The LHC Cryogenic System and Operational Experience from the First Three Years Run

Focused Review The LHC Cryogenic System and Operational Experience from the First Three Years Run Dimitri DELIKARIS *1 and Laurent TAVIAN *2 Synopsis: The LHC (Large Hadron Collider) accelerator helium cryogenic system consists of eight cryogenically independent sectors, each 3.3 km long, all cooled and operated at 1.9 K. The overall, entropy equivalent, installed cryogenic capacity totalizes 144 kw @ 4.5 K including 19.2 kw @ 1.8 K with an associated helium inventory of 130 ton. The LHC cryogenic system is considered among the most complex and powerful in the world allowing the cooling down to superfluid helium temperature of 1.9 K. of the accelerators high field superconducting magnets distributed over the 26.7 km underground ring. The present article describes the LHC cryogenic system and its associated cryogen infrastructure. Operational experience, including cryogen management, acquired from the first three years of LHC operation is finally presented. Keywords: high energy accelerators, large scale cryogenics, superconducting magnets (Some figures in this article may appear in colour only in the electronic version) 1. Introduction The LHC (Large Hadron Collider) consists in a deep underground, 26.7 km circumference, accelerator 1) equipped with high field superconducting magnets totalizing the unprecedented cold mass of 36,000 ton and operated in superfluid helium at the temperature of 1.9 K. The LHC cryogenic system 2) consists in eight 18 kw at 4.5 K helium refrigerators each of them respectively combined with eight 2.4 kw at 1.8 K refrigeration units, the latter based on several stages of hydrodynamic cold compressors process. From the operational point of view, this implementation subdivides the LHC accelerator into eight cryogenically independent sectors, each of 3.3 km long (Fig. 1). Each LHC sector is connected to a pair of refrigerators, the first one providing the cooling capacity at 4.5 K, the second one completing the cooling down to the operating temperature of 1.9 K 3). Based on the LHC sector cryogenic scheme, considering the string of superconducting magnets as a continuous cryostat, the cryogenic fluids are distributed by means of a dedicated compound cryogenic distribution line circling the LHC tunnel. Regarding the cryogen inventory, with the installation and operation of the LHC cryogenic system, the associated infrastructure for storage and management of the helium and nitrogen has been drastically upgraded in order to fulfill and secure the entire spectrum of operational requirements. The overall helium inventory of the LHC accelerator amounts to 130 t. The accelerator pre-cooling from ambient temperature down to 80 K is performed by vaporizing 10,000 t of liquid nitrogen stored at surface premises. Operational procedures and process control are duplicated for each LHC sector thus optimizing the steady-state operation by the cryogenic team. The availability results of the global LHC cryogenic system from the first 2010 2012 years physics run have been in constant progress, starting at 90% the first year and ending nearly to 95% in 2012, corresponding to an equivalent availability of more than 99% per individual LHC sector. Received November 15, 2014 *1 CERN, European Organization for Nuclear Research 1211 Geneva 23, Switzerland E-mail: Dimitri.Delikaris@cern.ch *2 CERN, European Organization for Nuclear Research 1211 Geneva 23, Switzerland E-mail: Laurent.Jean.Tavian@cern.ch DOI : 10.2221/jcsj.49.590 Fig. 1 Layout of the LHC cryogenic system. 590 TEION KOGAKU J. Cryo. Super. Soc. Jpn. Vol. 49 No. 12 2014

2. Functions, Constraints, Architecture 2.1 General Functions The superconducting magnet windings (in arcs, dispersion suppressors and inner triplets) are immersed 4) in a pressurized bath of superfluid helium at 0.13 MPa and a maximum temperature of 1.9 K thus allowing sufficient temperature margin for heat transfer across the electrical insulation. In the long straight sections (inner triplets and dipoles excepted) field strength and heat extraction do not require operation at 1.9 K allowing to keep the superconducting windings of the magnets in a bath of saturated helium at a temperature of 4.5 K. The LHC cryogenic system is designed in order to fulfill the following main functions: - be able to cope with the thermal load variations and large dynamic range induced by the accelerator operation - be able to cool-down and fill with liquid helium the large cold mass of the LHC (36,000 ton) in 15 days, respecting a maximum temperature gradient of 75 K in the magnetic structures; Reciprocally, the same thermal gradient limit applies for the forced emptying and warm-up of the accelerator during shutdown periods - be able to cope with the resistive transition of the superconducting magnets, minimizing cryogen losses and perturbations of the cryogenic plants by managing the Fig. 2 ప ᕤᏛ 49 ᕳ 12 2014 ᖺ resulting heat release and consequences such as fast pressure rises and flow increases. Additionally, the propagation of the resistive transition perturbation to neighboring magnets is limited thus allowing optimized time recovery leading to improve the operational availability of the accelerator. 