Gianluigi Arduini CERN - Beams Dept. - Accelerator & Beam Physics Group

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2 Gianluigi Arduini CERN - Beams Dept. - Accelerator & Beam Physics Group Acknowledgements: O. Brüning, S. Fartoukh, M. Giovannozzi, G. Iadarola, M. Lamont, E. Métral, N. Mounet, G. Papotti, T. Pieloni, L. Rossi, G. Rumolo, B. Salvant, R. Tomàs, J. Wenninger, F. Zimmermann and all the teams involved in LHC and LHC Injector operation S. Cittolin

3 LHC layout Total length: ~26.7 km 8 arcs (aka sectors): ~2.8 km each 8 long straight sections: ~700 m each RF 2-in-1 magnet design with separate vacuum chambers p-p, ion/ion, or p/ion collisions beams cross in 4 points

4 Interaction regions geometry In the IRs, the beams are first combined into a single common vacuum chamber and then re-separated in the horizontal plane, The beams move from inner to outer bore (or vice-versa), The triplet quadrupoles are used to focus the beam at the IP. beam1 194 mm D2 D1 Machine geometry in H plane Triplet IP Triplet D1 D2 beam2 beam2 ~ 40 m beam1 D1,D2 : separation/recombination dipoles ~ 260 m Common vacuum chamber

5 Separation and crossing Because of the tight bunch spacing and to prevent undesired parasitic collisions in the region where the beams circulate in the common vacuum chamber: o Parallel separation in one plane, collapsed to 0 to bring the beams to collision o Crossing angle in the other plane. a (mrad) 4 mm (450 GeV) 1.3 mm (4 TeV) q ~ 8 mm ATLAS -145 / ver. ALICE 145 / ver. CMS 145 / hor LHCb 90 / ver 4 TeV / 2012! Not to scale!

6 Luminosity , commissioning: 0.04 fb , exploring the limits: 6.1 fb , production: 23.3 fb first operational experience with low bunch intensity learn to handle intense beams (~1 MJ stored energy) 2010 production and Higgs hunt

7 Summary: 2010 to 2012 Impressive progress in performance L kn 2 b * f * 4 b e F Parameter Nominal Energy (TeV) N ( p/bunch) k (no. bunches) Bunch spacing / Stored energy (MJ) e (mm rad) b* (m) L (cm -2 s -1 ) Beam-beam parameter/ip Average beg. of fill

8 L kn 2 b * f 4 b e * F

9 Limits on b* The triplet quadrupoles in the high luminosity IRs define the machine aperture limit for squeezed beams, b* is constrained by: o the beam envelope, o a margin TCT to triplet, o the crossing angle e b triplet * TCT Triplet Triplet TCT ~ 8 mm TCT TCT=Tertiary Collimator

10 Collimation system Complex and high performance multi-stage collimation system. Collimation hierarchy has to be respected in order to achieve satisfactory protection and cleaning. Lower b* implies tighter collimator settings as well as alignment, beam sizes and orbit well within tolerance. We could do it only after having gained experience in orbit and optics controls and thanks to the small emittance delivered by the injectors. TCP TCS7 TCLA7 TCS6/ TCDQ TCT Aperture Primary halo Kicked beam beam Secondary halo Tertiary halo R. Bruce

11 Optics control Lower b* means enhancement of errors challenging R. Tomas et al. but proven to be feasible.

12 L kn 2 b * f 4 b e * F

13 Wake fields and Impedances Intense bunches generate electromagnetic (EM) fields when passing inside a structure (e.g. Carbon collimators opening of ~1 mm!!!) results in an EM force, called wake field in time domain, beam-coupling impedance in frequency domain. Abrupt transition With taper Avoid the abrupt transition for the beam fields at the location of the beam passage (taper) Reduce the resistivity of the material B. Salvant

14 Wake fields and instabilities Wake fiels can couple the head and tail of a bunch Direct EM interaction direct space-charge Induced (or image ) currents EM interaction through the pipe wall wall impedance Pipe wall x v=b c Test particle v=b c Source particle s Orbit (pipe axis of symmetry)

15 Wake fields and Instabilities Many bunches (up to 1380 with 50 ns spacing) ~36 cm ~15 m v ~ p Bunches can interact together (or even the head of each bunch can interact with the tail of the bunch) and in some cases begin to oscillate. Example with 36 bunches in the LHC: oscillation pattern along the bunch train (simulation result): s Coupled-bunch instabilities N. Mounet

16 Beam instabilities In 2012 instabilities have become more critical due to higher bunch intensity and tighter collimators settings. The LHC is one of the few machines where instabilities are more critical at high energy. Interplay between impedance (mostly due to collimators) and two-beams phenomena (mostly beam-beam) Cures: Transverse feedback ( damper ) that measures the oscillations and sends corrective deflections, Non-linear magnetic fields (sextupoles, octupoles, beam-beam) that produce a frequency spread among particles kill coherent motion

17 Beam-beam effects Long-range Strong non linear fields when counter-rotating beams are sharing vacuum chamber. spread in betatronic frequencies risk of overlapping resonances driven by magnetic errors Head-on T. Pieloni Minimize magnetic errors Paid off for the LHC Devise correction schemes and sorting Paid off for the LHC Initially expected to have limit at DQ BB ~ 0.005/IP S. Fartoukh

