities Specific Programme, Research Infrastructures, Collaborative Project, Design Study DELIVERABLE REPORT CORRECTOR MAGNETS DELIVERABLE: D2.3 Document identifier: Due date of deliverable: End of Month 36 (October 2014) Report release date: 28/11/2014 Work package: Lead beneficiary: Document status: WP2: Accelerator Physics and Performance CERN Final Abstract: In this document the specifications of the corrector magnets used to generate the crossing and separation bumps as well as those in the corrector package close to the Q3 quadrupole are presented. Previous estimates have been scrutinised and reviewed, thus obtaining the specifications reported here. Grant Agreement PUBLIC 1 / 15

3 Copyright notice: Copyright HiLumi LHC Consortium, For more information on HiLumi LHC, its partners and contributors please see The HiLumi LHC Design Study is included in the High Luminosity LHC project and is partly funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement HiLumi LHC began in November 2011 and will run for 4 years. The information herein only reflects the views of its authors and not those of the European Commission and no warranty expressed or implied is made with regard to such information or its use. Delivery Slip Name Partner Date Authored by S. Fartoukh, M. Fitterer, M. Giovannozzi, R. De Maria CERN 07/11/14 Edited by M. Giovannozzi CERN 07/11/14 Reviewed by M. Giovannozzi, Task coordinator G. Arduini, WP coordinator CERN 09/11/14 Approved by Steering Committee 17/11/14 Grant Agreement PUBLIC 2 / 15

4 TABLE OF CONTENTS 1. INTRODUCTION BRIEF REVIEW OF THE LAYOUT LAYOUT OF ORBIT CORRECTORS SPECIFICATION OF ORBIT CORRECTORS MAGNETS SPECIFICATION OF CORRECTOR S PACKAGE MAGNETS CORRECTORS LAYOUT SIMULATIONS RESULTS CONCLUSIONS AND OUTLOOK ACKNOWLEDGEMENTS REFERENCES ANNEX: GLOSSARY Grant Agreement PUBLIC 3 / 15

5 Executive summary In this paper the orbit correction strategy and crossing scheme adjustment for the HL-LHC orbit correctors in IR1/5 is reviewed. Both the HLLHCV1.0 and the latest layout HLLHCV1.1 are considered. The main objectives are to optimize the crossing scheme, in particular to reduce the strength of the orbit correctors at D2, and to obtain an estimate for the required strength to compensate orbit distortions due to various sources. In addition, the specification for the correctors located in the package close to the Q3 triplet quadrupole is presented. This completes and confirms the results obtained with previous layout versions. 1. INTRODUCTION One of the main objectives in preparation of the new HL-LHC layout version HLLHCV1.1 [1] was the review of the necessary orbit corrector strength and investigation of a possible reduction of the strength of the corrector at D2 and at the triplet quadrupole Q3, building up on a previous analysis presented in [2]-[4]. The orbit correctors in IR1 and IR5 in general have to provide sufficient strength for coping with the following aspects: Crossing and separation scheme Centring of the beam at the crab cavities Luminosity and Van der Meer scans Inner triplet (IT) misalignment and feed-down from transfer function errors Closed orbit generated by arc imperfections. The corrector strength for crossing and separation, luminosity and Van der Meer scans, and correction of IT errors depends only on the IT strength and the chosen crossing angle and separation. During the pre-squeeze the IT strength increases from injection to pre-squeezed optics and stays constant during the complete telescopic squeeze and for all squeezed optics (round, flat, sflat and sround). In most cases the studies are therefore only conducted for the injection optics at 7 TeV with β = 6 m, and round collision optics marking the two corner cases during the pre-squeeze, where the round collision optics configuration has been chosen as reference for all squeezed optics. The results for the other squeezed optics can be then obtained by linearly scaling with the crossing angle and separation, if changed. Currently the injection optics with β = 6 m represents the baseline option and most of the studies have thus been performed for this optics, although the maximum corrector strength would be reached for injection optics with β = 15 m as the Q2 strength is reduced even further for this optics. It is worth noting that in case of Van der Meer scans, no particular impact on the correctors strength has to be envisaged as during these special runs the crossing angle will be switched off and consequently also the crab cavities will not be active. The reduction of the Q2 strength in general results in an increase of the strength of the orbit corrector at Q1 and Q2 and would drive both correctors right to the limit of their range for injection optics with β = 15 m. The complete description of the detailed studies performed can be found in Ref. [5]. Similarly, the specifications of the multipole correctors have been reviewed with the proposal of the HLLHCV1.0 and HLHLCV1.1 layouts. The key studies have been reported in Ref. [3] Grant Agreement PUBLIC 4 / 15