2.2 Design Constraints The LHC accelerator is installed in the 26.7 km circumference tunnel originally build for the former LEP electron-positron collider 5, 6). The main design and installation decisions for the LHC cryogenic system resulted from the geographical and technical constraints taking in to account that the LHC re-use four (out of the required eight) large capacity helium refrigerators of LEP dully upgraded in order to cope with the LHC requirements. The architecture of the LHC cryogenic system was adapted to the limited number of access points and available underground areas. The cryogenic headers distributing the cooling power along the magnets continuous cryostat in the tunnel are contained in a compound cryogenic distribution line (QRL) as shown in Fig. 2. The QRL is running along the magnets string in the tunnel, feeding each standard lattice cell (106.9 m long) in parallel via a dedicated jumper connection. An additional important design constraint for the Transverse cross-section of the LHC tunnel. 591

cryogenic system consists in the LHC tunnel inclination, at the level of 1.41% with respect to the horizontal plane (due to geological reasons), leading to elevation differences of up to 120 m across the tunnel diameter. In order to avoid generating flow instabilities in the cryogenic headers, all fluids are transported over large distances in mono-phase state (superheated vapor or supercritical flow in the region of the phase diagram). Two-phase circulation of saturated liquid is only tolerated over short lengths. As a basic principle, the cryogenic equipment is installed at surface premises with the exception of the components required to be next to the magnets cryostat or have to be installed underground in order to avoid hydrostatic heads (typically the 1.8 K refrigeration units). The LHC cryogenic system was originally designed for a full automatic operation based on a yearly cycle (9 months operation followed by 3 months of maintenance activities). However, due to physics run optimization, the operation period was set to a continuous 3-year cycle (interrupted by a limited number of short technical stops per year) and followed by a long shutdown (typically 12 to 18 months) allowing preventive and corrective maintenance, major overhauling at manufactures premises as well as consolidation and upgrade works. The LHC cryogenic system was successfully adapted to the new operation period strategy and operational experience from the first three years run is summarized in section 5 of the present article. 2.3 General Architecture With respect to the general layout as shown in Fig. 1, cryogenic equipment and ancillary infrastructure are regrouped in five access LHC Points (P18, P2, P4, P6, P8). The general architecture of the LHC cryogenic system is presented in Fig. 3. According to their LHC access Point location, equipment distribution is shared in four sections: surface, shaft, underground area (Cavern) and tunnel. A refrigeration plant combines one 4.5 K refrigerator and one 1.8 K refrigeration unit. The implemented cryogenic system consists in two-refrigeration-plants per LHC Point (P4, P6, P8), with one exception, due to the lack of space at P2 and the need of large refrigeration capacity for magnet testing at P18, one refrigeration plant was respectively installed at P18 and one at P2. Cryogenic equipment at surface premises includes warm compressor stations, upper cold boxes, cryogen storage (helium and nitrogen), electrical substations and water cooling towers. In the underground areas are located the lower cold boxes, 1.8 K refrigeration units, interconnecting lines and interconnection boxes. 2.4 Temperature Levels Taking in account the basic principle that the thermodynamic cost of the refrigeration at 1.8 K is high, the thermal design of the LHC components was adapted in order to intercept the largest fraction of heat loads at higher and staged levels of temperature in the cryogenic Fig. 3 General architecture of the cryogenic system. 592 TEION KOGAKU J. Cryo. Super. Soc. Jpn. Vol. 49 No. 12 2014