18 G. Iadarola, G. Rumolo Electron cloud effects F. Ruggiero Secondary emission yield [SEY] SEY>SEY th avalanche effect (multipacting) SEY th depends on bunch spacing and population Electron cloud effects occur both in the warm and cold regions, their intensity increases rapidly for shorter bunch spacing. Observed as soon as we started to inject bunch trains ( ns spacing): Vacuum pressure rise (interlock levels, beam losses ) Single-bunch and multi-bunch instabilities beam size growth Incoherent beam size growth Heat load on the cryogenics

19 Electron cloud effects Fields induced by electrons act like wake fields (more complex) that couple different bunches along the train and head&tail of each bunch and have similar adverse effects on beam stability as impedances. As a result of that emittance blow-up and beam losses H. Bartosik

20 Cures for electron cloud effects At the time of the construction of the LHC: NEG coating would require activation to >200 o C impractical Conditioning by beam-induced electron bombardment ( scrubbing ) leading to a progressive reduction of the SEY as a function of the accumulated electron dose tested in the laboratory (on Cu surfaces) and in the SPS (Stainless Steel vacuum chambers) Chosen strategy for the LHC operation with bunch trains but it takes time!! (in particular for 25 ns operation) More recently a-c coating has been successfully tested in the SPS showing SEY as low as 1.1 possible implementation for HL-LHC (interaction regions)

21 Pile-up density 25 reconstructed vertices in a luminous region of 4.3 cm r.m.s. length Peaks of >40 events per bunch crossing observed with luminosities in the range of 7x10 33 cm -2 s -1 This is the maximum acceptable by the experiments with the present design upgrade is vital Pile-up proportional to luminosity per bunch more bunches (i.e. 25 ns) is better Z mm

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25 Expected peak performance 6.5 TeV Provided scrubbing successful 50 ns 25 ns Beta* [m] e*[mm] at start of fill Max. Bunch Population [10 11 p] Max. Number of bunches/colliding pairs IP1/ Bunch length (4 )[ns]/ (r.m.s.) [cm] 1.35/ /10.1 Max. Beam Current [A]/population[10 14 p] 0.36 / / 2.9 Max. Stored energy [MJ] Peak luminosity [10 34 cm -2 s -1 ] in IP1/ Half External Crossing angle IP1/5 [mrad] Beam-beam tune shift (start fill)/ip [0.001] Min. beam-beam separation ( ) d sep Maximum Average pile-up ( =85 mb) Pile-up density an issue for 50 ns Baseline scenario

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27 Why an LHC Upgrade? Motivations: Reduce statistical error by factor 3 Radiation damage limit of IR quadrupoles (~400 fb -1 ) Radiation damage to detectors Target (very ambitious) of the upgrade: fb -1 /y ( 10 today) experience: Head-on beam-beam limit higher than initially expected Single bunch with > 3x10 11 ppb with 2.5 mm emittance accelerated in the SPS Low b* optics successfully tested in MD Limit on total beam current in LHC due to several systems (RF, dump, vacuum, collimator robustness, machine protection, RP, ) at ultimate value (25 ns) Pile-up density replaces beam-beam as HL-LHC constraint

28 Ideas for the Upgrade Operation at pile-up density limit: choose parameters that allow higher than design pile-up Low b* Crab Cavities as tool to maximize overlap among colliding bunches (i.e. virtual luminosity) 2 b * kn f L F * 4 b e leveling mechanisms for controlling performance during run dynamic b* squeeze crossing angle and Long-range and beam-beam wire compensators

29 Ideas for the Upgrade L [10 34 cm -2 s -1 ] L kn 2 b * f * 4 b e F Virtual peak luminosity (F=1) leveling at 2.5x10 34 cm -2 s -1 t eff =30 h, T ta =5 h leveling at 5x10 34 cm -2 s -1 t eff =15 h, T ta =5 h F. Zimmermann t [h]

30 HL-LHC Performance Estimates O. Brüning Parameter Nominal 25ns HL-LHC 50ns HL-LHC Bunch population N b [10 11 ] Number of bunches Beam current [A] Crossing angle [mrad] Beam separation [ ] b * [m] Normalized emittance e n [mm] e L [evs] Relative energy spread [10-4 ] r.m.s. bunch length [m] Peak Luminosity [10 34 cm -2 s -1 ] Virtual Luminosity [10 34 cm -2 s -1 ]

31 Very low b* (10 cm) b* [m] L kn 2 b * f * 4 b e F IR4 IR5 IR6 Beam size [mm] and dispersion (IR4 IR6) at 3.5 TeV (for e=3.5 mm) of course it requires larger aperture triplets Tunes vs. d p ATS=Achromatic Telescopic Squeeze - S. Fartoukh

32 Hardware for the Upgrade Upgrade of the Experiments to cope with higher pile-up density New high field/larger aperture interaction region magnets Cryo-collimators and high field 11 T dipoles in dispersion suppressors New collimators (lower impedance) Additional cryo plants (P1, P4, P5) Crab Cavities to take advantage of the small b* SC links to allow power converters to be moved to surface Upgrade of the intensity in the Injector Chain (see M. Migliorati)

33 Summary The progress in the performance of the LHC has been so far breath-taking This has been possible thanks to the quality of the design, construction and installation and to the thorough preparation in the injectors Some of the (beam dynamics) challenges that had to be faced have been outlined Luminosity performance and choices for the upgrade are now constrained by the acceptable detector pile-up density but They are pushing even further the above challenges

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