Commissioning of the LHC with beam - CERNˆ’ No measurements at 3.5 TeV for physics beam ... −...
Transcript of Commissioning of the LHC with beam - CERNˆ’ No measurements at 3.5 TeV for physics beam ... −...
LHC
Emittance Preservation
V. Kain, G. Arduini, B. Goddard, B. J. Holzer, J. M. Jowett,
M. Meddahi, T. Mertens, F. Roncarolo, M. Schaumann,
R. Versteegen, J. Wenninger
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LHC
Introduction
Performance of collider is measured in luminosity
Will present analysis of emittance preservation from SPS extraction to
start of LHC collisions (mainly 50 ns p+)
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FfkN
L
*
2
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Small to maximize
luminostiy
Produce small in injectors → need to
keep small in the LHC
LHC
Injector Performance
o Impressive performance of injectors
− From design report (25 ns): PSB 2.5 mm – PS 3 mm – SPS 3.5 mm
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0.5 mm
0.5 mm
50 ns 2011: blow-up PSB to SPS 0.3 – 0.4 mm
Points to be addressed:
o PSB: poorer
performance ring 1,
ring 2
o PS: more blow-up with
higher intensity
o SPS: larger
measurement spread
at extraction
LHC
Analysed ~ 60 fills between mid July to mid August (50 ns, 1.2 ×1011, *=1.5 m)
Comparison of convoluted emittance from LHC luminosity with SPS wire scan for
144 bunches:
LHC Performance
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On average ~ 20 - 30 % growth between SPS flattop and
collisions
LHC
Results of analysis based on SPS/LHC wire scanners, LHC synchrotron light
monitor and luminosity of ATLAS and CMS
o Wire scanners in the SPS:
− Measured in the SPS routinely when setting up (intermediate 12 bunches + nominal 144
bunches)
− No synchronous measurement SPS-LHC
− Large spread in short time scales
o Wire scanners in the LHC:
− Measured routinely intermediate batch (12 bunches)
− Sometimes measured first nominal batch (144 bunches) – intensity limit at 2.5 × 1013 p+
− No measurements at 3.5 TeV for physics beam
Preparation of fill 2240
> 50 % spread
Measurement Limitations 2011 (1)
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LHC
Measurement Limitations 2011 (2)
o Continuous beam size measurement with Synchrotron Light Monitor (BSRT):
− Bunch-by-bunch: 3 s per bunch → 69 min (!) per ring;
− Good for relative measurement under same conditions
Absolute calibration not obvious (wire scanners are used for cross-calibration)
Cannot compare data at different energies – cannot see effect of ramp
Could not compare data for different beams/planes
o Emittance from luminosity
− Single emittance value for different beams and planes
− Not always fully optimized /not publishing the correct value
− Assumes Gaussian beams
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LHC
Improvements for 2012
o Long list of measurement improvements planned for 2012
o Instrumentation:
− Faster and better calibrated BSRTs
− Commission Beam-gas Ionization Profile Monitor (BGI)
− Pre-prepared wire scanner hardware settings for circulating intensity
− Bunch-by-bunch wire scans in the SPS
o Methods:
− Automatic wire scans during ramp
− Synchronous measurements across the accelerator complex
o Analysis and GUIs:
− Make measured betas available
− More reliable fits
− Write fit results into logging database
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LHC
Used:
o Wire scan data from SPS 12 bunches for fills 1960 - 2025
o Wire scan data from LHC beam 1 12 bunches for fills 1960 - 2025
Mismatch at injection?
