LHC CRYOGENICS – new experience of run with increased beam energy and intensity Krzysztof Brodzinski CERN, Geneva, Switzerland With contribution from: S. Claudet, D. Delikaris, L. Delprat, G. Ferlin, E. Rogez and L. Tavian

K.Brodzinski_ICEC26_ Outlook Introduction to LHC cryogenics Cryogenic operation overlook Long shut down 1 activities Preparation and operation with increased beam parameters 1.9 K circuit and heat load K beam screen circuits (cryogenics vs photon-electron cloud) Limitations and operational optimization scenarios Main failures during Run Availability and helium consumption Conclusions 2

K.Brodzinski_ICEC26_ Introduction  circumference  ~ 27 km,  constructed at ~ 100 m underground,  the accelerator ring inclination is 1.4 % Compressor station 4.5 K refrigerator 1.8 K pumping unit (cold compressor) Interconnection box LHC cryogenics: 8 x K 1800 sc magnets 24 km & K K 135 tons of helium inventory 3

K.Brodzinski_ICEC26_ Operation outlook 4

K.Brodzinski_ICEC26_ Feedback from LS1 Feedback from LS1 basing on Run2 (2015): Major overhaul and maintenance (mainly on warm compressor stations) – done and operational R2E (Radiation to Electronics): SEU (Single Event Upset) – eliminated (0 cases declared in 2015) ! Cold boxes, QRL bellows, DFBA SM – all repairs with success no leak left 71 QRL cryo valves replaced (38 on the BS circuit) - operational M M M M M M M M R R R R M – maintenance of 64 compressors R – Repairs of 4 refrigerators - QRL compensators replacement 3TU – 3 turbines replacement - DFBAs SM repairs - R2E campaign 3 TU P1 P2 P3 P4 P6 P7 P8 P5 P18 B B A B A B A A LL HL 5

K.Brodzinski_ICEC26_ Copper Stabilizer Continuity Measurement 6 Copper stabilizer  current conductor during resistive transition (quench). Consolidation + successful tests done during first LHC long shutdown. Example of non-conformity Cryogenic conditions adjustment for the test: -20 K and 4.5 bar in the cold mass (out from superconducting condition) -Supercritical helium in main supply line with J-T valve + bayonet magnets HX in use

K.Brodzinski_ICEC26_ Magnets training for 6.5 TeV Statistics for 1232 dipoles: 1064 did not quench, 168 did quench (9 - twice, times) 179 quenches in total Contribution G. Willering Courtesy of CERN TE-MPP team ~ 6 hours 7

K.Brodzinski_ICEC26_ K circuit IFJ-PAN_K.Brodzinski_ PK visit_K.Brodzinski_ K. Brodzinski, CryoOps, KEK - Japan 2012 ~ 214 m subsector F D Return (4.5 K, 0.3 Mpa) Screen (70 K, 1.6 Mpa)

1.9 K circuit heat load 9 K.Brodzinski_ICEC26_ Dynamic heat load on 1.9 K comes from resistive heating in the magnets interconnection splices and as beam induced heating Recalculation of the heat load for reference fills for sector 1-2 from equation (1) gives the following results where: Δh is the enthalpy difference upstream J-T valve and downstream the cold mass heat exchanger, m 1.8KPU is the flow measured by cold pumping unit and Q EH is electrical power applied on 1.9 K circuit 4 TeV operation (fill 3134): Q 1.9K = 717 W 6.5 TeV opearation (fill 4569): Q 1.9K = 878 W For one high load sector: Nominal 1.9 K = 1460 W, Installed K = 2400 W Conclusion: 1.9 K heat load for Run2 was considerably lower than nominal design and installed capacity  optimization in operation scenarios could be done

K.Brodzinski_ICEC26_ Beam screen at K and capacity Heat loads: Qs – static heat load, Q EH – electrical heater, Q BS – from beam operation (ARCs: EH is used now out of beam operation periods to avoid low velocity helium stream oscillations in BS circuit, LSS: in permanent use for stability ~50 W/sector) LHC Design Report vol. I, 2004, p.328 and p.316 Exercise for s1-2: Input: Qs=400 W, EH=49 W, Q qrl =630 W, P ref =7700 W, L totBS =6033 m Calculation: ( )/6033=1.1 W/m/a -> 2*53*1.1=116 W/half cell Exercise for 2-3: Input: Qs=400 W, EH=49 W, Q qrl =570 W, P ref =7600 W, L totBS =5971 m Calculation: ( )/5971=1.1 W/m/a -> 2*53*1.1=116 W/half cell 85 W/half cell 10

K.Brodzinski_ICEC26_ Beam screen circuit and HL at 6.5 TeV 11 During operation at 6.5 TeV with 25 ns of the inter-bunches spacing the contribution from photo-electron cloud was much higher than expected. Dedicated scrubbing did not treat the problem -> LHC was cleaned during the run. Courtesy of G. Iadarola Load per half cell: [W/hc = W/m *53*2]

K.Brodzinski_ICEC26_ BS – cleaning effect 12 In addition, the cloud was not homogeneously distributed between the sectors and in some cases pushed cryo-plants to the capacity limits. The cleaning effect is unexpectedly slower in more clouded sectors than in low clouded. The upgraded from LEP machine at P2 appeared as with the lowest capacity limit.

