Figure 1 The LHC cryogenic islands and plants layout

The LHC cryogenic operation for first collisions and physics run Brodzinski K, Barth K, Benda V, Bremer J, Casas-Cubillos J, Claudet S, Delikaris D, Ferlin G, Fernandez Penacoba G, Perin A, Pirotte O, Soubiran M, Tavian L, van Weelderen R, Wagner U Technology Department, CERN, CH-1211 Geneva 23, Switzerland The Large Hadron Collider (LHC) cryogenic system was progressively and successfully run for the LHC accelerator operation period starting from autumn 2009. The paper recalls the cryogenic system architecture and main operation principles. The system stability during magnets powering and availability periods for high energy beams with first collisions at 3.5 TeV are presented. Treatment of typical problems, weak points of the system and foreseen future consolidations will be discussed. INTRODUCTION LHC CRYOGENIC SYSTEM ARCHITECTURE The Large Hadron Collider [1], successfully re-commissioned in 2009, makes an extensive use of superconducting magnets located in the underground tunnel of about 27-km length. The accelerator ring is divided into eight sectors, each of about 3.3 km long. The LHC cryogenic system is designed to reach an equivalent cooling power of 144 kw at 4.5 K and requires for operation about 130 tons of helium. Most of superconducting magnets are operated in superfluid helium at 1.9 K. The LHC refrigeration capacity is provided by using eight large cryogenic plants installed on five cryogenic islands which are placed around the LHC ring circumference see Figure 1. Because of the integration constrains and design optimization a part of the cryogenic plant equipment is installed on the surface and second part underground [2]. Figure 1 The LHC cryogenic islands and plants layout Each plant consist a warm compressor station, 4.5 K main refrigerator, 1.8 K refrigeration unit and makes use of a cryogenic interconnection box. A local helium distribution system allows for helium transfer between cryogenic equipments within a cryogenic plant area. To distribute the helium to the magnet cooling loops eight independent cryogenic distribution lines (QRL) are used [3]. The complete LHC cryogenic system architecture is presented in Figure 2. 1

Figure 2 The LHC cryogenic architecture OPERATIONAL STABILITY The cool down process consists of three main phases [4, 5]. The first phase from ambient temperature to 80 K is performed by vaporizing large quantities of liquid nitrogen which pre-cools the helium flow through the refrigerator towards sector. This phase takes now approximately 21 days with about 1200 t of liquid nitrogen consumption per sector. The second phase from 80 to 4.5 K is performed using the cooling capacity of the 4.5 K refrigerators. The last third phase from 4.5 K to 1.9 K is carried out using the cooling capacity of the 1.8 K refrigeration units. The LHC magnet temperature evolution from first cool down in 2007 to the first long term beam operation, particle collisions to the current physics run is presented in Figure 3. Figure 3 The LHC sector cool down and magnet temperature evolution between 2007 and 2010 2

