optics measurements needed at top energy

1
optics measurements needed at top energy
thanks to Ralph Assmann, Rama Calaga, Stephane
Fartoukh, Massimo Giovannozzi, Per Hagen, Rhodri
Jones, John Jowett, Verena Kain, Jean-Pierre
Koutchouk, Stefano Redaelli, Stephane
Sanfilippo, Frank Schmidt, Ralph Steinhagen
2
basic considerations

• nominal optics similar to 450 GeV optics
• IR2 optics magnet errors different
• BPM offsets expected to change by less than 50 mm
• apertures already checked at 450 GeV/c
  normalized aperture 4x larger at 7 TeV
• damage and quench levels reduced
• it takes much longer (1 h) to return after beam
  loss

3
critical beam intensities
4
two extreme cases

• no need for detailed studies if optics hardly
  changes compared with injection
• but if optics strongly varies between 450 GeV/c
  and 7 TeV we may need to measure and correct
  optics already on the ramp

5
orbit change

• 50 units change in transfer function of arc
  dipoles
• about 700 units change for D1, D2, D3 and D4!
• - warm magnets saturation
• - cold magnets very low excitation at
  injection
• assume 1 transfer function accuracy
• ? 7x10-4 error in D1 to D4 ? 50 mm change each
• exact local correction not foreseen for LSS
• ? orbit different if Q4 correctors not touched
• 2 beams 2.5 mm separation bump reduced gt5 TeV
• change in rf frequency circumference
• orbit feedback will correct most of the changes

6
Animesh Jain, December 4, 2002 Stephane Fartoukh
7
7
rf frequency swing
Dfrf1 kHz ? Dx10 mm
dominant error may come from BPMs/pick-ups
p
208Pb82
radial loop gets input from orbit feedback
John Jowett
may expect 20-100 mm error for pilot
8
energy reconstruction error as function of BPM
noise for d10-5
Ralph Steinhagen
9
optics change

• tune change DQ0.1 (F.Z., Chamonix 2006)
• chromaticity change DQ320 units
• peak beta beat change up to 40 peak?
• - Db2 /-2.5 units (apertures 1 and 2), 7-8
  peak
• - Db3 7 units (spool-piece excitation 0.6 mm
  rms spool misalignment), 6 peak
• - WISE model 13-18 w/o geometric effects
• IP1-5 phase advance change Df0.14 x 2p
• coupling change Dk0.05 (Chamonix 2006)
• IR2 optics change (injection ? pre-squeeze)
• tune and coupling feedback likely to be active
• chromaticity feedback less probable

10
illustration b2 evolution with MB current for
the two apertures
Courtesy of N. Sammut
S. Fartoukh and M. Giovannozzi, FQWG 08/03/2005
11
illustration b3 evolution with MB current

• change of sign!
• 4 units (inj.)
• ? 3 units (7 TeV)

L. Bottura, 13.01.2004
12
expected b beating (av. 2s)
preliminary WISE results from Per Hagen
rough guesses
larger error at 7 TeV than at injection due to
uncertainty
13
IR2 injection and pre-collision optics

• Injection optics has to have phase advance of 90
  degrees between MKI and TDI, and a vertical phase
  advance of 360-20 degrees and 36020 degrees
  between the TDI and the two auxilliary
  collimators
• Not possible at 7 TeV without exceeding
  quadrupole strength limits
• Pre-collision optics has same b10 m but
  different phase advances and does not respect
  injection condition
• Need to change optics in the ramp, called
  pre-squeeze here for want of a better word
• At which energy ?
• Following examples for V6.5
• Need to be updated, indicative only for now

John Jowett
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Horizontal optics in pre-squeeze
Beam 1/2 thick/thin, Injection/Pre-collisionRed
/Blue
John Jowett
15
Vertical optics in pre-squeeze
Beam 1/2 thick/thin, Injection/Pre-collisionRed
/Blue
John Jowett
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Horizontal phase in pre-squeeze
Beam 1/2 thick/thin, Injection/Pre-collisionRed
/Blue
John Jowett
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Vertical phase in pre-squeeze
Beam 1/2 thick/thin, Injection/Pre-collisionRed
/Blue
John Jowett
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7 TeV measurements corrections

• tunes (excite beam) do we go to collision
  tunes?
• or tunecoupling feedback
• chromaticity (tune radial steering, or
  head-tail monitor, or rf phase modulation)
• orbit (stabilized with feedback?)
• coupling (minimize CTA or coupling line in tune
  spectrum) using orthogonal knobs or
  tunecoupling feedback
• beta function dispersion
• IP1-5 phase advance
• ORM measurements not needed for feedback
• b3 correction check? (likely not needed at 7
  TeV beam smaller already done at injection)

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dispersion

• aperture permits up to Dd /- 3.5x10-3
• tighten momentum collimators to be safe
• against optics errors, e.g. to 1.0 or 1.7x10-3?
• 20 mm BPM resolution
• for pilot in orbit mode

Stephane Fartoukh
20
beta function / linear optics

• phase beating from turn-by-turn data
• how to excite the beam at 7 TeV
• - aperture kicker 1.6s max. (LTC 15 June
  2005)
• - tune kicker 0.7s maximum
• - ac dipole available at this stage?
• machine protection issues?
• repeated kicks of same bunch problematic,
• - several pilots kicking single bunches?
• collimator scans
• and/or K modulation in collimator straights?