system. The interception temperature levels are defined as following: - 50 K to 75 K for thermal shield as a first major heat intercept, sheltering the cold mass from the bulk of heat in-leaks from ambient - 4.6 K to 20 K for lower temperature heat interception and for the cooling of the beam screens which protect the magnet cold bore from beam-induced loads - 1.9 K quasi-isothermal superfluid helium for cooling the magnet cold mass - 4 K at very low pressure (VLP) for transporting the superheated helium flow coming from the distributed 1.8 K heat exchanger tubes across the sector length to the 1.8 K refrigeration units - 4.5 K normal saturated helium for cooling special superconducting magnets in insertion regions and HTS current leads - 20 K to 300 K cooling for the resistive upper sections of HTS current leads Consequently, the LHC cryogenic system makes use of helium in several thermodynamic states as shown in Fig. 4 on the pressure-temperature diagram. 2.5 Heat Loads, Operating and Transient Modes Heat loads to LHC cryogenic system are classified in three domains: - static heat in-leaks resulting from the cryostats design - resistive heating occurring in the non-superconducting splices and in current leads - beam-induced head loads deposited in the magnets by the beams circulation and collisions effect, strongly depending on the energy, bunch intensity, number and length of the bunches as well as luminosity level at collision: synchrotron radiation from the bending magnet, mostly absorbed by the beam screens resistive dissipation of beam image currents induced in the resistive walls and geometrical singularities of the beam channel impingement of photo-electrons accelerated by the beam potential ( electron clouds ), mostly adsorbed by the beam screen nuclear inelastic beam-gas scattering corresponding to a continuous distributed loss of particles from the circulating beam, mostly absorbed by the cold mass helium bath continuous random loss of particles escaping the collimation system, mostly absorbed by the cold-mass helium bath over a length of a few tens of meters corresponding to the region of aperture restriction Fig. 4 Thermodynamic states of helium in the LHC cryogenic system. 49 12 2014 593

losses of secondary particles, mostly absorbed at 1.9 K in the magnet cold-mass helium bath close to the high-luminosity experimental areas radio-frequency losses in superconducting acceleration cavities Table 1 summarizes the cooling cryogenic capacity requirements for steady-state operating modes for both low and high-load LHC sector type (the latter includes high-luminosity insertions in experimental areas). Four steady-state operating modes and their corresponding requirements for the cryogenic operation of the machine are considered: - injection standby characterized by negligible resistive dissipation and beam-induced heat loads in the magnets - low beam intensity operation, characterized by negligible beam-induced heat loads in the magnets but still with full beam energy - nominal operation at 7 TeV beam energy, 2 0.582 A beam current and 10 34 cm -2.s -1 luminosity - ultimate operation at 7 TeV beam energy, 2 0.86 A beam current and 2.5 10 34 cm -2.s -1 luminosity Four transient modes are considered: - Cool-down from 300 K to 4.5 K in 15 days: in a first step, forced circulation of gaseous helium is used for the cooling of the magnets from room temperature down to 80 K (respecting a maximum thermal gradient of 75 K in the magnet structure) by means of liquid nitrogen pre-coolers with a capacity of 600 kw installed for each refrigeration plant. Below 80 K the cooling capacity is provided by the refrigerators turbo-expanders - Magnet filling and cool-down from 4.5 K to 1.9 K: in a first step the liquefaction capacity of the 4.5 K refrigerators is used to partially fill the cold mass volume with saturated helium at 4.5 K up to the level determined by the position of the highest interconnecting pipe. The filling of the remaining dead volume and the cooling to 1.9 K are performed by using the 1.8 K refrigeration unit which first condenses the remaining gaseous helium at constant pressure of about 0.1 MPa and finally cools the liquid down to 1.9 K. - Resistive transition: The discharge of helium from the magnet cold mass is made possible, by means of 400 cold safety relief valves, normally closed, with their inlet under pressurized helium II. Such components, which are not available off-the-shelf, had to be developed by industry following CERN specifications and their performance assessed on a specially designed test facility. Cold gaseous helium is discharged and recovered into the dedicated 2 MPa gas storage vessels at ambient temperature located at surface premises in all LHC cryogenic points (see also section 4.1 in this article) - Warm-up from 4.5 K to 300 K: forced circulation of gaseous helium is used for the warming-up of the magnets to ambient temperature in 15 days (respecting a maximum thermal gradient of 75 K in the magnet structure) by means of electrical heaters with a capacity of 600 kw integrated into the interconnection boxes of the refrigeration plants. 