G. Ferlin

Commissioning the cryogenic system of the first LHC sector

The LHC machine, composed of eight sectors with superconducting magnets and accelerating cavities, requires a complex cryogenic system providing high cooling capacities (18 kW equivalent at 4.5 K and 2.4 W at 1.8 K per sector produced in large cold boxes and distributed via 3.3-km cryogenic transfer lines). After individual reception tests of the cryogenic subsystems (cryogen storages, refrigerators, cryogenic transfer lines and distribution boxes) performed since 2000, the commissioning of the cryogenic system of the first LHC sector has been under way since November 2006. After a brief introduction to the LHC cryogenic system and its specificities, the commissioning is reported detailing the preparation phase (pressure and leak tests, circuit conditioning and flushing), the cool-down sequences including the handling of cryogenic fluids, the magnet powering phase and finally the warm-up. Preliminary conclusions on the commissioning of the first LHC sector will be drawn with the review of the critical points already solved or still pending. The last part of the paper reports on the first operational experience of the LHC cryogenic system in the perspective of the commissioning of the remaining LHC sectors and the beam injection test.

Performance Assessment of 35 Cold Hydrodynamic Compressors for the 1.8 K Refrigeration Units of the LHC

The cooling capacity below 2 K for the superconducting magnets in the Large Hadron Collider (LHC) at CERN will be provided by eight refrigeration units of 2400 W at 1.8 K, each of them coupled to one 18 kW at 4.5 K refrigerator. The supply of the series units was linked to successful testing and acceptance of the pre‐series units delivered by the two selected vendors. The two pre‐series units were temporarily installed in a dedicated test station to validate the overall capacity and to properly assess the performance of specific components such as the cold compressors. The cold‐compressor cartridges to be installed in the following six series, and the associated spare cartridges, were intensively and systematically tested in the test station. After a brief description of the test bench and the main achieved features of the pre‐series units, we will present the results of the tests of 35 cold‐compressor cartridges. These tests show isentropic efficiency in the 75 % range, with excellent reproducibility and...

A Facility for Accurate Heat Load and Mass Leak Measurements on Superfluid Helium Valves

The superconducting magnets of the Large Hadron Collider (LHC) will be protected by safety relief valves operating at 1.9 K in superfluid helium (Hell). A test facility was developed to precisely determine the heat load and the mass leakage of cryogenic valves with Hell at their inlet. The temperature of the valve inlet can be varied from 1.8 K to 2 K for pressures up to 3.5 bar. The valve outlet pipe temperature can be regulated between 5 K and 20 K. The heat flow is measured with high precision using a Kapitza-resistance heatmeter and is also crosschecked by a vaporization measurement. After calibration, a precision of 10 mW for heat flows up to 1.1 W has been achieved. The helium leak can be measured up to 15 mg/s with an accuracy of 0.2 mg/s. We present a detailed description of the test facility and the measurements showing its performances.

First assessment of reliability data for the LHC accelerator and detector cryogenic system components

The Large Hadron Collider (LHC) cryogenic system comprises eight independent refrigeration and distribution systems that supply the eight 3.3 km long accelerator sectors with cryogenic refrigeration power as well as four refrigeration systems for the needs of the detectors ATLAS and CMS. In order to ensure the highest possible reliability of the installations, it is important to apply a reliability centred approach for the maintenance. Even though large scale cryogenic refrigeration exists since the mid 20th century, very little third party reliability data is available today. CERN has started to collect data with its computer aided maintenance management system (CAMMS) in 2009, when the accelerator has gone into normal operation. This paper presents the reliability observations from the operation and the maintenance side, as well as statistical data collected by the means of the CAMMS system.