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Emittances conserved at injection within measurement
accuracy
LHC/SPS = 1.07 ± 0.11 LHC/SPS = 0.99 ± 0.12
LHC
o Comparing beam 1 with beam 2 wire scans of 12 bunches
Beam 1 versus Beam 2
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Using the measured beta functions at wire scanners:
Beam 1 and beam 2 emittances are consistent
LHC1/LHC2 = 0.96 ± 0.08 LHC1/LHC2 = 1.06 ± 0.08
LHC
Growth @ 450 GeV
o Emittances are growing at injection – reasonably consistent with IBS although
slighty faster
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Fill 1897, beam 1
BSRT gated over 12 bunches Fill 2028, beam 1
BSRT gated over single bunch
Simulations of IBS, uncoupled
T. Mertens
Horizontal emittance: ~ 10 % in 20 minutes
Filling about 30 minutes → D/ 0 – 10% in H
More studies to come in 2012 to understand
faster growth of bunch length and transverse
emittance
LHC
→ Effect on bunch-by-bunch luminosity
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Specific luminosity 10 %
lower for first bunches
consistent with growth
injection A. Ryd, CMS
LHC
o Cannot use BSRT data, BGI was not commissioned
o Dedicated fills are necessary: low number of bunches to do wire scans through
the ramp
First indication: Abort gap cleaning test fill (12+12+12 bunches, 50 ns)
Used measured at injection and flattop and linear interpolation between
The Ramp
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Blow-up during the ramp: measurement indicates > 20 % all planes
H0 = 1.6 mm
V0 = 1.4 mm
H0 = 1.6 mm
V0 = 1.3 mm
LHC
The Ramp - continued
Last BI MD in 2011:
Ramp of 4 bunches per ring – different emittances
Unfortunately no useful wire scan data for beam 1
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> 20 %
emittance D ~ 1 mm emittance D ~ 0.7 mm
Relative growth different,
but…different emittances grow by the same amount
Although larger than for 12 bunch trains
LHC
Possible sources for growth @ ramp o Ramp still needs to be further optimized
− Chromaticity,…
o Effect of reduced damper gain during ramp?
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Example proton Fill 2254 (1380 bunches per beam)
BBQ hor Beam 1amplitude
damper gain hor beam 1(linear scale) HIGH @450 GeVbefore prepare for ramp
ramp (energy)
drop of dampergain
increase of damper(electronic) gain in rampTo maintain approx.same damping rate
increased BBQ amplitude= more residual beam oscillations=> potentially leading to blow-up;but signal needed for tune feedbackwhich is switched on here
rampprepare forramp
Injection plateau
Reduce damper gain
before ramp for tune
feedback →Increase of BBQ
amplitude
W. Hofle
LHC
Effect of damper gain change
Emittance evolution during fill with single bunch (loss map fill).
Evolution during injection and switch of damper gain during ramp preparation:
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Effect of damper gain change not clear (data inadequate)!
Required test 2012: change gain and wait at injection
@ 450 GeV
@ 450 GeV
3.5 min
LHC
o Emittance evolution from BSRT
o Looked at the Abort Gap Cleaning Test Fill (12 + 12 + 12 bunches per ring)
− Integration: 3 s per bunch; averaged over 12 bunches → good resolution
o Blow-up during squeeze for beam 1 H
− Took measured beta at 3.5 m and 1 m from optics team into account!!
− Was there anything different in this fill?
Emittance blow-up at 3.5 TeV
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LHC
Blow-up during the squeeze
o Looked at more fills since Evian workshop
− if emittances of all bunches similar → bunch-by-bunch scan of BSRT gives emittance
evolution in time
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Looked at several fills for end of 2011 → BSRT data consistently shows blow-up for beam 1 H
between * 5 m and 1.5 m. No obvious source.
Needs to be followed up during squeeze commissioning 2012
Bunch emittances similar
LHC
Dependence on bunch intensity?
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Approximately constant absolute growth between SPS
extraction and LHC collisions for different bunch intensities
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LHC
Ions
o Ions also experience blow-up during the ramp
o Wire scans during the ramp not possible with physics beam
Data taken during quench test on 7th of December, single bunch analyzed
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0
0.5
1
1.5
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2.5
14:15 14:20 14:25 14:30 14:35 14:40 14:45 14:50
no
rmal
ize
d E
ps.
X (μ
m.r
ad)
Time
Wire Scanner data - B1 - horizontal plane
Ramp
Blow-up of ~ 20 %
Similar to proton trains
LHC
Summary
o Excellent performance of injectors: 1.9 mm for 1.5 × 1011 p+ per bunch
o Emittances grow 20 % - 30 % from SPS extraction to LHC collisions
o LHC injection: emittances preserved within measurement accuracy
o LHC injection plateau: emittance growth apparent (~ 10 % in 20 minutes for
horizontal plane)
− Continue to minimise time at injection
− Dedicated filling cycle? Reduce time in “prepare ramp”…
o LHC ramp: blow-up in H and V > 20 % (50 ns), more for single bunches
o LHC squeeze: blow-up for beam 1 H, > 20 %
o 2012: understand and (hopefully) correct blow-up
− Expect improvements from measurements, tools and analysis
− Need to sort out during commissioning: INSTRUMENTS, blow-up @squeeze, ramp
optimization, minimize time at injection
− MDs: effect of damper gain and working point,…, IBS
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LHC
Challenge for 2012
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Understand and correct LHC
blow-up
…Make (good) outliers
routine
G. Arduini
LHC
Beta errors for emittance
o Error emittance only taking error on beta function into account.
o Plot 3 sigma error
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