1. Joint LIU / HLLHC day -15 October 2015
25 ns operation and MDs in the LHC – findings and consequences for the LIU and HL-LHC?
G. Iadarola with input from: F. Antoniou, A. Apollonio, G. Arduini, H. Bartosik, N. Biancacci, L. Carver, S. Claudet, V. Kain. M. Kuhn, G.Ferlin, K. Li, L. Mether, E. Metral, A. Romano,
G. Rumolo, B. Salvant, M. Schenk, L. Tavian, G. Trad, Y. Papaphilippou, G. Papotti
Joint LIU / HLLHC day -15 October 2015

2. Outline Introduction e-cloud limitations with 25 ns beams:
Scrubbing runs with 25 ns beams
Intensity ramp-up with 25 ns at 6.5 TeV
Other limitations:
UFOs
Transverse instabilities at high energy
Achieved performance:
Availability
Losses and emittance blow-up (injection to collisions)
Beam evolution in stable beams
Luminosity observations
Summary and first implications for HL-LHC

3. Outline Introduction e-cloud limitations with 25 ns beams:
Scrubbing runs with 25 ns beams
Intensity ramp-up with 25 ns at 6.5 TeV
Other limitations:
UFOs
Transverse instabilities at high energy
Achieved performance:
Availability
Losses and emittance blow-up (injection to collisions)
Beam evolution in stable beams
Luminosity observations
Summary and first implications for HL-LHC

4. Recommissioning with beam
Introduction
The main goals of the 2015 run were to re-commission the machine after the Long Shutdown 1 (LS1) and explore operation at 6.5 TeV with 25 ns beams
Expected challenges from e-cloud effects (as anticipated from 25 ns pilot run in 2012)
 Plenty of time allocated for scrubbing
Decided to operate with ~nominal bunch parameters (injecting 1.1 x 1011 ppb in 2.5 um), relaxed machine configuration: b* = 80 cm in IP1 and IP5, relatively large crossing angles:
No blocking issues so far from beam induced heating (also thanks to mitigations put in place during LS1), or from beam-beam effects
5 April
24 June
5 July
Recommissioning with beam
(low intensity, 6.5 TeV)
Scrubbing Run 1
(450 GeV)
Intensity Ramp-up (*)
(50 ns – 6.5 TeV)
TS1
MD1
25 July
10 August
7 September
Scrubbing Run 2
(450 GeV)
Intensity Ramp-up (*)
(25 ns – 6.5 TeV)
Intensity Ramp-up
(25 ns – 6.5 TeV)
MD2 +TS2
(*) Limited to ~450b. by radiation induced faults in QPS electronic boards (fixed during TS2)

5. Outline Introduction e-cloud limitations with 25 ns beams:
Scrubbing runs with 25 ns beams
Intensity ramp-up with 25 ns at 6.5 TeV
Other limitations:
UFOs
Transverse instabilities at high energy
Achieved performance:
Availability
Losses and emittance blow-up (injection to collisions)
Beam evolution in stable beams
Luminosity observations
Summary and first implications for HL-LHC

6. Scrubbing for 25 ns operation
Scrubbing Run 1: 24/6 – 2/7
Scrubbing Run 2: 24/6 – 2/7
50 ns
L. Mether
After LS1 the SEY was practically reset to what was observed at the beginning of Run 1
During (1+2) weeks of scrubbing, regularly filling the machine with up to ~2500b. with 25 ns spacing
Main limitations: vacuum spikes at the TDI8, pressure rise in the MKIs and time required by cryogenics to handle transients on beam screen temperatures (should be better already in 2016)
Reduction of the SEY could be inferred from heat load measurements and confirmed by steadily improving beam quality

7. Scrubbing for 25 ns operation
Scrubbing Run 1: 24/6 – 2/7
Scrubbing Run 2: 24/6 – 2/7
50 ns
L. Mether
After LS1 the SEY was practically reset to what was observed at the beginning of Run 1
During (1+2) weeks of scrubbing, regularly filling the machine with up to ~2500b. with 25 ns spacing
Main limitations: vacuum spikes at the TDI8, pressure rise in the MKIs and time required by cryogenics to handle transients on beam screen temperatures (should be better already in 2016)
Reduction of the SEY could be inferred from heat load measurements and confirmed by steadily improving beam quality