K.Brodzinski_ICEC26_ How to deal with the cloud The e-cloud thermal effect requires from cryogenics: 1.Fast dynamic increase of cooling capacity during injection and rump AND fast dynamic decrease of the capacity during de-dump 2.Large continues demand for cooling capacity during related fill How to deal with “1” (*) Fast dynamic control logic applied on local BS cooling loops (acting by anticipation to thermal load, triggered with information about number of injected bunches) Thermal capacity buffer of ~ K prepared before each beam injection Modification of Cryo Maintain interlock (before T=30 K during 30 s, now T=40 K, 30 min) * see more in B. Bradu et al contribution to ICEC26 Beam screen cryogenic control improvements for the LHC run 2 How to deal with “2” Process optimization – already well done, not a lot of margin left (limitations on main refrigerators – see slide 12) Apply modifications in operation scenario for equal split of return flow (3 valves to be changed – see slide 15) 13

K.Brodzinski_ICEC26_ Operation scenarios 14 Thanks to in-built interplant piping and reduced demand for capacity Run1 was operated with two cryo-plants off. Run2 (2015) required high capacity for BS cooling so all 4.5K cold boxes were in operation but as 1.9 K load was relatively low and 3 cold pumping units were off (higher global availability proven) Run2 (2016) is foreseen to be operated with 4 cold pumping units off – tests underway

K.Brodzinski_ICEC26_ Cryo plants Run2 configuration 1.9 K circuit 4.5 K circuit Magnets supply 1.9 K circuit 4.5 K circuit Magnets supply He storage Compression Refrigerator (economizer) Refrigerator (liquefier) AB Currently 1.9 K return flow is unbalanced affecting operational stability and performance of the refrigerators. To resolve the issue 3 valves design must be more precise to control the flow –> replacement to be studied and approved (long term solution … ~2019) 15

K.Brodzinski_ICEC26_ Main failures 1/2 Major failure – an internal helium leak (order of a mbarl/s) in LHCB 4.5 K refrigerator at P8 declared on 18 June 2015 (TS1). Consequences: vacuum diffusion pump stopped, lowered capacity on P8 cryo plant, frequent cold condition losses because of poor quality of supercritical helium in the supply collector. Repairs done on “helicoflex” sealing on TU filter cap in January Second (smaller, 10^-3 mbarl/s) leak detected on DN400 AL/SS transition repaired with “vac-seal” varnish. Compensatory measures: -Cold compressor connected to plant B (for economizer mode) -Vac. roots pumping unit connected -2 nd spare roots in situ -Process adaptation to minimize the leak Vacuum signal Start up of cold turbines 16

K.Brodzinski_ICEC26_ Main failures 2/2 Main components or rotating machines which required replacement: 3 turbine failures on 4. 5 K refrigerators: P18: TU2 – replaced with LN2 pre-cooler during repair time P18: TU3 – replaced with spare P2: TU2 – replaced with LN2 pre-cooler during repairs 1 warm compressor failure: P4: HP compressor CP6 Systematic vibration measurements (1/month) allow for in advance detection of the problem to avoid serious damage and high repairs cost of affected machine From June 2009 until now: 8 warm compressors had to be repaired (7 of firm “A”, and 1 from firm “B”) 16 turbine accessory valve failures (needed for some turbines to control axial profile of the shaft temperature). Replacement study is launched at CERN (offers already received), prototypes to be installed on the cold boxes in PLC failures – progressive replacement was done by a new product with “anti-crash firmware”. 17

K.Brodzinski_ICEC26_ Availability – global cryogenics 18 Statistics for Run2 (2015) period from 5 th April to 14 th December 2015

K.Brodzinski_ICEC26_ Availability – main cooling machines K cold boxes 1.8 K pumping units 4 x PLC

K.Brodzinski_ICEC26_ Cryo Maintain statistics The most time consuming losses: PLC: 4 failures with 26.9 % Cold compressor: 8 failures 19.4 % Human factor 15.1% (mainly late TS1 recovery) Elec. Instrum. Tunnel 13.3 % 4 above contributors  75% of the down time The most frequent losses (main contributors): DFBMI: 39 He level losses with 2% (linked with P8 QSRB leak) DFBAF: 21 He level losses with 1% solution found (to be improved ex. with YETS) No obvious solution for improvement (to be studied) Legend: 20

K.Brodzinski_ICEC26_ Helium consumption and losses 21

K.Brodzinski_ICEC26_ Conclusions 22 LHC cryogenic system operated successfully for Run1 and Run2 (2015) with global availability between % for 8 independent cryo-plants First Long Shutdown consolidations visibly helped (0 radiation caused failures declared in 2015) Beam energy increase required special tests and preparation (copper continuity test and magnets training) where cryogenic provided requested safe operation Cold box capacity for 1.9 K circuit has relatively large margin and this saving is used to cover heat load from electron cloud on the beam screen circuit Gained operational experience allowed optimization in operation scenarios of the plants, however some of them approached the global capacity limit, continues study on improvements and optimization of the cryogenic process is underway Thank you for your attention! Questions ?

K.Brodzinski_ICEC26_ Back up 1: Electron cloud mechanism 23 G. Rumolo - CERN

K.Brodzinski_ICEC26_ Back up 2: Scaling laws 24 LHC Design Report

K.Brodzinski_ICEC26_ Back up 3: LHC standard cell 25