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Requirements of the LHC on its injectors What are the nominal & already achieved beams at PS exit?

THE LHC NOMINAL PROTON BEAM IN THE PSB AND PS MACHINES. M. Benedikt & E. Metral PS-OP shut-down lectures, MCR glassbox, 20/02/2001. Requirements of the LHC on its injectors What are the nominal & already achieved beams at PS exit? How is it obtained in the PS complex? General aspects

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Requirements of the LHC on its injectors What are the nominal & already achieved beams at PS exit?

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  1. THE LHC NOMINAL PROTON BEAM IN THE PSB AND PS MACHINES M. Benedikt & E. MetralPS-OP shut-down lectures, MCR glassbox, 20/02/2001 • Requirements of the LHC on its injectors • What are the nominal & already achieved beams at PS exit? • How is it obtained in the PS complex? • General aspects • Linac2 • PSB • PS • Future work • SPS and LHC filling

  2. Requirements of the LHC on its injectors (1/3) • 2 main challenges involved in the design of the LHC • Very high magnetic field to reach the collision energies in the TeV range • Very high luminosity necessary to provide significant event rates at this energy Beam current Brightness = transverse bunch density It is limited by : - Space-charge effects in the injectors... - Head-on beam-beam interaction at collision It is limited by : - Collective instabilities - Cryogenic load (synchrotron radiation and wall current) - S.C. magnet quench

  3. Requirements of the LHC on its injectors (2/3) Choice of the nominal LHC parameters

  4. Requirements of the LHC on its injectors (3/3) • LHC project leader  L. Evans • Project leader to prepare the PS complex to be a pre-injector (started in 1995)  K. Schindl (Deputy  M. Benedikt) •  Major upgrade needed • all along the injector chain

  5. What are the nominal & already achieved beams at PS exit?  The specifications are met in the PS complex

  6. How is it obtained in the PS complex? General aspects • 2main challenges had to be faced • High brightness production (2 as before) and conservation • Production of the train of very short bunches with the LHC spacing • Solutions • Double-batch filling of the PS (2  1.2 s)  Lowers the space charge effects at PSB injection (50 MeV) • Increase of the PSB ejection kinetic energy (PS injection) : 1  1.4 GeV  Lowers the space charge effects at PS injection • 1 triple + 2 double splittings to produce the desired number of bunches, longitudinal emittance and bunch spacing • Bunch rotation to produce the desired bunch length

  7. Linac2 • The initial transverse emittance is given by the duoplasmatron source • The beam is then adiabatically bunched and accelerated in a Radio Frequency Quadrupole (RFQ2) under high space charge conditions • Fine-tuning of the 50 MeV Drift Tube Linac (DTL) and of the transfer line to the PSB Normalised, at 1 Depends on extraction aperture, electrode shape and space charge   

  8. PSB (1/5) • General aspects • PSB delivers 2 batches to PS (2 consecutive 1.2 s cycles) • 3 PSB rings per batch (3,4 and 2) • 1 bunch per ring (H1) • Injection at 50 MeV • Horizontal plane • Multi-turn injection : 3 turns exactly  more stability and reproducibility (most homogeneous longitudinal distribution of the unbunched beam) • Adjustment of the horizontal injection steering and injection bump timing to minimise the horizontal emittance  BIX.SKSW2,3,4 • Special tune because of large tune shift  Qh = 4.28 • Tiny shaving ~ 30 ms after injection C275, Bdot = 5 Gauss/ms This is what sets the brightness

  9. PSB (2/5) • Vertical plane • Injection on orbit • Minimisation of vertical oscillations at injection (1/2 turn pick-up) to minimise vertical emittance  BI.DVT 50 and 70 • Special tune because of large tune shift  Qv = 5.44 • Shaving vertical  to have a well-defined emittance • Acceleration from 50 MeV to 1.4 GeV • Double harmonics operation to increase the bunching factor (bunch flattening) and thus decrease the space charge tune shift at injection C02 (H1, 1/ring) and C04 (H2, 1/ring). C04 voltage slowly reduced to zero at synchronisation/ejection • Available controlled blow-up  C16 (H9, 1/ring) • No coupling between the transverse planes • Standard settings of multipoles for resonance compensations C275  C765

  10. PSB (3/5) • Synchronisation Non-standard bunch spacing at ejection to fit the PS H7 RF system  adjustment with the phase offsets : BA3,4,2.PSYNCOFFSET (ring 3 is always used as the reference) • Ejection at 1.4 GeV  fast extraction towards the PS through the BT/BTP transfer line C805

  11. PSB (4/5)

  12. PSB (5/5) Beam parameters at PSB extraction Without blow-up

  13. PS (1/10) • General aspects • Double-batch injection : 1 batch of 3 bunches + 1 batch of 3 bunches 1.2 s later  6 bunches out of 7 buckets • Longitudinal beam slicing  complicated RF gymnastics • High brightness conservation  careful control of collective effects, injection oscillations, working point, chromaticities, non-linearities at extraction… PSB exit PS exit ~ 300 ns

