1. Chromaticity decay and snapback
CERN, LBOC meeting, 17th April 2012
Chromaticity decay and snapback
Nicholas Aquilina
TE-MSC-MDT
Acknowledgements:
E. Todesco, M. Lamont, G. Papotti, EICs

2. Overview Chromaticity decay during injection Chromaticity snapback
recall of 2011 situation
2012 beam measurements
analysis and proposed model based on 2012 beam measurements
Chromaticity snapback
Chromaticity decay during flattop
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3. b3 decay and snapback We need to model and correct the b3 component
Chromaticity depends on the b3 (1 unit b3 = 45 Q’)
decay
snapback
snapback
decay
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4. Chromaticity decay during injection
Parameter
Magnetic measurements
2011 beam measurements
2011 expected
at 4 TeV
τ‘ (s)
200
1000* - d'
0.66
0.5 c'
20 units
16 units
19 units
*First data points were after 15 injection
For 2012 operation, energy has been increased from 3.5 to 4 TeV, an increase of 20% is expected in the decay amplitude
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5. 2012 beam measurements Fill no 2395 2404 2406 Date 24/03/12 25/03/12
Lattice sextupoles
constant
Spool pieces
constant (no decay, no snapback correction)
dynamic (decay and snapback correction)
First data point after injection plateau
500 s
210 s
50 s
tinj
51 minutes
90 minutes
6 hours
Previous cycle
tFT
9 hours
17 minutes
tprep
24 minutes
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6. Fill 2395 – the “naked” ramp
decay
decay
snapback
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7. Fill 2406 (with dynamic correction)
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FiDeL model was based on this measurement

8. 2012 FiDeL model as obtained from Fill 2406
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9. 2012 FiDeL model as obtained from Fill 2406
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10. 2012 FiDeL model as obtained from Fill 2406
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11. 2012 FiDeL model as obtained from Fill 2406
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12. FiDeL model based on Fill 2406 Scaled for a pre-cycle case
2012 FiDeL parameters
FiDeL model based on Fill 2406
previous IFT
6740 A
previous tFT
17 minutes
previous tprep
23 minutes c
20 units d
0.44
tau
600 s
Scaled for a pre-cycle case
6740 A
10 minutes
20 minutes
17 units*
0.44
600 s
using powering history scaling law
*Decay amplitude for pre cycle is 17 units w.r.t. 19 units expected from scaling of 2011 beam measurements
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13. 2012 FiDeL model compared with Fill 2395
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14. 2012 FiDeL model compared with Fill 2395
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15. 2012 FiDeL model compared with Fill 2395
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16. 2012 FiDeL model compared with Fill 2395
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17. Implementation of new FiDeL
The new parameters have been used in the CCC since Friday 6th April
With new parameters chromaticity is kept within 1 unit (over a duration of 20 – 30 minutes)
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18. Fill 2478
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19. Fill 2479
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20. Fill 2480
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21. Snapback analysis from Fill 2395
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22. Snapback Parameters for snapback correction
Fitting parameters as obtained for the snapback
parameter
b3 Qh’ b1
b3 Qh’ b2
b3 Qv’ b1
b3 Qv’ b2
∆b3
0.63
0.66
0.74
0.75
∆I (A) 11 9 13 12
gSB
0.057
0.072
0.061
delta chroma 28 29
Parameters for snapback correction
2012 gSB
0.06
this implies that the snapback is slower
2011 gSB
0.176
based on series magnetic measurements – but evidence from beam that correction was not perfect
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23. Snapback modelling
2012 FiDeL fit (red curve) versus beam measurements (fill 2395 – the « naked ramp »)
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24. Decay during flattop at 4 TeV
Fitting parameters as obtained for the decay during flattop (Fill 2395)
No asymmetry between the vertical and horizontal plane
parameter
Qh’ b1
Qh’ b2
Qv’ b1
Qv’ b2 c
-4.95
-4.73
5.08
4.36 d
0.43
0.51
0.65
0.49
tau 48 63
191 89 vx
-11.74
-11.54
-10.69
-10.70
Parameters for decay correction c d
tau
4.78
0.52
100 s
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25. Decay during flattop at 4 TeV
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26. Decay during flattop at 4 TeV
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27. Fill 2452
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28. Fill 2466
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29. Conclusions Decay during injection Snapback Decay during flattop
decay amplitude increased by 5% from 2011 operation
time constant is around 600 s (more data was available during the first moments of injection)
correction is in place and it is working
chromaticity decay is kept within 1 unit
Snapback
gSB is found to be smaller, therefore slower snapback
not implemented yet
Decay during flattop
a correction using the spool pieces is in place and active
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