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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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slide1

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
requirements of the lhc on its injectors 1 3
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

requirements of the lhc on its injectors 2 3
Requirements of the LHC on its injectors (2/3)

Choice of the nominal LHC parameters

requirements of the lhc on its injectors 3 3
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
what are the nominal already achieved beams at ps exit
What are the nominal & already achieved beams at PS exit?

 The specifications are met in the PS complex

how is it obtained in the ps complex
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
slide7

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

slide8

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

slide9

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

slide10

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

slide12

PSB (5/5)

Beam parameters at PSB extraction

Without blow-up

slide13

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

slide14

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

slide15

PS (3/10)

Head-Tail resistive-wall instability

Beam-Position Monitor

(20 revolutions superimposed)

R signal

Time

(20 ns/div)

slide16

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

slide17

PS (5/10)

Magnetic field and double-batch injection

No 3.5 GeV/c

plateau

slide18

PS (6/10)

Longitudinal beam structure in the last turn of the PS

slide19

PS (7/10)

Normalised transverse emittances at 1

without bunch

rotation

!

slide20

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

slide21

PS (9/10)

Emittance measurements using the Semfils in TT2

with bunch rotation

=> Electrons are created ...

H - plane

slide22

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)

future work

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

slide24

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)
slide25

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

slide26

SPS and LHC filling (3/4)

Bunch disposition in the LHC, SPS and PS

slide27

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