  14. PS (2/10) • At low energy (1.4 GeV kinetic energy) • 1st injection => 3 bunches (H7) • Transverse matching between PSB and PS, orbit correction... • Careful control of the working point to avoid blow-up during the long flat-bottom  Qh ~ 6.21 and Qv ~ 6.23 • A single-bunch head-tail instability (due to the resistive wall-impedance) develops during the long flat-bottom => it is cured by x-y coupling (skew quadrupoles) • 2nd injection => 3 bunches (H7) => 3 + 3 = 6 bunches (H7) • Momentum adaptation PSB-PS => PSB synchro. made with PS beam • Triple splitting => 6 × 3 = 18 bunches (H21) • Acceleration from 1.4 to 25 GeV • on H21 • At transition : -jump + change of the chromaticities sign Inj 42 at C170 Inj 42 at C1370 C1380 3 groups of C10 cavities on H7,14,21 C1450 22 Gauss/ms C1560

  15. PS (3/10) Head-Tail resistive-wall instability Beam-Position Monitor (20 revolutions superimposed) R signal Time (20 ns/div)

  16. PS (4/10) C2120 • Longitudinal coupled-bunch instabilities between 6 and 20 GeV/ccured by controlled longitudinal blow-up • Horizontal orbit correction => PR.GSDHZ15,60-OC • At high energy (26 GeV/c momentum) • Synchronisation H1 => the worst • 1stdouble splitting => 18 × 2 = 36 bunches (H42) • 2nddouble splitting => 36 × 2 = 72 bunches (H84) • Bunch compression by a step voltage => longitudinal mismatch => bunch rotation and ejection after 1/4 of synchrotron period • Ejection at 26 GeV/c  fast extraction towards the SPS through the TT2/TT10 transfer line Cavities C200 (H420) It will change this year => new high-energy timings 1 cavity C20 1 cavity C40 1 cavity C40 (H84) 2 cavities C80 (H168)  with Ej 16 at C2395

  17. PS (5/10) Magnetic field and double-batch injection No 3.5 GeV/c plateau

  18. PS (6/10) Longitudinal beam structure in the last turn of the PS

  19. PS (7/10) Normalised transverse emittances at 1 without bunch rotation !

  20. PS (8/10) Only 1 measurement is still missing the transverse emittances in TT2 in the presence of bunch rotation Emittance measurements using the Semfils in TT2 without bunch rotation H - plane V - plane

  21. PS (9/10) Emittance measurements using the Semfils in TT2 with bunch rotation => Electrons are created ... H - plane

  22. Also observed in the PS PS (10/10) Baseline drift on electrostatic pick-ups in TT2 Without solenoid With solenoid ~ 50-100 G Apparently the beam is not affected  this is only a measurement problem for the PS (contrary to the SPS and LHC)

  23. The nominal beam is within reach, but one item is missing  the quantitative analysis of the non-linear effects due to the stray-field at PS extraction. It could create an optical mismatch  blow-up • 4 other subjects need to be investigated in the near future • Consolidation of the nominal beam  improvements in pulse-to-pulse injection mis-steerings, kicker ripples, PSB-PS transverse and energy matching, bunch to bunch intensity fluctuations, instrumentation… • Multi-gap/multi-spacing beams preparation for SPS MDs (e.g. 50 and 100 ns bunch spacing). In particular, cures for longitudinal instabilities have to be investigated (feedback systems, HOM damping) • The so-called initial beam should be prepared  good for collective effects  bad for injection mis-steerings => damper at injection certainly useful • The so-called ultimate beam should also be looked at Future work 1/6 of the intensity, 1/4 of the transverse emittance 1.6  the intensity

  24. SPS and LHC filling (1/4) • The cycle will consist of either 3 or 4 PS injections at 3.6 s intervals • 3-batch  2.38  1013 protons in 26% of the SPS circumference • 4-batch  3.17  1013 protons in 35% of the SPS circumference • The injection plateau will therefore lasts up to 10.8 s • The acceleration phase is about 8.3 s and brings the beam from 26 GeV/c to 450 GeV/c • A 1 s flat-top is presently assumed. This will be used to prepare the extraction equipment (bumpers, etc...) and perform any RF re-phasing necessary to put the beam on the correct location in the LHC • SPS issues Collective instabilities in both longitudinal and transverse planes programme for impedance reduction + electron-cloud studies • Fast extraction towards the LHC through TI2 (via the West extraction channel) or TI8 (via the new East extraction channel)

  25. SPS and LHC filling (2/4) LHC Proton Injection Cycle (21.6 s) This cycle is repeated 12 times for each LHC ring. 3 or 4-batch cycles will be interleaved in the form 334 334 334 333 to fill each ring with a total of 2808 bunches. The LHC filling time will be 12  21.6 s = 4.3 minutes per ring

  26. SPS and LHC filling (3/4) Bunch disposition in the LHC, SPS and PS

  27. SPS and LHC filling (4/4) • The sameMain Timing Generator will be used to pilot the PS complex, SPS and LHC • Several levels of super-cycles will be introduced 16 SPS levels Normal + spare 2 PS levels for each SPS level

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