6 and were based on the so-called SLLHCV3.1b layout. Among other differences, the more recent HLLHCV1.0 and HLLHCV1.1 layouts feature a complete implementation of the orientation of the triplets quadrupoles. This implies self-compensation of some multipoles within the IT magnets, with a potential reduction in the required strength of the correctors. 2. BRIEF REVIEW OF THE LAYOUT For IR1/5 the crossing scheme is in general fully symmetric except that the plane is alternated between IR1 and IR5 for beam-beam compensation. In this paper, we assume a horizontal separation and vertical crossing in IR1, and a vertical separation and horizontal crossing in IR5. The opposite choice might be advantageous for machine protection in case of an asynchronous dump, but the exact gain is still to be quantified and depends on the asymmetry in the aperture margin between the two planes at all potential bottlenecks [6]. Table 1: Optics parameters for layout HLLHCV1.0 and HLLHCV1.1. β* (H/V) [m] Half crossing angle [µrad] Half separation [mm] round 0.15/0.15 ±295 sround 0.10/0.10 ±360 ±0.75 Collision flat 0.30/0.075 ±275 sflat 0.20/0.05 ±335 vdm /30.0 ±295 Injection injection 6 6.0/6.0 ±295 ±2.0 (450 GeV)/ injection /15.0 ±0.75 (7TeV) Albeit the optics parameters did not change, the layout underwent several changes [1] (see Fig. 1). The main changes relevant for orbit correction are: Reuse of the Q4 quadrupole of the nominal LHC as Q5 for the HL-LHC (including the orbit correctors). Q4 is moved by about 12 m towards the arc in order to increase the β-function at the location of the crab cavities and to leave room for equipment like TCTs or other instrumentation, if needed. Four crab cavities per side and IP instead of three as for HLLHCV1.0. For HLLHCV1.0 the crossing scheme was closed after the MCBRD in order to have no orbit offset in the crab cavities for beam loading purposes and save aperture in TAXN and D2. This required a very large strength of the MCBRD. In order to reduce the strength of the MCBRD, the constraint of the orbit in the crab cavities has been relaxed for HLLHCV1.1 thanks to the latest results on crab cavities performance [7], [8] permitting to close the crossing scheme bump at the Q5 and allowing to share the strength between the Q4 orbit corrector and the MCBRD. By reusing Q4 as Q5 for HL-LHC, orbit correctors in both planes become available next to Q5 making it possible in the first place to extend the crossing bump to Q5 without using the correctors at Q5, Q6 and Q7 dedicated to the general closed orbit correction. The change of the position of Q4 in general influences how the corrector strength is shared between the MCBRD and MCBYY4 in case of an extension of the crossing scheme until the Grant Agreement PUBLIC 5 / 15

