1. LHC Emittance Measurements and Preservation
LBOC Meeting July 28, 2015
LHC Emittance Measurements and Preservation
Maria Kuhn – July 28, 2015
Many thanks to G. Baud, E. Bravin, B. Dehning, J. Emery, A. Guerrero, V. Kain, A. Langner, Y. Papaphilippou, E. Piselli, R. Tomas, G. Trad, and J. Wenninger.

2. Outline Accuracy of LHC emittance measurements with wire scanners
Wire scanner calibration
Orbit bumps
Photomultiplier saturation studies
LHC optics measurements
Wire scanner intensity thresholds
Emittance growth during the LHC cycle
Injection plateau and IBS
BSRT measurements
Comparison with emittance from luminosity
M. Kuhn - 28/07/2015

3. LHC Wire Scanner Calibration
M. Kuhn - 28/07/2015

4. Orbit Bump Calibration (1)
Using local orbit bumps to verify the wire position measurement calibration of the wire scanners
Orbit measured with BPMs and extrapolated to wire scanner
Not energy dependent
2015 results comparable to 2012 calibration results
If reproducible should change calibration factor in front-end
Also important for BSRT cross calibration
 overestimating B2V emittances by ~ 6 %
B2V1 ~ slope difference + 3 %
M. Kuhn - 28/07/2015

5. Orbit Bump Calibration (2)
Summary of results:
Mostly emittances are overestimated by LHC wire scanners
Exception: B1V2
Best would be to correct the calibration factor on the front-end
Would need another measurement to check reproducibility of results
Wire Scanner
Error on position
Error on emittance
B1H2
+ 3.6 %
+ 7.2 %
B1V2
- 2.6 %
- 5.2 %
B2H1
+ 4.5 %
+ 9 %
B2V1
+ 3.3 %
+ 6.6 %
M. Kuhn - 28/07/2015

6. Photomultiplier Saturation Studies Run 1
Photomultiplier (PM) gain and filter can have a strong influence on measured beam size
See measurements of 2012
Observed strong gain dependence at 450 GeV and 4 TeV during Run 1!
PM saturation studies at 450 GeV in 2012.
PM saturation studies at 4 TeV in 2012.
M. Kuhn - 28/07/2015

7. Photomultiplier Saturation Studies Run 2
Studies at 450 GeV with 6 bunches of different emittances:
Systematically changing gain and filter of PMs
2500 < profile amplitudes < 7500 (no ADC saturation)
Example scanner B2H1, other planes look similar
Studies at 6.5 TeV more difficult – very small range of settings
Nominal betas used
6.5 TeV
450 GeV
M. Kuhn - 28/07/2015

8. PM Saturation Studies @ 450 GeV
Beam sizes grow at 450 GeV. Effect needs to be removed:
Exponential fit of scans with reference settings to take out effect of growth at injection plateau
Average beam sizes per PM setting (combination of filter and gain)
 Beam size minus growth from exponential fit
No dependence on gain or filter at 450 GeV!
Changes during LS1:
One broken PM has been replaced and power supply schematics have been upgraded.
Also reduced PM gain dependency on intensity.
M. Kuhn - 28/07/2015

9. Resumé Wire Scanner Accuracy
Possibly “small” calibration error on position measurement
 Hence beam size measurement error of about - 3 to + 5 % depending on scanner
Results in % emittance measurement error
No PM saturation effects with single bunches
PM saturation might have to be revised for trains
NEWS: Emiliano et al will further investigate optimum working point in the lab
Fairly large statistical fluctuations of scan-to-scan emittance measurements at high energy
Limited precision on wire position measurement results in 10 – 20 % beam size spread from scan to scan
Recipe: average emittance over 3 wire scans to get reliable measurement
M. Kuhn - 28/07/2015