2.6 Cooling Scheme (LHC lattice cell) The elementary LHC lattice cell 109.6 m-long is shown in Fig. 5. In order to maintain the furthest magnet of each sector below 1.9 K, each cell is composed of a single bath of static pressurized superfluid helium, cooled from a quasi-isothermal heat sink in the form of a bayonet heat exchanger running through the magnet string and in which the latent heat of the stratified two-phase flow of saturated helium II absorbs the applied heat loads. Operational experience on magnet temperature regulation during a typical LHC run in 2012, achieving the requirements, is presented in Fig. 6 in relation with the magnetic field ramping phases, the two circulation beams population and energy. Table 1 Cryogenic capacity requirements for steady state operating modes Mode Injection standby Low beam-intensity Nominal Ultimate Temperature level Sector type 50 75 K [kw] 4.6 20 K [W] 4.5 K LHe [W] 1.9 K LHe [W] 4 K VLP GHe [W] 20 300 K [g/s] Low-load 23 710 160 620 330 10 High-load 25 840 220 650 350 17 Low-load 23 720 180 910 330 20 High-load 25 850 260 940 350 32 Low-load 23 5320 240 1200 330 20 High-load 25 5440 320 1420 350 32 Low-load 23 13600 330 1330 330 20 High-load 25 13600 420 1840 350 32 594 TEION KOGAKU J. Cryo. Super. Soc. Jpn. Vol. 49 No. 12 2014

Fig. 5 Cryogenic flow-scheme of the LHC lattice cell. Table 2 Installed refrigeration capacity in LHC sectors Temperature level High-load Low-load sector sector 50 75 K [W] 33000 31000 4.6 20 K [W] 7700 7600 4.5 K [W] 300 150 1.9 K LHe [W] 2400 2100 4 K VLP [W] 430 380 20 280 K [g.s -1 ] 41 27 Fig. 6 Operational experience on magnet temperature evolution during a typical LHC run. 3. Helium Refrigeration Plants 3.1 Installed Refrigeration Capacity The installed refrigeration capacity 7) in the LHC sectors is summarized in Table 2 for both low and high-load sector type. One refrigeration plant consists of one 4.5 K refrigerator and one 1.8 K refrigeration unit. 3.2 4.5 K Refrigerators The refrigeration of the LHC sectors requires mixedduty operation of the cryogenic helium refrigerators, in order to fulfill a variety of isothermal and non-isothermal cooling duties. This amounts to a total equivalent entropic capacity of 144 kw at 4.5 K (eight refrigerators of 18 kw at 4.5 K each), thus making the LHC the world's most powerful helium refrigeration system. Among the 8 refrigerators, four have been ordered new in order to be coupled to the High load sectors thus completing the existing four recovered from the LEP accelerator, upgraded in capacity and coupled to the Low load sectors of the LHC. The new four refrigerators have been supplied by two industrial contractors (two refrigerators each), Air Liquide (France) and Linde Kryotechnik (Switzerland) and despite technological variants, both systems were designed to reach similar Claude cycle efficiency, near to 29%, corresponding to 49 12 2014 595

performance coefficient of 230 W per W 8). The refrigerators system consisting of a compressor station and a cold box is based on three pressure cycles (0.1-0.4-2 MPa). Each compressor station has five to eight oil-lubricated screw compressors, water refrigerant for helium and oil as well as oil removal system achieving final oil content of a fraction of ppm. The installed electrical input power is about 5 MW per refrigerator. The cold boxes are vacuum insulated, housing the aluminum plate-fin heat exchangers and from 8 to 10 turbo-expanders, providing the cooling capacity. Switchable dryers are connected at the ambient temperature cold box inlet to remove humidity. Additionally in the cold boxes, switchable 80 K absorbers remove up to 50 ppm of air and one 20 K adsorber removes remaining traces of hydrogen and neon. 3.3 1.8 K Refrigeration Units In order to fulfill the LHC requirements, the efficient production of large 1.8 K refrigeration capacity 9, 10) in the kw range was a challenge, successfully achieved by means of combined thermodynamic cycles making use of sub-atmospheric cryogenic compressors and heat exchangers. In order to allow the development and validation of the related technologies, CERN has procured from industry several