1. Cryogenic operational experience from the LHC physics Run 2 (2015 – 2018 inclusive)
Laurent Delprat
CERN, Geneva, Switzerland
With contribution from: B. Bradu, K. Brodzinski, G. Ferlin

2. OUTLOOK Introduction to LHC cryogenic infrastructure
Run 2 overview – main transients
Physics production
Heat load and applied optimizations
Global capacity optimization
Local cooling loop optimization
Availability and helium consumption
Conclusions and perspectives

3. Cryogenic Infrastructure
circumference  ~ 27 km,
constructed at ~ 100 m underground,
the accelerator ring inclination is 1.4 %
LHC cryogenics:
8 x K
1800 sc magnets
24 km & K K
130 tons of helium inventory
Total for 8 sectors:
Compressors: 64
Turbines:
Cold Comp.: 28
Leads: ’200
I/O signals: 60’000
PID loops: 4’000
Compressor station
4.5 K refrigerator
1.8 K pumping unit (cold compressor)
Interconnection box

4. Run 2 Cryogenic Operation
2015 YETS: AUG tests for P8, late recovery afterwards, increase of TT avg ARC up to 50 K in one sector (S78).
2016 EYETS: 31L2 magnet replacement MBB2 (defective dipole).
2017 YETS: 16L2 issue with gaseous impurities – beam pipe partial warm-up (regeneration) for accidentally accumulated impurities via the vacuum pumping system.
Nota for 2018 sector 1-2: beam pipe partial warm up (regeneration) for accidentally accumulated gaseous impurities via the vacuum pumping system.

5. Physics production Beam parameter Nominal Run 2 Energy 7 TeV 6.5 TeV
Luminosity
cm-2.s-1
cm-2.s-1
Intensity
protons/beam
Production during Run 1 and Run 2
ATLAS
(fb-1)
CMS
Run 1
28.87
29.38
Run 2
158.71
162.62
Total
187.58
192.00
Nominal intensity: ^11 protons/bunch with 2808 bunches (cf. LHC Design Report)
Achieved figures: ^11 protons/bunch with 2556 bunches during Run 2 (see Benjamin Bradu presentation)

6. Beam-induced heat load – beam screen
One beam-screen local cooling loop = half-cell (hc) *
The LHC beam screen has a double function, to protect the 1.9 K cold mass from dynamic heat load generated by circulating beams and also to allow keeping ultra-high vacuum in the beam pipes [4]. The beam screen is actively cooled with helium at 3 bar between 4.6 and 20 K.
The dynamic heat load on the beam screen comes from three contributors: synchrotron radiation (sr), image current (ic) and photo-electron effect so-called electron cloud (ec). Knowing main beam parameters, the heat load from first two contributors can be precisely calculated [5, 6]. However, analysis of thermal effect coming from electron cloud is much more complex, depending mainly on surface condition, beam intensity, and inter-bunches spacing.
During Run2, the LHC was operated with 6.5 TeV/beam of energy and intensity up to protons/beam, running with 25 ns of the inter-bunches spacing (compared to 50 ns during Run 1). Such operation scheme generated particularly high values of dynamic heat load in four LHC sectors, exceeding significantly the design values [7].
*half-cell: LHC cryogenic half-cell of 53 m housing (among others) one local beam-screen cooling loop

7. Cryogenic operation scenario optimization
How to cope with significantly increased dynamic heat load ?
Hardware optimization: evolution of the cryoplants operation scenario
Software optimization: introduction of feed-forward process control* for smarter use of cooling capacity
Why was it possible to stop 1 QURC?
Main reason: resistance of the interconnections of the superconducting cables was lower than design  dynamic heat load lower
Second reason: static heat load lower than design as well for the QURCs
The LHC is equipped with eight large independent cryogenic plants feeding eight accelerator sectors and located in defined places on the accelerator circumference [1]. The reduced heat load during run1 allowed for stop of two cryoplants, one at P6 and one at P8. Such a solution was not possible to be applied for run2 because of increased heat load on the beam screen cooling loops. However, following several measurements and tests it was confirmed that operation of one 1.8 K pumping unit is sufficient to cope with the heat load coming from magnet cold masses of two adjacent sectors [2]. In practice, thanks to installed interplants bypasses, the operation scenario of the cryogenic system was optimized as presented in Figure 3 (hatched area – equipment not coupled to the accelerator).
Run 1 ( ): operation at reduced capacity
Two 4.5 K refrigerators and two cold pumping units kept off
Run 2 (2015): high capacity required for beam screen cooling
All 4.5 K refrigerators in operation
Low 1.9 K load => 3 cold pumping units kept off => higher global availability
Run 2 ( ): high capacity required for beam screen cooling
4 cold pumping units kept off
*see C1Or1B-05 – Cryogenic management of the LHC run2 dynamic heat load, by B. Bradu