10. Optics measurements
M. Kuhn - 28/07/2015

11. LHC Optics Measurements
Can use results from optics measurements with the turn-by-turn phase advance method and k-modulation for:
Outstanding measurements:
K-modulation at 6.5 TeV and after the squeeze
Turn-by-turn phase advance measurements at 450 GeV (repeated) and during the ramp!
Now using measured beta functions where possible!
Measurement error < 3 %
Maximum measured beta beat is 5 % at the wire scanners
IR4
IP1/2/5/8
Injection
Ramp
Flattop
After Squeeze b*
K-modulation x
Turn-by-turn
M. Kuhn - 28/07/2015

12. Motivation: More K-Modulation Measurements
Comparison of optics measurements with k-modulation and turn-by-turn phase advance method:
Consistent results
Small errors with k-modulation
Want precision emittance measurements;
Need most accurate optics measurements.
A. Langner
M. Kuhn - 28/07/2015

13. Motivation: More K-Modulation Measurements
Recent k-modulation measurement in the LHC (23. July)
IP1 at 3 m b*
Pilot bunch without tune chirp
Very promising results,
measurement error on tune oscillation in sub-percent level!
M. Kuhn - 28/07/2015

14. Wire scanner Intensity limits
M. Kuhn - 28/07/2015

15. Wire Scanner Intensity Limits
Limits so far:
Maximum 2.7 x 1013 protons per beam at 450 GeV
Corresponds to ~ 240 nominal bunches
Maximum 2.3 x 1011 protons per beam at 6.5 TeV
Corresponds to ~ 2 nominal bunches
New Limit: maximum 1.6 x 1012 protons per beam at 6.5 TeV
Corresponds to ~ nominal bunches
To be implemented soon
Run 2: rely even more on an operational BSRT
M. Kuhn - 28/07/2015

16. Emittance growth during the cycle
M. Kuhn - 28/07/2015

17. Emittance Evolution during the Cycle
Require: measured beta function through the entire cycle
Fill 3954 (July 4, 2015)
squeeze
Partly unphysical emittance evolution!
From Run 1 experience:
optics
M. Kuhn - 28/07/2015

18. Emittance Growth in Numbers: Fill 3954
Average emittance of 4 scans
SPS emittance at extraction: H = 2.16 mm, V = 2.0 mm
Adding additional 10 % uncertainty on wire scanner position measurement
Fill 3954, Bunch 1
Injection
Collision
De [mm]
De/e [%]
B1H
e [mm]
2.66 ± 0.14
2.95 ± 0.16
0.29 ± 0.21
11 ± 18
B1V
2.40 ± 0.11
2.80 ± 0.13
0.40 ± 0.17
17 ± 17
B2H
2.12 ± 0.08
2.14 ± 0.18
0.02 ± 0.20
1 ± 19
B2V
2.85 ± 0.07
3.14 ± 0.32
0.29 ± 0.32
10 ± 21
M. Kuhn - 28/07/2015

19. Emittance Growth vs. IBS – Beam 2
IBS simulations with MADX IBS module
Including: dispersion, radiation effects, measured bunch length through cycle, measured initial intensity and emittance
Bunch 1
IBS could fit evolution in horizontal plane.
Need optics through the ramp for vertical plane.
Nominal betas used
Bunch 2
M. Kuhn - 28/07/2015

20. Emittance Growth vs. IBS – Beam 1
Beam 1 horizontal grows more strongly than IBS suggests
Origin of growth in the vertical plane?
Nominal betas used
Bunch 1
Bunch 2
M. Kuhn - 28/07/2015

21. Beam Parameters Fill 3954
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22. Measurements Reproducible? (1)
Fill 3809 (June 1, 2015):
With measured beta
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23. Beam Parameters Fill 3809
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24. Measurements Reproducible? (2)
Fill 3939 (June 30, 2015):
With measured beta
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25. Bunch length reduction could explain emittance evolution
Beam Parameters Fill 3939
Bunch length reduction could explain emittance evolution
M. Kuhn - 28/07/2015