After more than one year of consolidation (due to the incident which took place on 19 September 2008 and caused damage to about 400 m of the accelerator [6]) the LHC was successfully re-commissioned and cooled to the nominal temperature. The temperature stability of the magnets operated with superfluid helium at 1.9 K remains within tolerance +10/-70 mk for the whole LHC. The gained experience over last 3 years and applied process optimizations, like improved sequencing and better use of the cooling capacity, allowed to reduce the total cool down time of one sector from 10 weeks to about 5 weeks. Following careful analysis CERN is confident that the present status of the LHC machine allows safe operation up to 50% of the nominal accelerator energy (present operation: 3.5 TeV per beam, nominal energy: 7 TeV per beam). This operation mode needs a reduced refrigeration capacity with respect to nominal operation. In order to minimize the electrical energy consumption, the cooling capacity for two sectors on cryogenic islands P6 and P8 may be provided by only one refrigeration plant. This is possible mainly by using redundant functions foreseen in the interconnection boxes. Stopping of two cryogenic plants allows to pull away the periodic maintenances of these installations. This optimization allows a saving of about 10 MW of the electrical power i.e. 15000 CHF/day. Due to the specific architecture of P1 and P1.8 as well as to higher load at P4 related to the RF cavities cooling, the reduced refrigeration scheme cannot be applied for these cryogenic islands. AVAILABILITY The electrical powering of the superconducting magnets and by consequence the accelerator operation depends strongly on provided cryogenic conditions of different machine equipment. The LHC cryogenic availability is defined by two conditions: Cryo Start (CS) and Cryo Maintain (CM). CS allows to start the electrical circuits powering, CM allows to keep the LHC equipment electrically powered. The CS and CM conditions depends on operation limits defined for different cryogenic equipment e.g. liquid helium level in electrical feedboxes, maximum and minimum pressures or temperatures in the RF cavities or magnets etc. The limits are stricter for CS condition see Figures 4a and 4b. The requirements to obtain correct CS and CM conditions are defined for single cryogenic equipment and for specific groups of equipments such as e.g. a sector. The loss of one single CS causes loss of general CS condition in which it contributes (cascade principle). There are about 3500 single CS/CM conditions affecting the LHC operation. Figure 4a Simple CS and CM indicator Figure 4b Cryogenic availability for electrical powering with different CS and CM statuses. When discussing the cryogenic availability for the accelerator operation, more attention is paid on the continuity of CM condition. The CS has to be ensured only for starting period and then is more used as the system stability diagnostics. Simple loss of CS condition is automatically phone-reported to the cryogenic operator. The loss of CM condition leads to direct slow discharge of electrical circuits and consequently to dump of the beam (if any). The curves in Figure 5 present the cryogenic availability in terms of CM condition. Over last 6-month period the weekly availability increased from ~50% to ~90% and monthly availability from ~45% to ~85% (including periodical technical stops needed to perform the necessary machine 3

maintenance). This good system behaviour is a result of operation experience return as well as periodic corrective and preventive maintenance executed on the cryogenic equipment. The availability calculation is done for all eight LHC sectors. Single weekly sector availability is about 98.5% what gives a total weekly LHC cryogenics availability at about 90%. Figure 5 The cryogenic availability for the LHC physics over last 6-month period TYPICAL PROBLEMS AND THEIR TREATMENT The cryogenic operation of the LHC gives currently a very good feedback in terms of the process stability and availability for the beam operation. Nevertheless the plants still have to face with different operation difficulties. A few examples of typical problems or failures are presented and discussed below. Instrumentation failures The LHC cryogenic system is equipped with about 10000 thermometers, ~700 pressure transducers, ~500 level transducers, ~1400 cryogenic control valves, ~2500 electrical heaters and other instrumentation like flow meters or current lead valves. The observed progressive degradation of the instrumentation is followed by the operation team and instrumentation experts. The repair of the defective instrumentation is done periodically during the accelerator technical stops, with exceptions of the indispensable instrumentation failures which are repaired as soon as possible. The most frequent instrumentation failures are related to the electronic cards. Since the beginning of 2008 about 140 cards have been replaced what gives about 1% of the total instrumentation inventory. The replacement was done mostly on the beginning of the operation period treating starting weaknesses. Cold compressor filter clogging The function of the 1.8 K refrigeration units (QURC) is to pump on helium bath to guaranty a saturation temperature of 1.8 K. The very low pressure pumping line is working at ~16 mbar and has a length of ~ 3.3 km per sector. In order to protect the cold compressor itself from any impurities, helium guard systems have been put in place on all sensitive interfaces with the atmospheric pressure. Nevertheless a small part of the system, some small fully joint welded pipes or capillaries connecting low pressure line with diagnostics, calibration or purge ports, have a direct surrounding of the atmospheric pressure creating potential risk of the impurities aspiration. In order to protect the cold compressor inlet from any type of the impurities dedicated filters are installed upstream of the cold compressor station on each cryogenic plant. The progressive clogging 4

