Preliminary results from SPS collimator MDs

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Preliminary results from SPS collimator MDs

• LTC 27.10.04
• R. Assmann for the collimation team

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People involved in collimator design/construction/
testing

• This has been an outstanding team effort over the
  last 12 months!
• Work by
• O. Aberle, G. Arduini, R. Assmann, A.
  Bertarelli, T. Bohl, L. Bruno, H. Burkhardt, S.
  Calatroni, F. Caspers, E. Chiaveri, B. Dehning,
  A. Ferrari, E.B. Holzer, J.B. Jeanneret, L.
  Jensen, M. Jimenez, R. Jones, M. Jonker, T.
  Kroyer, M. Lamont, M. Mayer, E. Metral, R.
  Perret, L. Ponce, S. Redaelli, G.
  Robert-Demolaize, S. Roesler, F. Ruggiero, D.
  Schulte, H. Tsutsui, P. Sievers, R. Steinhagen,
  V. Vlachoudis, L. Vos, J. Wenninger, F.
  Zimmermann, ...
• Not including work on LHC collimation design!

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Goals of SPS Tests

• SPS ring
• Show that the LHC prototype collimator has the
  required functionality and properties (mechanical
  movements, tolerances, impedance, vacuum, loss
  maps, ).
• 2. TT40 extractionShow that an LHC collimator
  jaw survives its expected maximum beam load
  without damage to jaw material nor metallic
  support nor cooling circuit (leak).
• Crucial project milestone Mechanical
  engineering(installation 18Aug04) Tolerances
  Prototype production Control and
  motorization Set-up of a single LHC
  collimator with beam

(APC April 04)
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Beam conditions

• Beams prepared by G. Arduini, J. Wenninger and OP
  team
• Low intensity MD Monday Oct 11th Bunch
  population 1.1e11 p Number of
  bunches 1-16 Beam energy 270
  GeV Emittance 1 mm H beam size at
  collimator 0.4 mm Beam orbit stability 10
  mm
• High intensity MD Monday Oct 18th Bunch
  population 1.1e11 p Number of
  bunches 288 Beam energy 270
  GeV Emittance 3.75 mm H beam size at
  collimator 0.7 mm
• Robustness test Monday Oct 25th

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Configuration
BLM team
Collimation team Collimator in P5 of SPS BLM
team 8 downstream BLMs Together 1 Hz DAQ and
plotting in control room
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Different issues

• Functionality and basic collimator control
• Set-up and beam-based alignment of jaw (includes
  BLM diagnostics)
• Halo dynamics
• Impedance and trapped modes
• Heating of collimator
• Vacuum and e-cloud (outgassing)
• Effects on BPMs and orbit feedback

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1. Functionality Mechanical movement and
tolerances

• Collimators moved in a FULLY operational way no
  limits or unexpected difficulties encountered!
• Closest gap of 1 mm achieved with circulating
  beam! Mechanical tolerances and angular alignment
  at the 100 mm level!
• ? Much smaller gaps than required in the 7 TeV
  LHC have been achieved with the LHC collimator
  prototype and circulating beam!
• Knowledge of full collimator gap (excluding human
  math errors) Absolute 100 mm
  Reproducibility 20 mm Anti-collision
  settings 1.188/1.146/1.160 mm
• ? Gap known to 100 mm with excellent
  reproducibility (20 mm) over 16 h (motor setting
  reproducibility)!
• ? Some sensors useful others less useful Reduce
  number of sensors!

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2a. Set-up and beam-based alignment of jaw
Gap width
Gap center
First basic set-up (100 mm accuracy) within 50
min!
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2b. Set-up and BBA Typical BLM signal for move
of jaw
Observation of BLM signal tails Up to 10-20
seconds in length BLM team Many measurements ?
Beam related true signal!
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2c. Studies of BLM systematics
Time
L. Ponce et al
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2d. Set-up and BBA High precision set-up

• LHC requirement Center gap around beam with 25
  mm accuracy for nominal b (beam-based
  alignment).
• SPS beam 120 h beam lifetime (de-bunched
  beam?) orbit stable to 5 mm ? ideal tuning
  conditions
• Observation Beam-based alignment ... ... to
  100 mm is OK! ... to 50 mm is
  difficult! ... to 10-20 mm is impossible?
• Understand effect to improve beam-based set-up!

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3a. Halo dynamics Re-shaping
Collimator jaw
Beam distribution
After 10-20 seconds New stable shape

• Problem Re-shaping of beam with collimator in!?
• Edge 1 jaw 1 creates sharp edge and stays in!
• Rectangular distribution close to edge unstable!
• Particles in sharp edge diffuse and are lost!
• No sharp edge for precise alignment of edge 2 jaw
  1 or jaw 2!
• Similar effects observed in ISR, SPS, ...