low-pressure heat-exchangers and hydrodynamic compressors each designed for a flow-mass of 18 g/s at 1 kpa suction pressure and a pressure ratio of 3. Design and optimization studies have been conducted in collaboration with industry in order to achieve the required full performance of the refrigeration cycles thus allowing the final successful procurement and installation of eight 2.4 kw at 1.8 K refrigeration units from two suppliers, namely, Air Liquide (France) and IHI (Japan) - Linde Kryotechnik (Switzerland) consortium. Each supplier has provided four 1.8 K refrigeration units. All eight 1.8 K refrigeration units are respectively combined with the eight 4.5 K refrigerators thus allowing the required cooling capacity production at 1.8 K. As shown in Fig. 7, for both industrial suppliers, the 1.8 K refrigeration units are composed of a cold compressor box (QURC) and of a warm compression station (QSCC). A set of 3 or 4 centrifugal cold compressors (CC) delivers gas through heat exchangers to screw compressors (WC), at a pressure between 35 and 60 kpa (350 and 600 mbar) to minimize the volumetric capacity of the latter. The cold compressors are equipped with active magnetic bearings operated at ambient temperature with rotational speeds from 200 to 800 Hz for the warmest stages. As the return temperature to the 4.5 K refrigerator is set to 20 K, re-cooling after the heat Fig. 7 Flow-scheme of the 1.8 K refrigeration components and main parameters for both type as delivered by IHI-Linde consortium and Air Liquide suppliers. exchangers with turbo-expanders was necessary within the 1.8 K refrigeration units. The cooling capacity (i.e. the mass flow) to be achieved defines the delivery pressure of the volumetric warm compressors. Switchable 80 K adsorbers (ADS) remove up to 50 ppm of air. At the inlet of the cold compressor set, a chamber (M) is mixing pumping flow coming from the tunnel with cold 1.8 K saturated vapor or with warm 300 K helium in order to adjust the inlet conditions (flow and temperature) of the cold compressor set. Following the coupling of each 1.8 K refrigeration unit to its dedicated 4.5 K refrigerator, the overall performance coefficient at 1.8 K corresponds to 900 W per W. 4. Cryogen Storage and Management 4.1 Helium The overall helium inventory for the LHC accelerator amounts to 130 t. In normal operation, some 87 t are contained in the helium vessels of the superconducting magnets, 3 t in the refrigeration plants, and 40 t in the distribution and recovery lines running down the access shafts and along the accelerator tunnel. An additional quantity of 20 t of liquid helium (strategic storage) is permanently stored on the CERN premises, thus securing any immediate needs in case of accidental losses. Consequently the overall LHC helium inventory reaches 150 t. The gaseous helium storage capacity makes use of previously existing infrastructure recovered from the former LEP accelerator, in the form of vertical gas tanks of 80 m 3 capacity at 2.1 MPa maximum operating pressure. It was upgraded with the installation of 58 new horizontal gas tanks of 250 m 3 at 2.1 MPa maximum operating pressure. 596 TEION KOGAKU J. Cryo. Super. Soc. Jpn. Vol. 49 No. 12 2014

The liquid helium storage capacity was upgraded with the installation of six new low-loss horizontal liquid helium tanks of 120,000 litter each. This upgrade was performed in two distinctive phases with the initial installation of two units, prior to the LHC operation start and followed at a later stage by the installation of four additional units. The liquid helium storage vessels equip four of the five points around the machine circumference where large cryogenic plants are installed, so as to be refilled by these plants upon warm-up of the machine. Figure 8 summarizes the helium inventory storage infrastructure at CERN. The combination of the available gaseous and liquid helium storage capacities allows, if necessary during technical shutdowns, the storage of the full LHC helium inventory on the CERN premises. The 120,000 litter liquid helium vessels, with a boil-off rate of the order of 1% per day, however constitute only medium-term storage as long as they are not equipped, for the time being, with permanent re-liquefiers. In order to mitigate this issue, an additional option was implemented in the CERN helium supply contracts, allowing the management of part of the LHC accelerator helium inventory in partnership with the industrial suppliers, the so-called virtual storage for bridging the gap between the 130 ton inventory and the long-term storage capacity of about 40 tons in the gaseous tanks. The concept of virtual storage consists in to include in the helium supply contracts clauses for sending back helium to the market when emptying the LHC and being supplied with equivalent quantities upon re-cooling and refilling of the LHC. In case of scheduled or accidental shutdown of a Fig. 8 Helium inventory storage and infrastructure for the LHC accelerator (with typical pictures of equipment). 49 12 2014 597