5-year operation experience with the 1.8 K refrigeration units of the LHC cryogenic system

Since 2009, the Large Hadron Collider (LHC) is in operation at CERN. The LHC superconducting magnets distributed over eight sectors of 3.3-km long are cooled at 1.9 K in pressurized superfluid helium. The nominal operating temperature of 1.9 K is produced by eight 1.8-K refrigeration units based on centrifugal cold compressors (3 or 4 stages depending to the vendor) combined with warm volumetric screw compressors with sub-atmospheric suction. After about 5 years of continuous operation, we will present the results concerning the availability for the final user of these refrigeration units and the impact of the design choice on the recovery time after a system trip. We will also present the individual results for each rotating machinery in terms of failure origin and of Mean Time between Failure (MTBF), as well as the consolidations and upgrades applied to these refrigeration units.

Operational Experience with the 1.75 K Tore Supra Refrigerator

The Tore Supra refrigerator is the fifth machine in the world operating at approximately 1.8 K but with a refrigeration power of 320 W at 1.75 K. Its principal characteristics are: the compression of helium at low temperature, a large number of operating modes, an entirely automatic control system.

Beam screen cryogenic control improvements for the LHC run 2

This paper presents the improvements made on the cryogenic control system for the LHC beam screens. The regulation objective is to maintain an acceptable temperature range around 20 K which simultaneously ensures a good LHC beam vacuum and limits cryogenic heat loads. In total, through the 27 km of the LHC machine, there are 485 regulation loops affected by beam disturbances. Due to the increase of the LHC performance during Run 2, standard PID controllers cannot keeps the temperature transients of the beam screens within desired limits. Several alternative control techniques have been studied and validated using dynamic simulation and then deployed on the LHC cryogenic control system in 2015. The main contribution is the addition of a feed-forward control in order to compensate the beam effects on the beam screen temperature based on the main beam parameters of the machine in real time.

Cryogenics for HL-LHC

The discovery of a Higgs boson at CERN in 2012 is the start of a major program of work to measure this particles properties with the highest possible precision for testing the validity of the Standard Model and to search for further new physics at the energy frontier. The LHC is in a unique position to pursue this program. Europes top priority is the exploitation of the full potential of the LHC, including the high-luminosity upgrade of the machine and detectors with an objective to collect ten times more data than in the initial design, by around 2030. To reach this objective, the LHC cryogenic system must be upgraded to withstand higher beam current and higher luminosity at top energy while keeping the same operation availability by improving the collimation system and the protection of electronics sensitive to radiation. This chapter will present the conceptual design of the cryogenic system upgrade with recent updates in performance requirements, the corresponding layout and architecture of the system as well as the main technical challenges which have to be met in the coming years.

COMMISSIONING OF THE CRYOGENICS OF THE LHC LONG STRAIGHT SECTIONS

The LHC is made of eight circular arcs interspaced with eight Long Straight Sections (LSS). Most powering interfaces to the LHC are located in these sections where the particle beams are focused and shaped for collision, cleaning and acceleration. The LSSs are constituted of several unique cryogenic devices and systems like electrical feed‐boxes, standalone superconducting magnets, superconducting links, RF cavities and final focusing superconducting magnets. This paper presents the cryogenic commissioning and the main results obtained during the first operation of the LHC Long Straight Sections.

1.9 K HEAT INLEAK AND RESISTIVE HEATING MEASUREMENTS ON LHC CRYOMAGNETS

The superconducting magnets of the Large Hadron Collider (LHC) distributed over eight sectors of 3.3‐km long are cooled at 1.9 K in pressurized superfluid helium. During the commissioning campaign of the sectors in 2008, cold standby periods at nominal operating temperature have allowed to measure the overall static heat inleaks reaching the magnet cold masses at 1.9 K by enthalpy balance in steady‐state operation. In addition, during electrical powering of the different magnet circuits, helium II calorimetry based on precision thermometry has been implemented to assess with an accuracy of 100 mW/m the additional heat loads due to resistive heating and to detect possible abnormal heat dissipation during powering.This paper describes the method applied to perform these measurements, compares the results with the expected specified values and discusses the impact of the measured values on cryo‐plant tuning and operational margins.