7 Q5. Moving Q4 towards the arc implies a larger orbit in the crab cavities for the same reduction of the MCBRD strength. As one of the main limitations is the orbit in the crab cavities, the movement of Q4 towards the arc entails a smaller reduction of the MCBRD strength if the crab cavities are not aligned along the crossing scheme. The number of crab cavities is only relevant for the knobs for the orbit adjustment in the crab cavity region LAYOUT OF ORBIT CORRECTORS The orbit correctors in the straight section of IR1/5 for the optics version HLLHCV1.0 and HLLHCV1.1 are shown in Fig. 1 for the IT region and Fig. 2 for the section from Q4 to Q7 inclusive. The maximum strength of all orbit correctors is summarized in Tables 2 and 3. The MCBRD and MCBYY orbit correctors in HLLHCV1.1 are of the same hardware type, but the name is kept for historical reasons. The main changes from HLLHCV1.0 layout to HLLHCV1.1 are: The Q4 of the nominal LHC is reused as Q5 for the HL-LHC by simply shifting it to the left or right respectively. The crossing scheme is extended until Q5 inclusive. Figure 1: Schematic layout of the orbit correctors in IR1/5 in the IT region (longitudinal positions are not to scale). The layout is the same for IR1/5 and left/right of the IP. Explicitly, the MCBRDH is always closest to the D2 left/right and Beam1/2 for the layout HLLHCV1.0, and the MCBRDV is always closest to the D2 for HLLHCV1.1. All orbit correctors are used for the crossing scheme adjustment and orbit correction. The MCBRDH and MCBRDV position are inverted between HLLHCV1.0 and HLLHCV1.1. The reason is that by moving the MCBRDV closer to the D2 the aperture in D2 is slightly increased. In the layout presented in this paper the Q4 of the nominal LHC is simply shifted in position, which entails that the plane of the orbit correctors is inverted at the Q5 of the HLLHCV1.1, and consequently not optimal for the orbit correction (see MCBY*5R5.B1 in Fig. 2 for which two vertical correctors are at a QF instead of two horizontal orbit correctors). Grant Agreement PUBLIC 6 / 15

8 Table 2: Maximum orbit corrector strength for HLLHCV1.0 layout. The plane of the orbit correctors at Q4, Q5, Q6 and Q7 is given for IR5 Beam 1 right and has to be changed accordingly for the other side and beam. IR1 and IR5 are fully symmetric in respect of orbit correctors and no change of plane has to be applied between the two IRs. The orbit correctors marked as com are common orbit correctors acting on both beams, while the ones marked as ind are the correctors acting only on one beam. Note that the MCBY[HV] and MCBC[HV] are of the same hardware type and the difference in length is only an inconsistency in the layout database, which is currently being resolved. The temperature indicated is a copy from the existing LHC correctors as strengths and temperatures were still under investigations at the time of the HLLHCV1.0 release. Presently all the new IR1/IR5 insertion magnets will be cooled down to 1.9K. com ind Orbit corrector Type Max. field [T] Length [m] Integrated strength [Tm] MCBX[HV].[12] MCBXFB MCBX[HV].3 MCBXFA MCBRD[HV].4 MCBRD MCBYYV.[AB]4 MCBYYH.4 MCBYY 2.5@4.5K MCBYH.5 MCBYH @1.9K MCBYV MCBCV.6 MCBCV 2.33@4.5K MCBCH.7 MCBCH 3.11@1.9K This choice was made on condition of minimal hardware changes. Recent studies and discussions revealed that anyway the beam screen should be changed and a carbon coating applied, making the requirement of minimal hardware changes obsolete. In view of this recent change the best configuration in terms of orbit correction should also be discussed again. Figure 2: Schematic layout of the orbit correctors in IR1/5 in the Q4 to Q7 region (longitudinal positions are not to scale). Representatively, IR5 Beam 1 right is shown. Orbit correctors used additionally for the crossing scheme are indicated in green and orbit correctors only used for orbit correction in pink. For Beam 2 the plane of correction is inverted for the orbit correctors at Q4, Q5 and Q6 due to the change of polarity of the quadrupoles. Furthermore, the MCBRDV has been placed closest to D2 for the layout HLLHCV1.1 in order to optimize the aperture in D2. On the other hand, recent analysis showed that by inverting the plane between Beam 1 and Beam 2 the crosstalk between apertures could be Grant Agreement PUBLIC 7 / 15