refrigeration plant, the LHC cryogenic system allows the recovery of clean gaseous helium directly to the 250 m 3 horizontal tanks up to 2.1 MPa, thus avoiding capital investment and maintenance costs on recovery and high pressure swing purification systems. In addition to the implemented gaseous and liquid helium storage capacity for the LHC, a new pipeline for pure gaseous helium runs around the 26.7 km of the machine tunnel, rated at 2 MPa maximum operating pressure thus allowing inventory transfers from sector to sector according to the operational needs. 4.2 Liquid Nitrogen The pre-cooling of the full LHC accelerator from room temperature to below 100 K requires in total 10,000 ton of liquid nitrogen to be delivered by road transport to the CERN site, from regional air separation plants located within some 150 km. This requires about 500 standard insulated semi-trailers arriving at CERN and therefore represents a major logistic operation. However, as the LHC accelerator is sub-divided in 8 cryogenically independent sectors, each requiring 1,250 ton of liquid nitrogen for pre-cooling using its 600 kw nitrogen-to-helium vaporizer heat exchanger, the pre-cooling is usually performed in sequential mode by providing 2,500 ton of liquid nitrogen over 11 continuous days thus allowing the simultaneous pre-cooling of two adjacent LHC sectors. The liquid nitrogen storage capacity was upgraded accordingly with the installation on 10 new vertical containers of 50,000 litter capacity each, used as buffers between delivery from the semi-trailers and consumption of the vaporizers. 5. Cryogenic Operation 5.1 Operation and methodology The complexity of the LHC cryogenic system, the large variety of components in the refrigeration plants and ancillary equipment, required the implementation of a dedicated structure for operation and technical support activities. Table 3 summarizes the inventory of the main components of the LHC cryogenic system. The operation team (20 FTE persons) is assisted by four support teams in the domains of mechanics, electricity-industrial instrumentation-process controls, Table 3 Main components of the LHC refrigeration plants Helium screw compressors 64 Cold compressors 28 Expansion turbines 74 I/O signals 60,000 PLCs 120 PID control loops 4,000 metrology and a methodology unit caring for schedule, maintenance methods, logistics and cryogen distribution. Cryogenic operation is based on monitoring by shift work 24 h/7 days in the CERN Control Center, supported on request, by two layers of sequential operation stand-by duty teams for field intervention in case of long duration, one layer of maintenance stand-by duty, and one layer of instrumentation, metrology, electricity and controls stand-by duty service. With respect to the European industrial standards EN 15341, a digital condition was defined allowing the quantified evaluation of the cryogenic availability with respect to the requested accelerator service. A global Cryogenics-Maintain (CM) digital condition is established as the combination of all the required conditions from the eight cryogenically independent LHC sectors thus allowing the availability definition per unit of time. The systematic reporting and analysis of the CM losses allowed the identification and classification of the non-availability events according to their origin in the following four categories: Utilities (ELectrical network, Cooling & Ventilation, Information Technology network), Cryogenics, Cryogenics following a Single Event Upset (SEU) from high energetic neutrons, and Users (magnet resistive transitions or assimilated). This approach allowed the improvement of the operational procedures and an optimized tuning of the cryogenic settings. 5.2 Results Over the first three-year operation period of the LHC cryogenic system, the number of CM losses for less than 8 hours (qualified as short stops ) was drastically reduced by 42% thus demonstrating the constant progress made in availability improvement. Regarding the CM losses for more than 8 hours, qualified as long stops and considered as critical issue, major efforts have been concentrated in improving the overall availability and the recovery of CM nominal conditions in the shortest possible delay. Figure 9 summarizes the evolution of the long stops over the last three-year LHC run, illustrating the progress made in overall availability terms during the last year, in particular for the 4.5 K refrigerators and giving clear indication that the part of 1.8 K refrigeration units and related sub-atmospheric circuits has to be improved. As an immediate action during the recurrent short and end-of-the-year technical stops of the accelerator, mitigation actions have been conducted mainly in the repair of air leaks in sub-atmospheric circuits, the upgrade of 1200 electronic cards for temperature sensors affected 598 TEION KOGAKU J. Cryo. Super. Soc. Jpn. Vol. 49 No. 12 2014

