Introduction to LHC cryogenic system (layout, architecture) Preparation before cool-down (Purge, flushing) Transient operations to reach nominal operating condition (Cool-down, filling) Tuning of the cryogenic system: – With an without electrical circuit powering – Coping with resistive transition Availability for powering Conclusion Contents 14 May 2008L. Tavian - LHC Cryogenics2

Introduction 14 May 2008L. Tavian – LHC Cryogenics3 Main cryogenic requirement: Cooling of 24 km of superconducting 1.9 K, 8.33 T

Why 1.9 K ? 14 May 2008L. Tavian – LHC Cryogenics4 With Nb-Ti as technical superconductor, to get sufficient current density above 9 T, cooling below 2 K is required Trade-off between magnet and cryogenic complexity

Why helium as refrigerant ? 14 May 2008L. Tavian – LHC Cryogenics5 Helium is the only available cryogen which is not solid below 15 K

14 May 2008L. Tavian – LHC Cryogenics6 Cryogenic system layout 5 cryogenic islands 8 helium cryogenic plants: 1 plant serves 1 sector ( K, K and 600 kW LN2 precooler) (Distribution line) (Interconnection box) Cryogenic plant

Cryogenic architecture 14 May 2008L. Tavian – LHC Cryogenics7

Typical even-point architecture 14 May 2008L. Tavian – LHC Cryogenics8

Photo gallery: Refrigerators 14 May 2008L. Tavian – LHC Cryogenics9 1.8 K refrigeration units ( K) 4.5 K Refrigerators ( K)

Photo gallery: Storage and distribution 14 May 2008L. Tavian – LHC Cryogenics10 GHe storage LN2 storage Cryo-magnet Distribution line (QRL) Interconnection box Vertical transfer line

Superfluid helium cooling principle 14 May 2008L. Tavian – LHC Cryogenics11 Heat exchanger tube in copper with a diameter DN50. Overall thermal conductance: ~ 100 W/m.K (i.e., for 1W/m, a temperature difference of 10 mK) Principle of the LHC He II Cooling Scheme

Normal operating conditions 14 May 2008L. Tavian – LHC Cryogenics12

Magnet-cell (107 m) cooling scheme 14 May 2008L. Tavian – LHC Cryogenics m

Other tunnel equipments 14 May 2008L. Tavian – LHC Cryogenics14 RF cavities (P4) Feed box (DFB)Standard cell (107 m) Inner triplet Standalone magnet

Introduction to LHC cryogenic system (layout, architecture) Preparation before cool-down (Purge, flushing) Transient operations to reach nominal operating condition (Cool-down, filling) Tuning of the cryogenic system: – With an without electrical circuit powering – Coping with resistive transition Availability for powering Conclusion Contents 14 May 2008L. Tavian - LHC Cryogenics15

Sector preparation: Purge & leak test 14 May 2008L. Tavian – LHC Cryogenics16 Removal of air in circuit by evacuation and He filling: 400 m3 of circuits distributed over 3.3 km, He taken from medium pressure storage, at least 3 cycles to get air and humidity content below 50 ppm to be compatible with the cryoplant purification system (dryer and adsorber), 1 to 2 cycles per day, ~10 kCHF of He per sector purge.

Sector preparation: circuit flushing 14 May 2008L. Tavian – LHC Cryogenics17 Removal of dust and debris by flushing the helium circuits with high helium flow at 300 K: all circuits flushed in series, QRL headers first to avoid migration of dust to the machine circuits (magnets and beam screens), use of refrigerator compressors for flow production, the complete return flow is passing trough a filter to stop the debris, 1 to 2 week per sector depending of the circuit cleanliness.

Filter inspection after flushing 14 May 2008L. Tavian – LHC Cryogenics18 Debris & Kapton Dust Possible short in magnet diodes Travellers Foam (welding plug) But the sooner the best !

Introduction to LHC cryogenic system (layout, architecture) Preparation before cool-down (Purge, flushing) Transient operations to reach nominal operating condition (Cool-down, filling) Tuning of the cryogenic system: – With an without electrical circuit powering – Coping with resistive transition Availability for powering Conclusion Contents 14 May 2008L. Tavian - LHC Cryogenics19

300 – 5 K cool-down of Sector 14 May 2008L. Tavian – LHC Cryogenics20 Cool-down of 4625 t per sector over 3.3 km: From 300 to 80 K: 600 kW pre-cooling with LN2, up to ~5 t/h, 6 LN2 trailer per day during 10 days (1250 t of LN2 in total); LN2 logistics critical. From 80 to 5 K: Cryoplant turbo-expander cooling, cryoplant tuning critical (4 different types). LHe filling: 15 t of LHe in total (4 trailers). Up to 40 PID loops to control the cool-down speed of the cells, standalone magnet and DFB.

Logitics during sector cool-down 14 May 2008L. Tavian – LHC Cryogenics21 Unloading of LHe & LN2: ~ 200 kCHF of LN2 per sector cool- down, ~ 600 kCHF of LHe per sector filling.

300-5 K cool-down time (w/o filling) 14 May 2008L. Tavian – LHC Cryogenics22 Total w/o external factors & cryoplant stops Cool-down time hampered by: external factors (leaks, electrical short-circuits, electrical control plateaus), cryogenic stops (utility loss, cryogenic problems), cryogen logistics management (week-ends, nights…), cryoplant and tunnel cooling loops tuning and limitations. Room for improvement !

