Mirko Pojer

Impact of the Voltage Transients After a Fast Power Abort on the Quench Detection System in the LHC Main Dipole Chain

A Fast Power Abort in the LHC superconducting main dipole circuit consists in the switch-off of the power converter and the opening of the two energy-extraction switches. Each energy-extraction unit is composed of redundant electro-mechanical breakers, which are opened to force the current through an extraction resistor. When a switch is opened arcing occurs in the switch and a voltage of up to 1 kV builds up across the extraction resistor with a typical ramp rate of about 80 kV/s. The subsequent voltage transient propagates through the chain of 154 dipoles and superposes on the voltage waves caused by the switch-off of the power converter. The resulting effect caused intermittent triggering of the quench protection systems along with heater firings in the magnets when the transient occurred during a ramp of the current. A delay between power converter switch-off and opening of the energy-extraction switches was introduced to prevent this effect. Furthermore, the output filters of the power converters were modified in order to damp faster the voltage waves generated after the power-converter switch-off and to lower their amplitude. Finally, snubber capacitors were added in parallel to the extraction switches to help the commutation process by reducing the arcing effect and thus smoothing the voltage transient. A set of dedicated tests has been performed in order to understand the voltage transients and to assess the impact of the circuit modifications on the quench detection system. The results have been compared to the simulations of an electrical model of the LHC main dipole circuit developed with the Cadence suite (PSpice based).

Quench performance and field quality of the LHC preseries superconducting dipoles

The preseries production of the LHC main superconducting dipoles is presently being tested at CERN. The foremost features of these magnets are: twin structure, six block two layer coils wound from 15.1 mm wide graded NbTi cables, 56 mm aperture, polyimide insulation and stainless steel collars. The paper reviews the main test results of magnets tested to day in both normal and superfluid helium. The results of training performance, magnet protection, electrical integrity and the field quality are presented in terms of the specifications and expected performance of these magnets in the future accelerator.

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.

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.

Chapter 16 Commissioning and Operation

At the point of commissioning and subsequent operation of the HL-LHC, the LHC itself will have been operational for over 10 years and a wealth of knowledge and experience will have been built up. The key operational procedures and tools will have been well established. The understanding of beam dynamics will be profound and refined by relevant measurement and correction techniques. Key beam-related systems will have been thoroughly optimized and functionality sufficiently enhanced to deal with most challenges up to that point. Availability will have been optimized significantly across all systems. This collected experience will form the initial operational basis following the upgrade.

A Non-Linear Finite Element Model for the LHC Main Dipole Coil Cross-Section

The production of the dipole magnets for the Large Hadron Collider is at its final stage. Nevertheless, some mechanical instabilities are still observed for which no clear explanation has been found yet. A FE modelization of the dipole cold mass cross-section had already been developed at CERN, mainly for magnetic analysis, taking into account conductor blocks and a frictionless behavior. This paper describes a new ANSYS model of the dipole coil cross-section, featuring individual turns inside conductor blocks, and implementing friction and the mechanical non-linear behavior of insulated cables. Preliminary results, comparison with measurements performed in industry and ongoing developments are discussed

Conductor limited quenches of LHC superconducting main dipoles

In the framework of the series tests of superconducting magnets for the LHC, a special procedure was developed at CERN to perform conductor limited quenches at temperatures around 4.4 K. All results obtained on pre-series and series main dipoles tested to date will be presented with their analysis. These quenches allow fine diagnostics concerning the electrical integrity of the conductors and of the splices. They also allow the determination for each magnet of the temperature margin at nominal operating conditions of the LHC at superfluid helium. The comparison between the quench current and the critical current directly measured on short samples of superconducting cables used for the winding is discussed.

Performance of the first LHC pre-series superconducting dipoles

Within the LHC magnet program, a preseries production of final design, full-scale superconducting dipoles has presently started in industry and magnets are being tested at CERN. The main features of these magnets are: two-in-one structure, 56 mm aperture, six-block two layer coils wound from 15.1 mm wide graded NbTi cables, and all-polyimide insulation. This paper reviews the main test results of magnets tested to date in both supercritical and superfluid helium. The results of the quench training, conductor performance, magnet protection, sensitivity to ramp rate, and magnetic field quality are presented and discussed in terms of the design parameters and the aims of the LHC magnet program.

Status Report on the Superconducting Dipole Magnet Production for the LHC

In August 2006, about 95% of the production of the 1232 LHC superconducting dipole cold masses, whose coils are wound with Cu/Nb-Ti cables, is completed. One of the 3 manufacturers, having produced one third of the required magnets, completed its production in the end of 2005. The acceptance of the magnets takes place after the 1.9 K performance tests and has been issued for more then 1000 magnets so far. More then half of the dipole magnets are already installed in the tunnel. The paper reviews the main features of the dipoles, the most important steps of the manufacturing and the most critical operations. The quality control and the critical non-conformities that have led, for instance, to a swift campaign of investigations and repairs of few subcomponents (diode assembly, cold bore tube to welding flare fillet weld) are discussed. The status of the production and the performance of the tested dipoles will be presented. Finally the expected schedule for the completion of the production will be shown.

Experimental characterization of resistive joints for use inside ATLAS toroids

The authors have investigated, both experimentally and theoretically, the thermo-electrical behavior of the ATLAS magnets resistive joints. These magnets exploit an Al-clad NbTi Rutherford superconducting cable, and the splices between different sections are performed by TIG-welding the Al matrices of the two cables to be connected. This technique is simple from a construction point of view, and we have shown that its performance is adequate for a safe operation of the magnets. The two main concerns during the design of these joints are the temperature rise due to Joule dissipation and the eddy currents induced under nonstationary conditions. We have devised a reliable model of these joints, that allows estimating their resistances and the induced eddy currents; later we have built and measured several sample joints to give experimental confirmation. The model requires, along with the joint geometry, the knowledge of the Rutherford-matrix interface resistance as well as the RRR of the aluminum matrix. In this paper we present the latest experimental data about the joint specific resistances, confirming the first results, and independent measurements of the interface resistance and Al RRR. All these quantities are characterized as a function of an applied magnetic field between 0 and 4 T.