Beam losses versus BLM locations at the LHC

Transcription

1 Geneva, 12 April 25 LHC Machine Protection Review Beam losses versus BLM locations at the LHC R. Assmann, S. Redaelli, G. Robert-Demolaize AB - ABP Acknowledgements: B. Dehning

2 Motivation - Are the proposed BLM locations suitable for detecting slow beam losses at the LHC? Design Philosophy for BLM locations (see next talk by B. Dehning) 1. BLM at each collimator Where largest losses occur! 2. BLM at each quadrupole Maximum functions! 3. Additional special locations Large dispersion, aperture restrictions (e.g., dispersion suppressor and separation dipoles, ) Goal of this study: Assess these design criteria with tracking results Find if there are unexpected loss locations S. Redaelli, LHC MPS Review page 2

3 Overview of my talk: 1. Slow losses from the collimators 2. Simulation tools 3. Beam loss patterns 4. Conclusions S. Redaelli, LHC MPS Review page 3

4 Mechanism to produce slow losses in a two-stage collimation system Primary collimator Secondary collimator Wall of the beam pipe Primary halo Secondary halo Tertiary halo? p Centre of the beam pipe Circulating beam Multi turn effect ~ single turn effect Well tuned machine with collimators at nominal settings and a stable circulating beam. Beam proton diffuse outwards with a rate fixed by the beam lifetime ( b ): Slow regular losses No other aperture bottlenecks are hit before the primary collimators. (no failures!) Loss rate of beam protons in the cleaning insertion determined by the beam lifetime. Time scale: some seconds. S. Redaelli, LHC MPS Review page 4

5 Some secondary and tertiary halo particles escape from the cleaning insertion can be lost around in the ring! 1. Large betatron kicks 2. Large energy errors Precise distribution of losses Requires detailed tracking of particle s trajectories Requires aperture model for the full ring The appropriated TOOLS have been setup in the framework of the Accelerator Physics collimation studies (AB-ABP) to understand: How many particles are lost? Where are they lost in the ring? How does the losses compare with the quench limits? Main focus for this talk ABP collimation team: R. Assmann S. Redaelli G. Robert-Demolaize S. Redaelli, LHC MPS Review page 5

6 Tools for halo tracking and loss maps Halo generation and tracking done with SixTrack + K2 Halo production in the two stage collimation system and multi-turn tracking of secondary and tertiary halos ( E/E, field errors, correction schemes, ) Trajectories of secondary and tertiary halo part s Aperture model for the full LHC ring, 1 cm longitudinal spatial resolution. Reconstruction of beam trajectory provides longitudinal and transverse distributions of losses Off-line treatment of effects such as closed orbit, misalignments, kicks from D1 D4 magnets IR7 IR8 1 cm Aperture wall Trajectory of a halo particle Magnets locations (thin lenses): s 1m Interpolation: s=1 cm (27 points!) S. Redaelli, LHC MPS Review page 6

7 The LHC aperture model y Horizontal coordinae [m] IR3 IR Longitudinal coordinae [km] 4.4 cm x The aperture model includes Beam screen of all superconducting magnets (different size, tilts) Aperture of warm magnets BPM aperture Detector region aperture Linear interpolation for the transitions Kick of separation magnets (D1, D2, ) S. Redaelli, LHC MPS Review page 7

8 Example of halo particle lost in the vacuum chamber 6 D3 TCP D4 TCS Q5.L X [ mm ] s Trace back trajectory until the loss point is found (5D) Count the number of lost particles in the bin s =.1 m Look at x, y, x, y distributions S. Redaelli, LHC MPS Review x [ mm ] page 8 Y [ mm ] X [ mm ] 6 y [ mm ]

9 Calculation of the proton loss rate per unit length From aperture model dn(ds) : number of particles lost around the full ring From tracking N abs : number of particles lost in the cleaning insertion (dn(ds)/n abs is the cleaning inefficiency!) For slow losses, all the particles that drift out of the beam core interact first with the primary collimators: Assumptions on: 1. Total beam intensity 2. Beam lifetime Quench limit of superconducting magnets: S. Redaelli, LHC MPS Review page 9

10 Performance of the collimation system Cleaning inefficiency Particle leakage: fraction of particles that escape from the cleaning insertion Cleaning inefficiency, η ( a c ) Injection (45 GeV/c) N tot = 512 N abs = Cleaning inefficiency, η ( a c ) Top energy (7 TeV/c) N tot = 5172 N abs = (7.5 ) (8.4) a c [ σ r ] a c [ σ r ] System designed to perform better at 7 TeV. S. Redaelli, LHC MPS Review page 1

11 Loss maps at injection (45 GeV/c) Warm insertion Cold region 1 13 Q5 Q4 IP7 Q4 Q5 Q6 DS ARC Loss rate per unit length [ p/s/m ] Warm Cold Coll Quench limit Losses in collimators / warm magnets are up to 1-1 times larger than in the cold section! Preliminary results for perfect machine/cleaning TCP (6 ) and TCS (7 ) only S. Redaelli, LHC MPS Review page 11

12 Loss maps at top energy (7 TeV/c) Warm insertion Cold region Q5 Q4 IP7 Q4 Q5 Q6 DS ARC Less losses at the quadrupoles: beam size smaller at 7 TeV/c! Loss rate per unit length [ p / s / m ] Warm Cold Coll Quench limit Slow losses are easier to detect at the collimators! Mandatory to have BLM s for EACH collimators, as foreseen. Perfect machine/cleaning TCP (6 ) and TCS (7 ) only S. Redaelli, LHC MPS Review page 12