9 minimized and the field quality improved, suggesting an inversion of the corrector polarity with the beam [9]. Table 3: Maximum orbit corrector strength for HLLHCV1.1 layout. The plane of the orbit correctors at Q5, Q6 and Q7 is given for IR5 Beam 1 right and has to be changed accordingly for the other sides, IRs and beam. The orbit correctors and corrector strength changed in respect of HLLHCV1.0 are marked in bold. Note that the MCBY[HV] and MCBC[HV] are of the same hardware type and the difference in length is only an inconsistency in the layout database, which is currently being resolved. com ind Orbit corrector Type Max. field [T] Length [m] Integrated strength [Tm] MCBX[HV].[12] MCBXFB MCBX[HV].3 MCBXFA MCBRD[HV] MCBYY[HV].4 MCBYY MCBYV.[AB]5 MCBYV @1.9K MCBYH.5 MCBYH MCBCV.6 MCBCV 3.11@1.9K MCBCH.7 MCBCH 3.11@1.9K SPECIFICATION OF ORBIT CORRECTORS MAGNETS The determination of the maximum magnets strength for the HL-LHC orbit correctors has been performed by means of Monte Carlo simulations. Such numerical simulations were designed to evaluate the required strength as a function of a number of external parameters that were varied on the basis of assumed distributions (see Ref. [5] for more detail). The effects considered have been: Error in the transfer function of the triplet quadrupoles Transverse misalignments of the triplet quadrupoles Closed orbit generated outside the IR In terms of functionalities, the dipole correctors should ensure: The generation of the crossing and separation schemes The generation of a transverse offset of the IP Centring of the beam in the crab cavities The generation of the bumps required to perform the optimisation of the luminosity, the so-called luminosity scans It is worth noting that between the layout version HLLHCV1.0 and HLLHCV1.1 a main change occurred. While for HLLHCV1.0 the crossing bump was closed at the location of the D2 separation dipole, for HLHLCV1.1 this constraint has been abandoned. The original reason for the constraint was based on the need to have zero orbit in the crab cavities to avoid any beam loading effect. Further analysis showed that a residual closed orbit not exceeding a certain upper bound can be tolerated [7], [8] and this argument has been used to extend the bump. It is worth mentioning that closing the crossing bump beyond the D2 decreases the strength requirement for the orbit correctors in the D2. The summary of the different contributions to the orbit correction budget justifying the requirements presented in Table 3 is listed in Table 4. Grant Agreement PUBLIC 8 / 15

10 Table 4: Layout HLLHCV1.1: Summary of different contributions to the orbit corrector budget in the crossing plane for round and flat collision and injection optics at 7 TeV (see [5] for more details). Corrector strength [Tm] Round optics: β = 0.15/0.15 m, xing = ±295 µm, separation = ±0.75 mm MCBX1 MCBX2 MCBX3 MCBRD MCBYY4 MCBY5 MCBC6/7 X-scheme Offset IT error Crab Lumi Arc /0.84 Sum /0.84 Margin % /70.0 Max /2.80 Corrector strength [Tm] Flat optics: β = 0.30/0.075 m, xing = ±275 µm, separation = ±0.75 mm MCBX1 MCBX2 MCBX3 MCBRD MCBYY4 MCBY5 MCBC6/7 X-scheme Offset IT error Crab Lumi Arc /0.84 Sum /0.84 Margin % /70.0 Max /2.80 Corrector strength [Tm] Injection optics (7 TeV): β = 6.0/6.0 m, xing = ±295 µm, separation = ±2.0 mm MCBX1 MCBX2 MCBX3 MCBRD MCBYY4 MCBY5 MCBC6/7 X-scheme Offset IT error Crab Lumi Arc /0.84 Sum /0.84 Margin % /70.0 Max /2.80 It must be noted that the requirements on the orbit correctors could be further reduced if the possibility of a beam-based remotely controlled alignment of the crab cavity would be made possible on a fill-to-fill basis. This possibility should be further investigated as part of the design of the crab cavities. Grant Agreement PUBLIC 9 / 15

