Crab Cavity failures with 2 CCs:
gain in luminosity ,
impact on availability,
and required Crab Cavity SYSTEM
availability
M. Valette, D. Wollmann, A. Appolonio, J. Uythoven
CERN-TE-MPE
6th HL-LHC Meeting – 15 November 2016
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Outline
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Crab Cavities and increased Luminosity
Monte Carlo Model
Simulations results with and without CCs
Existing RF Systems’ impact on availability
CC failures’ impact on availability
CC availability budget
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Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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CCs and increased luminosity
 Crab Cavities allow compensating the geometrical factor and
increasing instantaneous luminosity.
𝑛𝑏𝑓𝑟𝑒𝑣𝑁𝑝2
𝐿=
∗
∗2
4π σ𝑡𝑟𝑎𝑛𝑠
1
1+[
θ𝑐 σ𝑧 2
]
∗
2𝜎𝑡𝑟𝑎𝑛𝑠
 The new baseline includes 2 CC/beam/side in CMS and
ATLAS which allows partly compensating the crossing angle.
 The virtual peak luminosity would be12.6E34 cm-2.s-1, which
allows levelling at 5E34 cm-2.s-1 for 5.2h.

Source: HL-LHC parameter table.
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Crab Cavity failures with 2 CCs: gain in luminosity and impact on availability - M. Valette
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Monte Carlo model
 The HL-LHC is supposed to deliver 250 fb-1 per year, it
is important to make previsions to verify its feasibility.
 A model was developed by A. Apollonio to reproduce
LHC production from observed failure rates and make
predictions for the HL-LHC and FCC depending on
operational scenarios.
 see “LHC Availability Dependencies and Lessons Learnt from
LHC Run 2” by J. Uythoven, Wednesday morning
 A 160-days long production year of the LHC is
simulated with random failures affecting Luminosity
production.
 1.000 such years are simulated for every set of parameters in
order to have good statistics.
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Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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Monte Carlo model - schematic
Fill
Exponential
distribution
Exponential
distribution
LogNormal
distribution
Fault Dump
Fault
time B
Turnaround
B
Produced
Next Fill
Luminosity
Probability
of a dump
Operator
Dump
Fault
time A
Optimal Fill length Exponential
distribution
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Turnaround
A
LogNormal
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Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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Monte Carlo model - parameters
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HL-LHC luminosity parameters:

Levelled luminosity: 5E34 cm-2

Levelling time: 5h and luminosity lifetime (exponential): 5h
 Optimal Fill length: 9.2h
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Availability data is taken as of summer 2016, between TS1 and TS2:

Proportion of dumped Fills: 50%, Average time of dump: 9h
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After a successful Fill:
 Downtime: 2.2h, Turnaround time: 5.2h
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After a fault:
 Downtime 5.6h, Turnaround time 4h
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Not taken into account : the effect of higher radiation levels causing R2E
problems, higher fault rates due to ageing of systems and faults due to
new equipment.
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Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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Yearly luminosity production and time distribution for the HL-LHC with CCs
The simulation yields an average 280 fb-1 per year with 2016 LHC availability and increased
luminosity due to CCs which never fail.
The average physics efficiency is then 47.9% with 250 fills per year.
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Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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Parameter
With CC
Without CC
Levelling time
5h
2h
Optimal Fill
length
9.2h
7.15h
Yearly
Luminosity
280 fb-1
225 fb-1
Physics
efficiency
47.9 %
43 %
Turnaround
31 %
34.7 %
Yearly luminosity production and time distribution for the HL-LHC without CCs
Without Crab Cavities the instantaneous Luminosity can only be levelled (e.g. with the 𝛽 ∗ ) at 5E34
cm-2.s-1 for 2h instead of 5h. The optimal Fill length is then 7.15h => an average of 225 fb-1 per year.
Due to a shorter optimal fill length and by keeping the fraction of dumped fills constant the average
physics efficiency is consequently lower (43%).
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Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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Monte Carlo model - schematic
Increased
dump
probability
Fill
Increased
Exponential
fault rate
distribution
Increased
Exponential
down time
distribution
LogNormal
distribution
Fault Dump
Fault
time B
Turnaround
B
Produced
Next Fill
Luminosity
Probability
of a dump
Operator
Dump
Fault
time A
Optimal Fill length Exponential
distribution
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Turnaround
A
LogNormal
distribution
Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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Power converters
RF
Collimation
Accelerator Fault Tracker : LHC RF availability between TS1 and TS2
For the studied period the LHC accelerating cavities have been very reliable:
12 faults over 45 days of operations. Average downtime of 0.7h, MTBF of 90h
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Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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=> MTBF 18h
=> MTBF 48h
Source : The 15th KEKB Accelerator Review Committee (Y. Yamamoto)
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Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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CC failures’ impact on availability
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Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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CC failures’ impact on luminosity
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Conclusion
 Monte Carlo models simulating effects of crab
cavities on HL-LHC availability show, that crab
cavity faults
 strongly impact HL-LHC physics efficiency.
 every second day, causing magnet quenches, are not
acceptable  gain in integrated luminosity due to crab
cavities absorbed.
 Crab cavity faults (time scales, kick strength, … )
have to be fully understood to allow for
mitigations, avoiding (damage and) quenches of
superconducting magnets.
 Experience with crab cavities in SM18, SPS and
Uppsala University will provide important input to
extrapolate to HL-LHC physics efficiency.
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Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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Thank you for your attention.
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Crab Cavity failures with 2 CCs: gain luminosity and impact on availability - M. Valette
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