13 Losses in the cold region - Injection energy (45 GeV/c) 1 1 Q6 Q7 Q8 Q9 Q1 Q11 Q12 Q13 Q14 Loss rate per unit length [ p / s / m ] Quench limit Loss peaks at the quadrupoles, where is largest Losses in dispersion regions! BLM not foreseen everywhere! Perfect machine/cleaning TCP (6 ) and TCS (7 ) only S. Redaelli, LHC MPS Review page 13

14 D x = Losses in dispersion regions are expected because secondary halo particles can experience large energy errors! Energy distribution for protons impinging on a 5 cm Copper block D x = D x arc Energy errors due to single-diffractive scattering! See R. Assmann s talk. S. Redaelli, LHC MPS Review page 14

15 1 11 Loss rate per unit length [ p / s / m ] Q7 As expected, for all quadrupoles: Loss peaks at the warm/cold transition! (confirm simulations by R.Assmann, B.E. Holzer, V.Kain) Losses in the first half of the magnet (peak of in the middle) Q9 Loss rate per unit length [ p / s / m ] Q8 Loss rate per unit length [ p / s / m ] S. Redaelli, LHC MPS Review page 15

16 Transverse distributions of losses depend on the location! Beta function [ m ] Dx [ m ] βx βy Q6 Q11 Q Y [ mm ] Y [ mm ] Y [ mm ] X [ mm ] S. Redaelli, LHC MPS Review X [ mm ] page S loss : s1=2217.8m; s2=2218m. N loss = 1728 Q6 S loss : s1=2425m; s2=2445m. N loss = 719 Q X [ mm ] S loss : s1=246m; s2=248m. N loss = 174 Q12 N N N YP [ µrad ] XP [ µrad ] YP [ µrad ]

17 Losses also further downstream of the arc 7-8! Loss rate per unit length [ p / s / m ] IP8 Quench limit S loss : s1=2334m; s2=2336m. N loss = Large betas + separation induce losses in the triplet also at injection Same longitudinal profile for the other MQ s Y [ mm ] X [ mm ] S. Redaelli, LHC MPS Review page 17

18 Losses at top energy - cold region Same loss patterns as at injection However, in some locations longitudinal and transverse distribution of losses is different (betatron losses smaller, energy errors dominate!) Perfect machine/cleaning TCP (6 ) and TCS (7 ) only 1 9 Loss rate per unit length [ p / s / m ] Quench limit Y [ mm ] Q8 - injection S loss : s1=229m; s2=23m. N loss = X [ mm ] Y [ mm ] Q8 - top energy S loss : s1=229m; s2=23m. N loss = X [ mm ] S. Redaelli, LHC MPS Review page 18

19 Losses at the superconducting triplets are induced by the large beta functions (> 4m) and by the crossing schemes. 1 9 Q6 Q5 Q4 D2 D1 MQX 4 S loss : s1=266m; s2=26625m. Loss rate per unit length [ p / s / m ] IP1 Y [ mm ] Hori -4 Vert X [ mm ] Tertiary collimators have been added to shield the triplets. S. Redaelli, LHC MPS Review page 19

20 Conclusions Loss patterns around the full LHC ring can now be precisely calculated! Simulations: Tracking of particle halo trajectory and aperture model ( s = 1 cm!) Preliminary results (primary and secondary collimators only, no absorbers) As expected: Largest losses arise in the cleaning insertions Large loss peaks at the quadrupoles (warm/cold transitions) Large losses at local aperture restrictions However: Losses at unforeseen locations (e.g., dipoles with high D x ) Longitudinal and transverse loss distributions change during energy ramping! Re-evaluation of the BLM location is in progresses! Errors must be studied in detail! Alignment, closed orbit, non-linear fields Failure scenarios other than regular slow losses require dedicated studies S. Redaelli, LHC MPS Review page 2

21 Reserve slides S. Redaelli, LHC MPS Review page 21

22 1 11 Loss rate per unit length [ p / s / m ] Hori. halo Vert. halo Quench limit Losses due to energy errors in the dispersion suppressor but also further donwstream in the arc! Additional BLM s should be foreseen for dipoles were the dispersion is high! Dx [ m ] S. Redaelli, LHC MPS Review page 22

23 X' [ σ ] Generation and tracking of halo particles Annulus distribution at the beginning of the ring (one plane only). Amplitude chosen to have impacts on the primary collimators Nominal settings: Primary collimators (TCP) 6 Secondary collimators (TCS) 7 Initial distribution Opening of primary collimators X [ σ ] X [ σ ] Schematic view s [ arbitrary units ] S. Redaelli, LHC MPS Review page 23 TCP TCS Horizontal (vert) halo Initial distribution in X (Y) Interaction with horizontal (vert) primary collimators N p 1 7 N turn = 2

24 Amplitude of losses versus beta function values D x < 1.5 m D x > 1.5 m 1 N [ p ] β y [ m ] Neglecting the contribution from dispersion, losses occur at the peak values of! S. Redaelli, LHC MPS Review page 24

25 Amplitude of losses versus dispersion values β y > 5 m β y < 5 m 1 N [ p ] D X [ m ] For small values of, the losses are driven by energy error (large D x )! S. Redaelli, LHC MPS Review page 25

26 Distribution of available LHC aperture at injection (45 GeV/c) S. Redaelli, LHC MPS Review page 26

27 Distribution of available LHC aperture at top energy (7 TeV/c) S. Redaelli, LHC MPS Review page 27

28 Preliminary beam losses with tertiary collimators to protect the triple: 1 9 Q6 Q5 Q4 D2 D1 MQX Loss rate per unit length [ p / s / m ] IP No more losses at the triplets with tertiary collimators at 8.4! S. Redaelli, LHC MPS Review page 28