Antonio Perin

DESIGN, PRODUCTION AND FIRST COMMISSIONING RESULTS OF THE ELECTRICAL FEEDBOXES OF THE LHC

A total of 44 CERN designed cryogenic electrical feedboxes are needed to power the LHC superconducting magnets. The feedboxes include more than 1000 superconducting circuits fed by high temperature superconductor and conventional current leads ranging from 120 A to 13 kA. In addition to providing the electrical current to the superconducting circuits, they also ensure specific mechanical and cryogenic functions for the LHC. The paper focuses on the main design aspects and related production operations and gives an overview of specific technologies employed. Results of the commissioning of the feedboxes of the first LHC sectors are presented.

Consolidation of the 13 kA Interconnects in the LHC for Operation at 7 TeV

The accident in the LHC in September 2008 occurred in an interconnection between two magnets of the 13 kA dipole circuit. Successive measurements of the resistance of other interconnects revealed other defective joints, even though the SC cables were properly connected. These defective joints are characterized by a poor bonding between the SC cable and the copper stabilizer in combination with an electrical discontinuity in the copper stabilizer. A quench at the 7-13 kA level in such a joint can lead to a fast and unprotected thermal run-away and hence opening of the circuit. It has therefore been decided to operate the LHC at a reduced and safe current of 6 kA corresponding to 3.5 TeV beam energy until all defective joints are repaired. A task force is reviewing the status of all electrical joints in the magnet circuits and preparing for the necessary repairs. The principle solution is to resolder the worst defective joints and, in addition, to apply an electrical shunt made of copper across all joints with sufficient cross-section to guarantee safe 12-13 kA operation at 7-7.5 TeV. In this paper the various actions that have lead to this solution are presented.

Conceptual design of the Cryogenic Electrical Feedboxes and the Superconducting Links of LHC

Publisher Summary This chapter reviews the conceptual design of the cryogenic electrical feedboxes and the superconducting links of Large Hadron Collider (LHC). Powering the superconducting magnets of the LHC arcs and long straight sections is performed with more than 1,000 electrical terminals supplying currents ranging from 120 A to 13,000 A and distributed in 44 cryogenic electrical feedboxes (DFB). Where space in the LHC tunnel is sufficient, the magnets are powered by locally installed cryogenic electrical feedboxes. Where there is no space for a DFB, the current will be supplied to the magnets by superconducting links (DSL) connecting the DFBs to the magnets on distances varying from 76 m to 510 m. The complex task of powering the LHC superconducting magnets in the very limited underground available space will be performed with a combination of locally installed cryogenic electrical feedboxes and the use of superconducting links for the locations where the space limitations do not allow the installation of DFBs.

Validation and performance of the LHC cryogenic system through commissioning of the first sector

The cryogenic system [1] for the Large Hadron Collider accelerator is presently in its final phase of commissioning at nominal operating conditions. The refrigeration capacity for the LHC is produced using eight large cryogenic plants and eight 1.8 K refrigeration units installed on five cryogenic islands. Machine cryogenic equipment is installed in a 26.7-km circumference ring deep underground tunnel and are maintained at their nominal operating conditions via a distribution system consisting of transfer lines, cold interconnection boxes at each cryogenic island and a cryogenic distribution line. The functional analysis of the whole system during all operating conditions was established and validated during the first sector commissioning in order to maximize the system availability. Analysis, operating modes, main failure scenarios, results and performance of the cryogenic system are presented.

STUDY OF MATERIALS AND ADHESIVES FOR SUPERCONDUCTING CABLE FEEDTHROUGHS

Powering superconducting magnets requires the use of cryogenic feedthroughs for the superconducting cables capable of withstanding severe thermal, mechanical and electrical operating conditions. Such feedthroughs shall provide the continuity of the superconducting circuit while ensuring a hydraulic separation at cryogenic temperature. A study about the adhesive and polymers required for the production of thermal shock resistant feedthroughs is presented. The strength of the busbar to adhesive joints was first investigated by compression/shear tests as well as pin-and-collar tests performed with four epoxy adhesives. After the selection of the most appropriate adhesive, pin-and-collar tests were performed with four different polymers. Based on the results, a superconducting cable feedthrough for 6 busbars of 6 kA and 12 busbars of 120 A was constructed and successfully tested.