Fig. 9 Statistics of the refrigeration plants long stops. by SEU (high energetic neutrons) and the consolidation of 1258 cooling valves for the current leads of the accelerator magnets. Concerning the protection against SEUs, during the first long shutdown, a major program (Radiation to Electronics, R2E) of equipment relocation was conducted in order to mitigate this important issue. With respect to the overall availability of the LHC cryogenic system and according to the origin of the availability losses classified in the four main categories as described in section 5.1, Fig. 10 illustrates the final global availability results, ending at 94.8% (all refrigeration plants combined) corresponding to a better that 99.3% availability for each of the eight refrigeration plants. 5.3 Helium inventory management The evolution of the helium losses during the first three-year physics run of the LHC, in tons and as a percentage expressed as fraction of the total inventory (strategic storage excluded), is summarized in Fig. 11. In 2010, the largest part of the helium losses were generated by the multiple interruptions of operation having their origin in cryogenic faults proper, in utility failures, in magnet resistive transitions, as well as instrumentation problems typically generated by high energetic stray neutrons affecting digital electronics (SEU, Single Event Upset). With the learning curve and experience acquired from 2010 run, the operational losses were reduced in 2011; increased losses have been however experienced during the first end-of-the-year technical stop due to the installation and commissioning of the additional liquid helium tanks in that period. Finally in 2012, strong efforts have been sustained in all three major domains of activity generating helium losses. An extensive campaign was conducted for localising all detectable leaks around the machine, the losses in normal operation slightly decreased and the losses during the second end-of-the-year technical stop were significantly reduced. Fig. 10 Global availability results of the LHC cryogenic system and origin of main losses. Fig. 11 Helium losses during the first three-years run. The combination of all three actions led to a substantial overall improvement, reducing the losses by nearly a factor of two, down to 16% of the inventory as shown in Fig. 11. 6. The First Long Shutdown During the first Long Shutdown (LS1) of the LHC accelerator scheduled in 2013 2014, the full preventive and corrective maintenance of the LHC cryogenic system was performed including the major overhauling at manufacturers premises for the compression stations (compressors and electrical motors). Identified leaks on several insulation vacuum sections have been repaired on the refrigeration plants infrastructure and ancillaries as well as in the cryogen distribution line in the tunnel. Based on the operational experience over the first three years run, specific equipment such as heating control electronic units, frequency drivers, magnetic bearing controllers, have been either consolidated, upgraded or relocated in order to cope with the Radiation to Electronics (R2E) latest measurements (thus allowing higher tolerance to high energetic neutrons and 49 12 2014 599

consequently improving the cryogenic availability for the accelerator). With respect to the large spectrum of the performed activities during the first long shutdown, a complete re-commissioning of the LHC cryogenic system is required and actually on-going, prior to the cool-down of the accelerator in preparation of the second three-years physics run expected to start early in 2015. 