Filling & cool-down to 1.9 K 14 May 2008L. Tavian – LHC Cryogenics23 LHe filling and cool-down completion by using the 1.8 K refrigeration unit Complex operation of cold compressors (up to 4 in series) Filling and cooling of electrical feed boxes and their current leads (as well as RF cavity modules, if any) 1 week per sector Cold compressors

Cold compressors 14 May 2008L. Tavian – LHC Cryogenics24 Definition of reduced parameters: m: mass-flow Tin: Inlet temperature Pin: Inlet pressure N: Rotational speed Subscript 0: Design condition and Complex operation of hydro-dynamic compressor: - high rotational speed: up to 800 Hz, - reduced operation range at constant pressure ratio.

Introduction to LHC cryogenic system (layout, architecture) Preparation before cool-down (Purge, flushing) Transient operations to reach nominal operating condition (Cool-down, filling) Tuning of the cryogenic system: – With an without electrical circuit powering – Coping with resistive transition Availability for powering Conclusion Contents 14 May 2008L. Tavian - LHC Cryogenics25

Tuning of the cryogenic system 14 May 2008L. Tavian – LHC Cryogenics26 Commissioning at cold of instrumentation (temperature, cryo-heaters, Lhe level, valves) Commissioning of sub-systems like electrical feed boxes and superconducting links which are cold tested for the first time in the tunnel Tuning of the control loops to get the required stability and conditions for allowing the magnet powering Validate the conformity of the equipment cooling 1 to 2 weeks partially in parallel with the last cold-down phase

Commissioning of instrumentation 14 May 2008L. Tavian – LHC Cryogenics27 Always on-line with the instrumentation database *: including some redundant sensors installed but not connected

Commissioning of the control system 14 May 2008L. Tavian – LHC Cryogenics28 Local Cryogenic control room Central Control Room OWS [1..x] PVSS DS Sector OWS [1..x] PROFIBUS DP networks PROFIBUS PA networks WFIP Networks (7) RadTol electronics UNICOS PLC S7-400 Siemens FECs (FESA) RM sector 81 RM sector 78 PLC QUIC Schneider Return Module CRYO Instrumentation expert tool PVSS DS TT, PT, LT, DI, EHCV Courtesy E. Blanco

Commissioning of sub-systems 14 May 2008L. Tavian – LHC Cryogenics29 DFB commissioning : Level measurement and current lead temperature control and stability ~ 1 week per sector

Tuning of control loops 14 May 2008L. Tavian – LHC Cryogenics30 Tuning of all control loops and the corresponding control logic to get the required stability and conditions for allowing first the ELQA activity and then the magnet powering

Fill-up and boil-off of standalone magnets to confirm the level and the complete wetting of the magnet coils with liquid helium. Validation of the conformity of the cooling circuits: – Non conformities identified in some feeding pipes of the 1.8 K bayonet heat exchangers. Use of the built-in redundancy to cool the non-conforming cells. Validation of cooling conformity 14 May 2008L. Tavian – LHC Cryogenics31

Boil-off of standalone magnets 14 May 2008L. Tavian – LHC Cryogenics32

Cooling loop redundancy 14 May 2008L. Tavian – LHC Cryogenics33 Slope

S5-6 magnet temperature stability 14 May 2008L. Tavian - LHC Cryogenics34 ~ 140 temperature measurements superimposed ! (Validation of the thermometry quality)

Introduction to LHC cryogenic system (layout, architecture) Preparation before cool-down (Purge, flushing) Transient operations to reach nominal operating condition (Cool-down, filling) Tuning of the cryogenic system: – With an without electrical circuit powering – Coping with resistive transition Availability for powering Conclusion Contents 14 May 2008L. Tavian - LHC Cryogenics35

Quench recovery 14 May 2008L. Tavian – LHC Cryogenics36 Recovery time: 6 h Recovery time: 4.5 h Quench relief valve

Introduction to LHC cryogenic system (layout, architecture) Preparation before cool-down (Purge, flushing) Transient operations to reach nominal operating condition (Cool-down, filling) Tuning of the cryogenic system: – With an without electrical circuit powering – Coping with resistive transition Availability for powering Conclusion Contents 14 May 2008L. Tavian - LHC Cryogenics37

Cryo-availability 14 May 2008L. Tavian – LHC Cryogenics38 Availability for sector HWC : 80 % during weeks w/o resistive transition To be improved during beam commissioning: more than 95 % per sector Note : A cryogenic system takes time (hours) to recover any kind of stop ! (Considering independent origins of problems) HWC BC

Introduction to LHC cryogenic system (layout, architecture) Preparation before cool-down (Purge, flushing) Transient operations to reach nominal operating condition (Cool-down, filling) Tuning of the cryogenic system: – With an without electrical circuit powering – Coping with resistive transition Availability for powering Conclusion Contents 14 May 2008L. Tavian - LHC Cryogenics39

Time for getting nominal conditions: – Presently 10 weeks for getting nominal conditions, – In routine operation, about 1 month is foreseen, LHC cryogenics is the largest, the longest and the most complex cryogenic system worldwide. Operation for the needs of Sector HWC is now demonstrated. Based on experience, together with procedures and tools being put in place, availability must be improved for the next phase: The Beam Commissioning. Conclusion 14 May 2008L. Tavian - LHC Cryogenics40