Status of the Consolidation of the LHC Superconducting Magnets and Circuits

The first LHC long shutdown (LS1) started in February 2013. It was triggered by the need to consolidate the 13 kA splices between the superconducting magnets to allow the LHC to reach safely its design energy of 14 TeV center of mass. The final design of the consolidated splices is recalled. 1695 interconnections containing 10 170 splices have to be opened. In addition to the work on the 13 kA splices, the other interventions performed during the first long shut-down on all the superconducting circuits are described. All this work has been structured in a project, gathering about 280 persons. The opening of the interconnections started in April 2013 and consolidation works are planned to be completed by August 2014. This paper describes first the preparation phase with the building of the teams and the detailed planning of the operation. Then, it gives feedback from the worksite, namely lessons learnt and adaptations that were implemented, both from the technical and organizational points of view. Finally, perspectives for the completion of this consolidation campaign are given.

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.

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.

Critical current densities at 77 K and 4.2 K of Bi(2223) tapes prepared by cold and hot deformation

Monofilamentary Bi(2223) tapes have been prepared by the Powder-In-Tube technique using various alternatives for the final thermomechanical processing. In particular, both cold and hot deformation processes were investigated and their critical current densities at 77 K and 4.2 K were measured. For long cold rolled Bi(2223) tapes, transport j/sub c/ values up to 30000 A/cm/sup 2/ were obtained at 77 K and 0T. The value of j/sub c/ was found to increase from 17000 A/cm/sup 2/ at the oxide center to 46000 A/cm/sup 2/ at the borders of the oxide layer. At 4.2 K and 28 T, j/sub c/ values of 45000 and 30000 A/cm/sup 2/ were measured for B parallel and perpendicular to the tape surface, respectively. The j/sub c/ (B) hysteresis was found to disappear at higher fields. The present state of hot deformed Bi(2223) tapes is presented. For rolling temperatures up to 850/spl deg/C, a maximum of j/sub c/ (77 K, 0T)=18000 A/cm/sup 2/ was observed at 800/spl deg/C, j/sub c/ being considerably lower for higher rolling temperatures. The results obtained so far show generally lower j/sub c/ values than for cold rolled tapes. This is essentially due to microcracks and to sausaging effects, which are more pronounced than for cold rolled tapes. For static hot deformation at 800/spl deg/C of short, pressed Bi(2223) tapes an enhancement of j/sub c/ (77 K, 0T) by 20% up to >40000 A/cm/sup 2/ was observed. In addition, the decrease of j/sub c/ for B perpendicular to the tape surface is less pronounced compared to cold deformed tapes.<<ETX>>

Consolidation of the LHC Superconducting Magnets and Circuits

The first Large Hadron Collider (LHC) Long Shutdown (LS1) started in February 2013. It was triggered by the need to consolidate the 13-kA splices between the superconducting magnets to allow the LHC to reach safely its design energy of 14 TeV center of mass. The Superconducting Magnets and Circuits Consolidation (SMACC) project has principally covered the consolidation of the 10170 13-kA splices but also other activities linked to the superconducting magnets such as the exchange of 18 main cryomagnets, the installation of the additional safety relief devices, the repair of known helium leaks, and other consolidation activities. All these works have been structured in a project, gathering about 280 persons. The opening of the interconnections started in April 2013 and consolidation works were completed by September 2014. This paper first describes the preparation phase with the building of the teams and the detailed planning of the operations. Then, this paper carried out is summarized, and the main results achieved are presented. Finally, it gives feedback from the worksite, namely lessons learnt and adaptations that were implemented, both from the technical and organizational points of view.