8. Intensity ramp-up with 25 ns beams: beam dynamics
Scrubbing Run provided sufficient mitigation against beam degradation at 450 GeV but full suppression of the e-cloud was not achieved
During the physics intensity ramp-up we had to learn how to run the machine in the presence of the e-cloud
Tricky to ensure beam stability at 450 GeV: need for high chromaticity and octupoles settings and for full transverse damper performance
Slightly changed working point at injection to better accommodate large tune footprint from Q’, octupoles and e-cloud
Octupole knob at -1.5
Q’=15/20, 5 x 1011 e/m3
Q’v=15
Qv=.305
Q’v=15
Qv=.300
Q’v=10
Qv=.305
(.28, .31)
(.275, .295)
A. Romano

9. Intensity ramp-up with 25 ns beams: heat loads
Even with relatively low number of bunches, strong transients of the beam screen temperatures were observed, leading to loss of cryo-conditions (during injection, ramp and at the beam dump):
Intensity ramp-up performed in “mini-steps” for fine tuning of cryo-regulations
During the first stages, injection speed often decreased to control beam screen temperatures
After careful analysis, it was decided to modify the interlock rules in order to allow for larger temperature excursion  since then practically no loss of cryo-conditions during injection and ramp
SR1
SR2
TS2
50 ns
25ns

10. Intensity ramp-up with 25 ns beams: heat loads
Around ~1450b. we started approaching the limit of the available cooling capacity on the arc beams screens
Increased longitudinal emittance blow-up on the ramp (bunch length target from 1.2 ns to 1.35 ns) and optimized filling scheme to gain additional margin
Since last weekend running with 1825b. per ring:
Sector 12 and 23 close to the margin of available cryo cooling capacity (some margin still available for the others - important to understand this difference)
Cryo limit

11. Dipoles of the instrumented half-cells
Intensity ramp-up with 25 ns beams: heat loads
Scrubbing observed with physics fills at 6.5 TeV
Hopefully gaining margin to further increase the number of bunches
Scrubbing “memory” kept while running with 25 ns beams - deconditioning was observed after few weeks of low e-cloud operation
Dipoles of the instrumented half-cells

12. Outline Introduction e-cloud limitations with 25 ns beams:
Scrubbing runs with 25 ns beams
Intensity ramp-up with 25 ns at 6.5 TeV
Other limitations:
UFOs
Transverse instabilities at high energy
Achieved performance:
Availability
Losses and emittance blow-up (injection to collisions)
Beam evolution in stable beams
Luminosity observations
Summary and first implications for HL-LHC

13. UFOs UFO events observed quite often during operation at 6.5 TeV
Over the last month (operating with >700b.) ~24% of the fills which reached stable beams were dumped by a UFO (9 dumps / 38 fills) – One UFO induced quench
Conditioning is observed on the UFO rate in spite of the increasing number of bunches
BLM thresholds being optimize to find a good compromise between availability and quench protection
SR1
SR2
TS2
Low
intensity
25 ns
50 ns
450b.
1800 b.
G. Papotti

14. Transverse instabilities at high energy
Critical in 2012 (with brighter and more intense bunches): instabilities observed at the end of the squeeze for many of the physics fills
Situation easier in 2015 (with lower bunch intensity/brightness and larger b*):
Running with large chromaticity, octupoles and transverse damper gain
 no instability observed along the cycle for physics fills with up to 1825b.
Extensive work during commissioning and MD time to study stability limits versus Q’ and octupoles settings
Model (ADT d-time:100 turns)
25 ns beams: stability thresholds 5x larger w.r.t. to single bunches
 probably effect of the e-cloud
N. Biancacci, L. Carver, E. Metral

15. Outline Introduction e-cloud limitations with 25 ns beams:
Scrubbing runs with 25 ns beams
Intensity ramp-up with 25 ns at 6.5 TeV
Other limitations:
UFOs
Transverse instabilities at high energy
Achieved performance:
Availability
Losses and emittance blow-up (injection to collisions)
Beam evolution in stable beams
Luminosity observations
Summary and first implications for HL-LHC