No sharp edges!
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3b. Measurement of repopulation rate jaw
positions
Right jaw
Dump beam on collimator
2.7 mm 6.5 s
Left jaw
G. Robert-Demolaize et al

• Move from 7.7 mm ( 19s) back and forth to 2.7
  mm ( 6.5s).
• Wait different times in between.
• Observe beam loss.

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3c. Measurement of repopulation rate - BLM signals
G. Robert-Demolaize et al
DC coll at 6.5 s
DC coll at 19 s
G. Robert-Demolaize et al
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3d. Measurement of repopulation rate low
intensity analysis
G. Robert-Demolaize et al
? Shows how much beam diffuses out of sharp edge
versus time!
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3e. Measurement of repopulation rate high
intensity analysis
G. Robert-Demolaize et al
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3f. Beam distribution close to the edge after 30
seconds
1 mm
50 mm
G. Robert-Demolaize et al
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3g. DC beam loss versus collimation depth
10 s
20 s
S. Redaelli et al
More beam losses with collimator jaws further in
Enhanced diffusion rate?
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4a. Impedance and trapped modes

• Impedance is a limitation for the LHC
  collimators.
• Impedance depends on collimator gap.
• Measurement is simplified as impedance can
  controlled through gap.
• Different measurements triedTune shifts, orbit
  kicks, trapped modes, growth rates, ...

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Collimator MDs 2 (some) BBQ results
LARGE gap
SMALL gap

• Collimator cycled between
• 51 mm and 3.86 mm (5h04)
• 51 mm and 2.86 mm (5h35)
• 51 mm and 2.46 mm (5h43)
• 51 mm and 2.06 mm (5h50)
• 51 mm and 1.86 mm (5h58)

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Collimator MDs 2 (some) BBQ results
BBQ system
245 MHz system
245 MHz system confirms data (F. Caspers/T.
Kroyer) Also Standard tune measurments (H.
Burkhardt)

• Collimator cycled (at ca 4h33) between the gap of
  51 mm and 2 mm.
• Tune frequency was changing by 10 Hz, i.e.
  2.3?10-4 (? frev)

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Collimator MDs 2 (some) BBQ results
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4b Trapped modes

• Collimators were equipped with RF pickups to
  measure trapped modes.
• Measurement with and without beam by F. Caspers
  and T. Kroyer.

• Observation
• There are trapped modes.
• They are excited by beam.
• No effect on collimator temperature or beam
  stability observed.
• Detailed analysis required.
• Fritz has won a bottle of Champaign and 20
  straws...

F. Caspers T. Kroyer
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5. Heating of collimator (high intensity)
No sign of problematic heating... Maximum
increase 10 deg for closing gaps quickly!? No
over- or under-design of collimator cooling...
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6. Vacuum and e-cloud

• No sign of vacuum pressure increase.
• No sign of local e-cloud at the collimator.

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7. Effects on BPMs and orbit feedback

• Scraping of up to 5e12 protons at 270 GeV.
• No effect observed on downstream BPMs and overall
  orbit feedback (R. Steinhagen J. Wenninger).
• Orbit feedback stabilized to 10 mm orbit drifts.
  However, increased noise observed on BLM
  (equivalent to 10 mm jaw steps and consistent
  with BPM noise).

FB ON
FB ON
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Robustness test in TT40

• Beam accident just before sending first beam to
  collimator.
• Septum/power supply problem (? Jan). Safe
  extraction of nominal LHC beam for another trial?
• Collimator in TT40 saw no beam (except showers
  from accident).
• Concerns about collimator safety
• Lots of calculations and lab measurements ? We
  are convinced it will survive without damage
  (both jaw and water cooling circuit)!
• We had vacuum/radiation protection/cooling water
  experts on stand-by in the control room for an
  eventually needed emergency intervention.
• Was not needed for the collimator but was helpful
  for fast recovery of the SPS.
• Try again the robustness testWe are convinced
  there will be no problem. However, there can
  always be a bad surprise
• ? Test now with SPS beam so we can still
  correct problems!
• ? If there is an unexpected problem it would be
  a real mess to only find it in the LHC!

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Preliminary conclusions

• Collimator design has been validated successfully
  in beam operation
• Fully functional device with no significant
  hardware problems.
• Real world performance provides knowledge base
  for possible savings? Reduce number of sensors
  to required minimal level (save budget).
• Small gaps (smaller than in LHC) established with
  good accuracy and tolerances (circulating beam
  for 1 mm gap).
• Impedance measurements confirm predictions? Phas
  e 2 collimators are required for above 50 of
  nominal intensity (resources???).
• Set-up and beam-based jaw alignment worked at
  intermediate precision? Need to advance
  quantitative understanding of halo dynamics for
  b below 1m with high precision jaw set-up
  (resources???).
• Collimator design does not require any
  modifications (except maybe some material for
  absorbing trapped modes, if found dangerous).
• Collimators will now be produced and we are
  convinced that phase 1 collimators will be robust
  and powerful tools for the LHC (robustness test
  should still be done)!

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Gap center and width versus time
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