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resolution of turn-by-turn data

• rms BPM resolution for pilot 200 mm 1s
• phase error (Rogelio Rama)
• correction works for Dfpeak3Dfrmslt1o (Rama)
• example 400 turns with 1.6s kick Dfpeak7.6o

limited kicker strength BPM noise ? 7-TeV
optics measurements with pilot bunch extremely
difficult
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Rama Calaga
20 s kick, 200 mm (1s) BPM noise, 400 turns, no
decoherence
20s kick and 400 turns or 6.3s kick and 4000
turns give 0.5o peak phase error
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Rogelio Tomas Rama Calaga
0.5 degree peak phase error
with 0.5o peak phase error, peak b beat can be
corrected for all seeds
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Rogelio Tomas Rama Calaga
1 degree peak phase error
with 1o peak phase error, peak b beat cannot be
corrected for many seeds
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proposed procedure

• priority machine protection
• close collimators by factor 2 (during ramp)
• measure local beam sizes at all collimators with
  collimator scans to roughly check optics
  re-adjust, so that secondaries indeed are
  secondaries
• if inferred beating is in safe range (lt100)
  raise bunch intensity to gt 3x1010 for b
  measurements with aperture kicker or ac dipole ?
  50 mm BPM noise ? Dfpeak1.9o
• or (if resolution insufficient) use ac dipole!?

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SR monitors undulator light ? dipole
light   Main change light intensity. No change
in measured beam size expected. Current
plan  Fine tune telescopes at 450 GeV 7
TeV. Use 450-GeV settings up the ramp. Requires
time with "stable" circulating beam. Most can be
done parasitically if emittance blow-up limited.
Cross calibration with wire scanners required at
both energies. 
Rhodri Jones
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D1/D2 transfer function

• recall
• D1-D2 transfer function errors can have a
  significant effect on closed orbit during squeeze
  (10 units ? 3s orbit change at triplet)
• local correction requires careful analysis and
  distinction between
• D1/D2 transfer function errors
• Triplet alignment errors
• Triplet gradient errors with crossing-angle bump
  offsets

O. Bruning, Chamonix XII F. Zimmermann,
LHCCWG10
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IR1 layout
D2
D2
D1
D1
common BPMs
separate BPM
separate BPM
BPM distances to IP 21.35, 31.53, 58.32, 151.14 m
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D1/D2 suggestion

• assume alignment was done at injection (i.e. BPM
  readings for straight reference line identified
  quad offsets determined)
• main error after ramp is due to D1 D2 TF
  uncertainty (perform Q1/Q2/Q3 K modulation to
  verify quad misalignments?!)
• suggested approach correct incoming beams for 0
  orbit and slope upstream of D2, then use D1 and
  D2 strengths to bring both beams onto the same
  orbit at 7 TeV

O. Bruning, Chamonix XII F. Zimmermann,
LHCCWG10
30
D1/D2 problems (LHCCWG10)

• low-b quads misalignment of mechanical
  magnetic axes 0.1-0.2 mm in x, 0.5 mm in y beam
  needs to be steered through the mechanical
  center!
• BPM offsets may cause error of up to 5 mrad (?),
  compared with total deflection angle of 1.5 mrad
  this is larger than desired precision of 3x10-4
• BPMs might have different offsets for beam1 and
  beam2 (?)

S. Fartoukh, M. Giovannozzi, F.Z.
31
longitudinal blow up?

• longitudinal emittance at injection to the LHC
  will be 0.6 eVs for nominal intensity
• design emittance at 7 TeV 2.5 eVs
• emittance blow up at the middle of the ramp (or
  at 7 TeV?) to combat IBS rates and to stabilize
  the beam longitudinally
• if instability occurs, may need to implement the
  blow up in the commissioning

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draft 7 TeV procedure
close b collimators by factor 2 on ramp
measure b phase Df IP1-5 aperture kicker and
higher intensity bunch or ac dipole
orbit, tune coupling (pre-)corrected by
feedback manual adjustment if needed
correct optics? (and iterate if correction is
needed)
(pre-)correct chromaticity (decoherence, radial
steering)
ORM for feedback calibration?
collimator beam-size scans, and adjustments
settle D1/D2 (D3/D4) transfer function, prior to
squeeze
measure dispersion Q with radial steering
e blow up?
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summary

• optics errors at 7 TeV can be significant and are
  different from 450 GeV/c orbit optics also
  differ
• we should close collimators on ramp and perform
  collimator scans
• feedback may take care of (most of ) orbit, tune
  and coupling correction
• limited aperture kicker 200-mm BPM noise do not
  allow for clean optics measurements with pilot
  bunch need higher intensity and/or ac dipole
• dispersion Q can be measured by radial
  steering
• other issues D1/D2 (D3/D4) transfer function,
  feedback calibration, beam parameters