16. Luminosity evolution
SR1
SR2
TS2
50 ns
25ns
25ns

17. Machine availability
Period after TS2 (focused on intensity ramp-up with 25 ns beams, 4501800 bunches)
Stable beam fraction 35% - remarkably close to 2012 performance (36.5%)
~ 44 h quench recovery
(“child fault”)
A. Apollonio, Availability Working Group

18. Intensity and emittance preservation (injection to collisions)
HL-LHC loss budget
From injectors
F. Antoniou
Excellent transmission along the cycle losses well within the 5% HL-LHC budget
3-3.5 um convoluted emittances at beginning of collisions (similar values in H and V)
Injected emittances of the order to 2.5 um (see H. Bartosik)
~35% emittance blow-up, against HL-LHC allocated budget of 20%
Much less blow-up observed with single bunches
 Possible culprits: e-cloud, high Q’ and octupoles, ADT noise (to be further investigated)

19. Beam evolution in “stable beams”
Overall good emittance preservation in “stable beams”
Slow damping observed in the vertical plane
Some additional blow up w.r.t. expectations from IBS + Synchrotron Radiation (SR)
Horizontal
Vertical
Expected (IBS+SR)
 Strong damping in longitudinal
 Burn-off consistent with model
Bunch length [ns]
Bunch intensity [p+]
F. Antoniou

20. Integrated luminosity (12h)
Luminosity observations
2012 (4 TeV, 50 ns) - Fill 3249
2015 (6.5 TeV 25 ns) – Fill 4485
2015 configuration with low bunch intensity, low brightness and relaxed b* results in:
Low peak luminosity
Long luminosity lifetime
Integrated luminosity over a “typical” fill is very similar to what we used to get in 2012
2012 (4 TeV)
50 ns, 1380 b.
1.7e11 ppb, 1.6 um (inj.)
b* = 60 cm
2015 (6.5 TeV)
25 ns, 1825 b.
1.1e11 ppb, 2.5 um (inj.)
b* = 80 cm
Peak luminosity
7200 (ub.s)-1
4500 (ub.s)-1
Integrated luminosity (12h)
0.18 fb-1
0.17 fb-1

21. Outline Introduction e-cloud limitations with 25 ns beams:
Scrubbing runs with 25 ns beams
Intensity ramp-up with 25 ns at 6.5 TeV
Other limitations:
UFOs
Transverse instabilities at high energy
Achieved performance:
Availability
Losses and emittance blow-up (injection to collisions)
Beam evolution in stable beams
Luminosity observations
Summary and first implications for HL-LHC

22. Summary and first implications for HL-LHC
SEY in the arcs was found reset to beginning of Run 1
~3 weeks of scrubbing with 25 ns beams provided sufficient mitigation against beam degradation at 450 GeV but full suppression of the e-cloud was not achieved
Intensity ramp-up dominated by impact of e-cloud effects:
Beam degradation under control with strong Q’, octupoles, and high damper gain
Cryogenics had to learn how to deal with high heat loads
Heat load in Sector 12 presently limits to ~1800 bunches/ring but scrubbing is progressing with the physics fills
UFOs still dump ~24 % of the physics fills, but UFO rate quickly improving
Besides e-cloud, operation is quite smooth:
machine availability over the last months is comparable to 2012 statistics
despite having still lower peak luminosity w.r.t. 2012, we get similar integrated values per fill thanks to better luminosity lifetime
Operation with 25 ns beams does not look impossible but SEY reduction looks quite time consuming
Long intensity ramp-up to be expected after Long Shutdowns, also due to UFOs
This time should be taken into account when making integrated luminosity projections

23. Thanks for your attention!

24. Losses
To be updated

25. Reminder of doublet motivations
50 ns beam
~1400 bunches
1.7 x 1011 p/b
50 ns
Scrubbing
25 ns Scrubbing
Doublet Scrubbing
25 ns beam
~2800 bunches
1.15 x 1011 p/b
No e-cloud in the dipoles
Doublet beam
~900 doublets
0.7 x 1011 p/b