11 4. SPECIFICATION OF CORRECTOR S PACKAGE MAGNETS 4.1. CORRECTORS LAYOUT The nominal layout of the LHC ring includes a correctors' package located in the inner triplet in order to cope with alignment errors or magnetic field quality issues. In Fig. 3 a detailed layout is shown, including the triplets (Q1, Q2, and Q3), the first separation dipole (D1), as well as the correctors. The layout change between version 6.4 and 6.5 is also reported. Figure 3 Layout of the nominal LHC inner triplet, including the corrector magnets. For the case of the HL-LHC it is planned to implement a similar correction system with some improvements in the sense that additional magnets will be requested in view of correcting the a 5, b 5, and a 6 field components. For reference, the multipole expansion used to describe the magnetic field reads as [1] ( ) x + iy B y + ibx = Bref bn + ian n= 1 Rref where B x, By, Bref are the transverse magnetic field components and the reference field, respectively. The coefficients a b n, n are the so-called skew and normal field components and R is a reference radius at which the multipoles are expressed. It is worth recalling that in ref the framework of the LHC studies the magnetic errors are split into three components, namely mean (S), uncertainty (U), and random (R), such that a given multipole is obtained by ξu bn = bn + b S n + ξ U R bnr 1.5 where ξ U,ξ R are Gaussian distributed random variables cut at 1.5 σ and 3 σ, respectively. The ξ U variable is the same for all magnets of a given class, but changes from seed to seed and for the different multipoles. On the other hand, ξ R changes also from magnet to magnet. A sketch of the proposed correction system is shown in Fig. 4 [10] SIMULATIONS RESULTS The correction strategy follows the one that was established for correcting the field imperfections of the existing triplet and D1. The basic principles of the method are described Grant Agreement PUBLIC 10 / 15 n 1

12 in Ref. [3] and are based on the correction of the resonance driving terms generated by the magnetic errors in the IT and D1 magnets. It is worth noting that also in this case the overall layout is inherited from the Phase I studies [11]. Figure 4: Upper: Sketch of the latest HL-LHC IT and D1 layout with associated correctors. Lower: zoom of the triplet corrector package (CP). The horizontal scale indicates the distance to the IP. The specification of the strength requirements for the non-linear correctors in the proposed IT has been performed by means of numerical simulations. The algorithm presented in Ref. [3] has been applied to sixty different realisations (also called seeds) of the magnetic field errors of the triplets and of the superconducting D1 separation dipoles. Therefore, the correctors strength depends on the target field quality of the new triplets, but also of the new separation dipoles. The estimate of the IT and D1 field quality applied in the numerical simulations is reported in Table 5, where also the values that were used for the initial specification [3] are reported. Table 5: Multipoles used for the field quality of the triplets and of the D1 separation dipole. The values are in units of 10 4 at R ref =50 mm. Normal Skew IT quadrupoles D1 dipole Multipole Mean old/new Unc. old/new Random old/new Mean old/new Unc. old/new Random old/new / / Grant Agreement PUBLIC 11 / 15

13 The latest results are reported in Table 6, where also the old figures are listed for the sake of comparison. Normal Skew Multipole Computed Specification mt m at 50 mm old/new mt m at 50 mm / / / / / / / / As the field quality estimates used for the two studies are exactly the same apart for the value of the uncertainty and random components of b 6 for the IT, the sizeable reduction of the correctors strength is due to the orientation of the IT cold masses. In any case, the key outcome of this study is that the initial specifications of the system of correctors close to Q3 are confirmed. 5. CONCLUSIONS AND OUTLOOK The final aim of the studies presented in this paper is to give an estimate of the maximum orbit corrector strength needed in IR1 and IR5. To assess the maximum strength, the following contributions have been taken into account, explicitly the orbit corrector strength required for: The crossing and separation scheme (x-scheme) The transverse shift of the IP for compensation of misalignment of the detector (offset) Correction of transverse and longitudinal misalignment and feed-down from transfer function errors of the IT (IT error) Beam based alignment of crab cavities (crab) Luminosity scans (lumi), Correction of closed orbit generated by arc imperfections (arc). In summary, the corrector strengths are within limits in respect of the preliminary specifications given in the past and incorporated in the layouts HLLHCV1.0 and HLLHCV1.1. Beside the corrector strength, also the effect of hysteresis has been studied. Hysteresis effects are in particular relevant for luminosity scans where the corrector strength is not changed monotonously and could be also relevant during commissioning where the corrector strength is not necessarily changed monotonously during one fill, possibly leading to insufficient orbit Grant Agreement PUBLIC 12 / 15