8. Cryogenic System Configuration
1500 W / 53 cells for one sector = 28.3 W / half-cell => rounded to 30 W / half-cell.
Nevertheless, run2 scenario required re-balancing of the flows and capacities between two adjacent cold boxes (one working as refrigerator, the other as liquefier).
The capacity differences were adjusted using interplant bypasses on thermal shield circuit.
Beware: on the A refrigerator, flow is recovered at the 30 K level, whereas on the B refrigerator side, flow is recovered from the thermal shielding circuit at the 70 K level => not equivalent!
The related circulating helium flow diagram is presented in Figure 4.
Rebalancing on line D is not possible as too many variable parameters are to be taken into account + thermal inertia is too important on the refrigerator (line D return valves not adapted) wrt to the dynamic response of the tunnel, with today’s equipment
Think: when well balanced, there is no more utilization of the charge / discharge circuit through the GHe storage.
The estimated gain of applied solution allowed to spare ~1500 W at 4.6 K – 20 K for the beam screen cooling for each of two LHC sectors i.e. to increase the cooling capacity of one 53 m long half-cell by nearly 30 W [3].
Corresponding cooling power saved ≈ K – 20 K for each sector
i.e. ≈ 30 W / half-cell (53 m)

9. Run 2 CryoMaintain Downtime Origins
Supercritical helium degradation
Electrical feedboxes helium level oscillations
Beam screen temperatures evolution
57 losses
63 losses
25 losses
Most frequent losses
63% of all losses
 145 out of 230 losses
Most time consuming losses
77% of total cryo downtime
 485 out of 630 hours
Tunnel instrumentation
PLCs failures
Cryoplants stops
Most frequent losses:
DFBs level oscillations: 44 losses in 2015, 8 losses in 2016, 6 in 2017, 5 in 2018
ScHe quality degradation due to partial failure of insulation vacuum in one 4.5 K refrigerator in LHC Point 8: 56 losses in 2015, 0 in 2016, 0 in 2017, 1 in 2018
Beam screen temperatures evolution beyond thresholds: 25 losses in 2015, 0 in 2016, 0 in 2017, 0 in 2018
Most time consuming losses:
PLC failures (4 in 2015, 72h50min /// 2 in 2016, 27h46min /// 1 in 2017, 5h38min /// 2 in 2018, 29h00min
Cryoplants stops (not related to PLC failures): 72h29 in 2015, 21h31min in 2016, 70h24min in 2017 /// 26% of cryo downtime in 2015, 65% of cryo downtime in 2016, 75% in 2017 – for 2018: 10 stops, 132h44min
Tunnel instrumentation: 38h42min in 2015, 9h19 in 2016, 5h21min in 2017, nothing in 2018
53 hours
135 hours
297 hours

10. LHC Cryo Availability from Run 1 to Run 2
*Average Run 2 values
15 days
116 days
234 days M
LHC sectors cold standby and beam commissioning
LHC physics production
Cryo availability of 97.0% for 8 independent sectors  99.6% for each cryoplant !!!

11. LHC He consumption since 2007
2012 : 1500 kg/month of helium recurrent losses
2015 : 987 kg/month of helium recurrent losses
2016 : 850 kg/month of helium recurrent losses
2017 : 550 kg/month of helium recurrent losses
2018 : 833 kg/month of helium recurrent losses
Average: 3 kg/day/running cryoplant

12. Conclusions and perspectives
LHC Run 2 is considered as successful for physics production rate, availability and operation reliability of the cryogenic system,
Non standard optimization of global capacity with one 1.8 K pumping unit operated on two sectors was successfully applied for Run 2 and is recommended for next physics production periods,
Highly efficient feed-forward process control solution was successfully adapted for LHC Run 2 – it appears inevitable for application on HiLumi LHC where strong variations of beam induced heat load will appear during transients,
LHC Long Shutdown #2 started: major overhaul of the compressors, electrical motors, planned maintenance and consolidations give strong confidence for smooth Run 3
(see C4Or1B-01 – Experience from the outsourcing of the Cryogenic
Operation & Maintenance at CERN, by F. Ferrand)
Thank you for your attention !

14. LHC Cryogenics Operation Timeline
≈ 130 W / half-cell*
Beam-induced
heat load
≈ 10 W / half-cell*
13 TeV
8 TeV
Beam
Energy
7 TeV
RUN 1
LONG SHUT
DOWN 1
RUN 2
Time
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
1st cooldown
Year End Technical Stops
*half-cell: LHC cryogenic half-cell of 53 m housing (among others) one local beam-screen cooling loop

15. LHC / HL-LHC Plan
Courtesy L. Rossi, 8th HiLumi Collaboration Meeting 2018

16. Courtesy L. Rossi, 8th HiLumi Collaboration Meeting 2018