26. Reminder of doublet motivations
50 ns beam
~1400 bunches
1.7 x 1011 p/b
50 ns
Scrubbing
25 ns Scrubbing
Doublet Scrubbing
The 25 ns had to lower the SEY below 2.1 for the 50 ns, the doublet should lower the SEY below 1.4
Scrubbing curve is logarithmic with the electron dose
From scrubbing curves in the laboratory, the electron dose to decrease the SEY from 2.1 to 1.6 is more than one order of magnitude lower than what needed to decrease it further to below 1.4
We can apply this strategy if
The trains of doublets can survive the e-cloud instabilities at injection (chromaticity, transverse damper, octupoles)
It is possible to fill LHC with enough trains of doublets (though degraded) to produce significantly more heat load in the dipoles than the 25 ns beam at the knee of the e-cloud S-curve.
The enhanced heat load should still be within the capacity of the cryogenic system, i.e the 25 ns beam should have margin (at least at 450 GeV)…
25 ns beam
~2800 bunches
1.15 x 1011 p/b
No e-cloud in the dipoles
Doublet beam
~900 doublets
0.7 x 1011 p/b
The S-curves here are at injection, in reality the 25 ns point is even higher by a certain factor (~2) because of the e-cloud at high energy (photoelectrons), but then it loses a factor 1.5 due to the number of bunches. The cooling power line is lower at 6.5 TeV (about the half if we consider 1.3 W/m/aperture)
Doublet scrubbing is profitable if: 1) the SEY should be such that we are well into the knee of the S-curve with the 25 ns beam, heat load lower than cooling capacity; 2) LHC can be filled with doublet trains with good enough quality as to obtain the required enhancement of the scrubbing flux.
It could help is if the energy spectrum of the electrons with the doublets is higher and perhaps the scrubbing dose is larger for the same heat load.
Even though the cooling power is lower at 6.5 TeV and at injection we could in principle tolerate higher heat loads, the beam degrades much more quickly at 450 GeV and the scrubbing flux decays rapidly

27. Reminder of doublet motivations
50 ns beam
~1400 bunches
1.7 x 1011 p/b
50 ns
Scrubbing
25 ns Scrubbing
Doublet Scrubbing
The 25 ns had to lower the SEY below 2.1 for the 50 ns, the doublet should lower the SEY below 1.4
Scrubbing curve is logarithmic with the electron dose
From scrubbing curves in the laboratory, the electron dose to decrease the SEY from 2.1 to 1.6 is more than one order of magnitude lower than what needed to decrease it further to below 1.4
We can apply this strategy if
The trains of doublets can survive the e-cloud instabilities at injection (chromaticity, transverse damper, octupoles)
It is possible to fill LHC with enough trains of doublets (though degraded) to produce significantly more heat load in the dipoles than the 25 ns beam at the knee of the e-cloud S-curve.
The enhanced heat load should still be within the capacity of the cryogenic system, i.e the 25 ns beam should have margin (at least at 450 GeV)…
25 ns beam
~2800 bunches
1.15 x 1011 p/b
No e-cloud in the dipoles
Doublet beam
~900 doublets
0.7 x 1011 p/b
The S-curves here are at injection, in reality the 25 ns point is even higher by a certain factor (~2) because of the e-cloud at high energy (photoelectrons), but then it loses a factor 1.5 due to the number of bunches. The cooling power line is lower at 6.5 TeV (about the half if we consider 1.3 W/m/aperture)
Doublet scrubbing is profitable if: 1) the SEY should be such that we are well into the knee of the S-curve with the 25 ns beam, heat load lower than cooling capacity; 2) LHC can be filled with doublet trains with good enough quality as to obtain the required enhancement of the scrubbing flux.
It could help is if the energy spectrum of the electrons with the doublets is higher and perhaps the scrubbing dose is larger for the same heat load.
Even though the cooling power is lower at 6.5 TeV and at injection we could in principle tolerate higher heat loads, the beam degrades much more quickly at 450 GeV and the scrubbing flux decays rapidly

29. Alick MACPHERSON, Evian 2012