14 reproducibility between fills. The studies revealed that a uniformly distributed relative error of 10 3 of the corrector strength leads to an orbit deviation of approximately 7 μm corresponding to 1σ beam size in case of round collision optics. The hysteresis model should therefore be improved and included in the control software. In the studies presented in this paper it has been assumed that the Q4 of the nominal LHC is reused as Q5 for the HL-LHC by shifting it to the left/right respectively. The orbit correctors dedicated to the orbit correction (and not the crossing and separation scheme) would thus have the wrong polarity. This choice was made on the requirement of minimal hardware changes. As the beam screen has to be changed and a carbon coating applied, this requirement became obsolete and a configuration should be sought for the next layout version respecting the polarity of the orbit correctors. In addition, recent studies revealed that the field quality of the MCBRD correctors could be improved by changing the polarity with the beam, which should be considered as well in the next layout version. It must be noted that the requirements on the orbit correctors could be further reduced if the possibility of a beam-based remotely controlled alignment of the crab cavity would be made possible on a fill-to-fill basis. This possibility should be further investigated as part of the design of the crab cavities. The correctors located in the special package close to Q3 did not show any particular deviation from the initial specifications, which are therefore confirmed. Future studies will be devoted to the assessment of the HL-LHC performance in terms of DA whenever non-perfect corrections of the resonance driving terms are applied. Furthermore, detailed analysis of the coupling level achieved with the dedicated magnets in the correctors package will be made, as, unlike in the nominal LHC ring, the proposed location of the skew quadrupole magnet feature unequal beta values in the two transverse planes. This might not be ideal. 6. ACKNOWLEDGEMENTS We wish to thank Gianluigi Arduini, Roderik Bruce, Oliver Brüning, Hélène Mainaud Durand, Stefano Redaelli and Jörg Wenninger for useful discussions. Grant Agreement PUBLIC 13 / 15

15 7. REFERENCES [1] S. Fartoukh and R. De Maria, Database of Baseline Scenarios and Variants: Milestone: MS17, CERN-ACC , [2] S. Fartoukh, R. Tomas and J. Miles, Specification of the closed orbit corrector magnets for the new LHC inner triplet, CERN-sLHC-PROJECT-Report-0030, [3] M. Giovannozzi, S. Fartoukh and R. De Maria, Initial models of correction systems, CERN- ACC , [4] M. Fitterer, Budget for HL-LHC orbit correctors, HSS meeting, , HSSMeeting pdf. [5] M. Fitterer, S. Fartoukh, M. Giovannozzi, R. de Maria, Crossing scheme and orbit correction in IR1/5 for HL-LHC, in preparation. [6] R. Bruce, private communication. [7] R. Calaga, Crab cavity operational aspects, 25th HiLumi WP2 Task Leader Meeting, , [8] P. Baudrenghien, Crab cavity operational aspects, 25th HiLumi WP2 Task Leader Meeting, , [9] E. Todesco, Options of the D2/Q4 correctors, 30th HiLumi WP2 Task Leader Meeting, , [10] R. De Maria, S. Fartoukh, M. Giovannozzi, 2013, Triplet orbit corrector layout and strength specifications [online]. Available from: [Accessed 30 September 2013]. [11] S. Fartoukh, 2010, Optics Challenges and Solutions for the LHC Insertion Upgrade Phase I, slhc-project-report Grant Agreement PUBLIC 14 / 15

16 ANNEX: GLOSSARY Acronym DA FQ IP IR MS IT D1 D2 Definition Dynamic aperture Field quality Interaction point Interaction region Matching section Inner triplet First separation dipole just downstream of the triplet quadrupoles Second separation dipole downstream of D1 Grant Agreement PUBLIC 15 / 15