Requirements of the LHC on its injectors

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

• 2 main 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
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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
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PS (2/10)

• At low energy (1.4 GeV kinetic energy)
• 1st injection gt 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 gt it is cured by x-y coupling
  (skew quadrupoles)
• 2nd injection gt 3 bunches (H7) gt 3 3 6
  bunches (H7)
• Momentum adaptation PSB-PS gt PSB synchro. made
  with PS beam
• Triple splitting gt 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/c cured by controlled longitudinal
  blow-up
• Horizontal orbit correction gt PR.GSDHZ15,60-OC
• At high energy (26 GeV/c momentum)
• Synchronisation H1 gt the worst
• 1st double splitting gt 18 2 36 bunches (H42)
• 2nd double splitting gt 36 2 72 bunches (H84)
  
• Bunch compression by a step voltage
• gt longitudinal mismatch
• gt 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 gt new high-energy
timings
1 cavity C20
1 cavity C40
1 cavity C40 (H84) 2 cavities C80 (H168)
?
with
Ej 16 at C2395
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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
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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
gt 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
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 gt damper at
  injection certainly useful
• The so-called ultimate beam should also be looked
  at

1/6 of the intensity, 1/4 of the transverse
emittance
1.6 ? the intensity
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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
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SPS and LHC filling (3/4)
Bunch disposition in the LHC, SPS and PS
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SPS and LHC filling (4/4)

• The same Main 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