of these filters was observed due to unexpected entry of air impurities. Such impurities cause increase of the pressure drop over the filters and consequently increase of the magnet temperature. The periodic cleaning of the filters was required by means of local warm helium circulation. The increase of the temperature to about 80 K is enough to clean completely the filter. This type of cleaning requires the stop of the QURC and creates additional unavailability. The time related to the filter cleaning and recovery of the nominal conditions is about 18 24 hours. The evolution of all QURC filters clogging over last few months period is presented in Figure 6. Figure 6 QURCs filter blockage over last 6-month period During periodical technical stops dedicated to machine maintenance the problem of the QURC filters clogging was progressively resolved on all LHC cryogenic plants see evolution on Figure 6. The present filters autonomies (max. allowable filter pressure drop is 15 mbar) is about 10 weeks, what is fully compatible with regular technical stops scheduled presently every 6 weeks. Further improvements are underway. Helium losses The helium losses since the first cool down in 2007 are estimated at about 40 t/year. This gives about 30% of the total LHC inventory and remains within the estimates: 50% of the total inventory during commissioning time and 25% during stable operation. The investigation and related actions to minimize the losses are underway. Electrical feedbox valves Another hardware problem of the LHC cryogenics appeared in the control of the cooling helium flow of the current leads. There are 1258 current leads in the LHC. Each lead is equipped with a valve which controls its cooling flow. The design of the valves turned out to be not adapted for LHC application already during the commissioning period. After a few weeks of operation, some of the valves develop excessive friction in the valve bearings, which makes difficult and in some cases impossible to achieve the control requirements. To follow the degradation and treat the problem with minimum impact on the availability a special valve degradation road map has been created. The road map includes 4 steps with corresponding instructions how to mitigate the effects of the progressive valves degradation. The main action is to apply specific PID parameters to the valve controller. This solution is applicable for the valves classified at step 1, 2 and 3 on the degradation roadmap. The actions for the first 3 steps can be performed without field 5

intervention, directly from the control system without any impact on the operational availability. The valves that reach degradation step 3 are classified to be replaced during the nearest technical stop. When the actions on the control parameters are not sufficient, before replacing the valve, an intermediate step involves a special hardware tuning of the valve electronics. This solution was validated but until now it was never applied as all cases could be treated at the control system level. In parallel to the mentioned short term treatment, the progressive definitive replacement of the valves with a new design is planned. A few valves with the new, friction free, design were tested with very good response in real LHC operation since February 2010. The general evaluation of the valves degradation is presented in Figure 7. Figure 7 The DFB valves degradation evolution over last 16-week period CONCLUSIONS The LHC cryogenic operation is successfully running since autumn 2009. The first long term operation with maximum beam energy of 3.5 TeV as well as related collisions has no visible impact on the cryogenic system stability and availability. The thermal stability is provided within tolerance +10/-70 mk regarding all LHC 1.9 K magnets (still being improved). After a few weeks of operation the weekly availability reached the target of 90% and remains at this level within ~5% of tolerance. For the time being, the periodical technical stops of 3-4 days duration are needed each 4-6 weeks in order to perform the necessary maintenance of the LHC technical systems including cryogenics to guaranty maximum availability during foreseen operation periods and physics run. The very positive response of the cryogenic system let us hope for reliable long time operation of the system with present beam energy and also for the future run at nominal beam and collision parameters. REFERENCES 1. LHC Design Report, CERN-2004-003. 2. Claudet S., Design, construction, installation and first commissioning results of the LHC cryogenic system, 10 th European Particle Accelerator Conference, Edinburgh 2006, United Kingdom. 3. Brodzinski K., Chorowski M., Ciechanowski M., Fluder C., Fydrych J. et al., Reception tests of the cryogenic distribution line for the Large Hadron Collider, 21th International Cryogenic Engineering Conference, Praha 2006, Czech Republic. 4. Liu L., Riddone G., Serio L. And Tavian L., Cooldown of the first sector of the Large Hadron Collider: comparison between mathematical model and measurements, 2007 Cryogenic Engineering Conference and International Cryogenic Materials Conference, Chattanooga, Tennessee, USA. 5. Serio L., Bouillot A., Casas-Cubillos et al., Validation and performance of the cryogenic system through commissioning of the first sector, 2007 Cryogenic Engineering Conference and International Cryogenic Materials Conference, Chattanooga, Tennessee, USA. 6. Report of the Task Force on the incident of 19 th September 2008 at the LHC, CERN-LHC-PROJECT-Report-1168. 2009. 6