26. Measurements Reproducible? (3)
A more recent Fill 4039 (July 24, 2015):
With measured beta
M. Kuhn - 28/07/2015

27. Beam Parameters Fill 4039
M. Kuhn - 28/07/2015

28. Measurements Reproducible? (4)
A more recent Fill 4040 (July 24, 2015):
With measured beta
M. Kuhn - 28/07/2015

29. Beam Parameters Fill 4040
M. Kuhn - 28/07/2015

30. Average Absolute Emittance Growth
To better visualize the average growth overlaying all ramps
B1H: ~ 0.2 mm blow-up, reproducible fluctuation during the ramp.
B1V and B2H no blow-up.
B2V: ~ 0.2 mm blow-up, large error bars from in-out-scan deviations.
M. Kuhn - 28/07/2015

31. Average Relative Emittance Growth
B1H: ~ 10 % blow-up, reproducible fluctuation during the ramp.
B1V and B2H no blow-up.
B2V: ~ 10 % blow-up, large error bars from in-out-scan deviations.
M. Kuhn - 28/07/2015

32. BSRT Measurements at 450 GeV
Example Fill 3808, June 1, 2015
BSRT measurement
Wire scanner measurement
Nominal betas used
6 nominal bunches with different beam sizes.
M. Kuhn - 28/07/2015

33. BSRT Measurements at 450 GeV
A more recent Fill 4034 (July 22, 2015):
BSRT and wire scanner measure large growth in the vertical planes.
BSRT smaller scatter than wire scanner.
M. Kuhn - 28/07/2015

34. BSRT Measurements at 6.5 TeV
Stable beams Fill 3996
July 14, 2015
476 bunches
~ 3.5 hours in collision
Average emittance growth from BSRT and ATLAS/CMS luminosity
ATLAS
Measured betas used (tbt)
CMS
M. Kuhn - 28/07/2015

35. Emittance from Luminosity
Comparison of emittance from wire scans and luminosity
Fill 3954, one bunch in collision
Average of 4 measurements with similar timestamps
According to experts ATLAS/CMS luminosity low by ~10 %, assume error on luminosity ±10 %
5 % error on crossing angle and 10 % error on b* should be added
Preliminary: ATLAS and wire scanner results agree within errors!
Better than during Run 1
Need measured optics for final comparison
Fill 3954, Bunch 1
Injection
Collision
Growth
WS e [mm]
2.51 ± 0.10
2.75 ± 0.20
0.24
10 %
ATLAS e [mm]
2.97 ± 0.36
0.46
19 %
CMS e [mm]
4.01 ± 0.47
1.5
64 %
M. Kuhn - 28/07/2015

36. Comparison SPS – LHC - Luminosity
Convoluted emittance (H + V) of SPS wire scans at 450 GeV
Emittance per plane of LHC wire scans at 450 GeV
Lacking scans in the LHC!
Convoluted emittance from ATLAS and CMS luminosity
Results seem to indicate less growth through the cycle than in
Nominal betas used
M. Kuhn - 28/07/2015

37. Reminder: 2012 Emittance Blow-up
Overall average transverse normalized emittance blow-up through the LHC cycle:
~ 0.4 – 0.9 mm from injection into the LHC to start of collision (convoluted e) for the first injected batch of 144 bunches per beam
M. Kuhn - 28/07/2015

38. Summary
Good progress with understanding the wire scanner emittance measurements
In general seem to be in a better shape than during Run 1
More optics measurements are required
And upload into DB
For some fills the emittances seem to be growing in all planes through the cycle and mainly during injection plateau and ramp
Origins:
IBS ?
With the still not fully calibrated luminosity data: better agreement between wire scans and emittance from luminosity
Luminosity measurements (ATLAS) over the last weeks indicate small growth through the cycle
M. Kuhn - 28/07/2015

39. To Do Optics measurements in point 4 b* measurements
Repeat turn-by-turn phase advance measurements at 450 GeV
Turn-by-turn phase advance measurements during the ramp!
K-modulation at 6.5 TeV and after the squeeze
b* measurements
Repeat orbit bump scans – to check reproducibility
Understand additional sources of growth!
LHC OP please don’t forget to do wire scans at injection in all planes.
Thank you!
M. Kuhn - 28/07/2015