7. Conclusion The design, procurement, installation and commissioning of the LHC cryogenic system was the successful result of several years of multidisciplinary research and development activities at CERN, the collaborating National Institutes and the specialized industry thus allowing the complete and successful validation of the challenging and innovative technical choices for the LHC accelerator cooling scheme. With the installation and operation of the LHC accelerator, the cryogen inventory at CERN has drastically increased and the storage facilities have been accordingly upgraded in order to fulfill the operational and shutdown requirements. Immediately from the first year of cryogenic operation, the overall availability reference baseline of 90% was achieved. Common efforts from the operation, technical and industrial support teams allowed during the last year of the first three-years run to reach a global availability result of 94.8% (all refrigeration plants combined) corresponding to a better than 99.3% availability for each of the eight cryogenically independent sectors and their dedicated refrigeration plant. Undoubtedly, judging from the first operational experience and the availability results obtained, the LHC cryogenic system has successfully fulfilled all the accelerator technical requirements and necessary conditions for a highly reliable operation. Acknowledgment The authors wish to warmly thank all their colleagues from the operation, technical and industrial support teams, involved in the installation, commissioning, operation, maintenance and consolidation of the LHC cryogenic system at CERN, each member of these teams being co-responsible of the good performance obtained. References 1) Ph. Lebrun: The Large Hadron Collider, A Mega science Project, CERN-LHC-Project-Report-374 Geneva, 5 Apr. 2000 2) Ph. Lebrun: Cryogenics for the Large Hadron Collider, IEEE Trans. Appl. Supercond. 10 (2000) 1500-1506 3) L. Tavian: Large Cryogenics Systems at 1.8 K, CERN-LHC- Project-Report-412, Geneva, 23 Sep. 2000 and Austrian Academy of Sciences Press, 2002, EPAC (2000) 212-216 4) Ph. Lebrun: Superfluid helium cryogenics for the Large Hadron Collider project at CERN, Proc. ICEC15 Genova, Butterworth-Heinemann (1994) 1-8 5) The LEP design report Vol. II, The LEP main ring, CERN Report LEP/84-01 6) The LEP design report Vol. III, LEP2, CERN Report AC/96-01 7) Ph. Lebrun, G. Riddone, L. Tavian and U. Wagner: Demands in refrigeration capacity for the Large Hadron Collider, Proc. ICEC16, Elsevier Science, Oxford, UK (1997) 95-98 8) S. Claudet, Ph. Gayet, Ph. Lebrun, L. Tavian and U. Wagner: Economics of large helium cryogenic systems: experience from recent projects at CERN, CEC-ICMC 99, Montreal, Canada (1999) 9) Ph. Lebrun, L. Tavian and G. Claudet: Development of large-capacity refrigeration at 1.8 K for the Large Hadron Collider, Proc. Kryogenika'96, Icaris, Praha, Czech Republic (1996) 54-59 10) S. Claudet, Ph. Lebrun and L. Tavian: Towards cost-to-performance optimisation of large superfluid helium refrigeration systems, CERN-LHC-LHC-Project-Report-391, Geneva, 27 Jun. 2000 and ICEC18 (2000), Mumbai, India Dimitri DELIKARIS Dimitri Delikaris holds a doctorate in High Energy Physics from the University of Paris XI. He joined CERN (the European Organisation for Nuclear Research) in 1988 as member of the DELPHI collaboration on the LEP electron-positron collider. In 1991, he joined CERN s Cryogenics group, and as section leader was responsible for the cryogenics operation of the LEP (ALEPH & DELPHI experiments and accelerators sc magnets) as well as for the SPS fixed target detectors. In 1999, he designed the cryogenic helium refrigeration system for the sc solenoid magnet of the CMS experiment installed in the LHC (Large Hadron Collider) accelerator. Over the last fifteen years, he is managing the cryogen (helium and nitrogen) supply at CERN. Since 2000, he is Deputy-Head of the Cryogenics group at CERN and acts in parallel since 2009 as Industrial Services Coordinator (Field Support Units) for the Organisation. Laurent TAVIAN Laurent Tavian holds a Master in engineering from L Ecole Nationale Supérieure des Arts et Métiers. He started his professional career at the Commissariat à l Energie Atomique (CEA) where he partcipated to the construction, the commisioning and the operation of the cryogenic system of the Tore Supra Tokamak. He joined CERN (the European Organisation for Nuclear Research) in 1990 as member of the Cryogenics Group. He first participated to the design and development of the LHC cryogenic system and its superfluid helium refrigeration. In 1998 he became the leader of a Section in charge of the LHC cryogenic plants and infrastructure. In 2002, he became the Leader of the Cryogenic Group for Accelerator in charge of the construction and commissioning of the LHC cryogenic system. Since 2008, he is at the Head of the CERN Cryogenics Group in charge of overall CERN cryogenic activities. 600 TEION KOGAKU J. Cryo. Super. Soc. Jpn. Vol. 49 No. 12 2014