P. Hagen

Second Generation Coil Design of the Nb3Sn low-beta Quadrupole for the High Luminosity LHC

As part of the Large Hadron Collider (LHC) Luminosity upgrade program, the U.S.-LHC Accelerator Research Program collaboration and CERN are working together to design and build a 150-mm aperture Nb3Sn quadrupole for the LHC interaction regions. A first series of 1.5-m-long coils was fabricated and assembled in a first short model. A detailed visual inspection of the coils was carried out to investigate cable dimensional changes during heat treatment and the position of the windings in the coil straight section and in the end region. The analyses allow identifying a set of design changes which, combined with a fine tune of the cable geometry and a field quality optimization, were implemented in a new second-generation coil design. In this paper, we review the main characteristics of the first generation coils, describe the modification in coil layout and discuss their impact on parts design and magnet analysis.

Trends in Field Quality Along the Production of the LHC Dipoles andDifferences Among Manufacturers

More than two thirds of the dipoles of the Large Hadron Collider have been manufactured and their magnetic field has been measured at room temperature. In this paper we make a review of the trends that have been observed during the production. In some cases, the trends were traced back to displacements of conductors with respect to the nominal lay-out. The analysis allows detecting the most critical zones in the superconducting coil as far as field quality is concerned. The second part of the paper makes the point of the observed differences in field quality between the three manufacturers. The analysis allows evaluating which multipoles are more affected, what magnitudes of displacements are necessary to explain these differences (the manufacturers all producing the same baseline), and what could be the origin of such differences

The LHC Main Quadrupoles During Series Fabrication

By the end of August 2005 about 320 of the 400 main LHC quadrupole magnets have been fabricated and about 220 of them assembled into their cold masses, together with corrector magnets. About 130 of them have been cold tested in their cryostats and most of the quadrupoles exceeded their nominal excitation, i.e. 12,000 A, after no more than two training quenches. During this series fabrication, the quality of the magnets and cold masses was thoroughly monitored by means of warm magnetic field measurements, of strict geometrical checking, and of various electrical verifications. A number of modifications were introduced in order to improve the magnet fabrication, mainly correction of the coil geometry for achieving the specified field quality and measures for avoiding coil insulation problems. Further changes concern the electrical connectivity and insulation of instrumentation, and of the corrector magnets inside the cold masses. The contact resistances for the bus-bar connections to the quench protection diodes and the elimination of insulation problems of the main bus-bars required special attention. To this must be added actions for solving of interface problems to the neighboring magnets in the machine and to the cryogenic feed line

Testing Results for Nb-Ti, 120-mm-Aperture, Low-B Quadrupole Models for the LHC High-Luminosity Insertion

The design and construction of a 120-mm wide-aperture, Nb-Ti superconducting quadrupole magnet for the Large Hadron Collider (LHC) insertion region is part of a study towards a luminosity upgrade of the LHC at CERN, envisaged for 2020-22. The main challenges for this accelerator quality magnet are to operate reliably with the high heat and radiation loads that are predicted in the insertion magnet regions. Calculations give approximately 500 Watts over the 30-m-long string of insertion magnets, while today LHC is operating for a nominal heat load of 12 Watts. To extract this heat, the model magnets incorporate new features: Open cable insulation, open ground insulation, open magnet structure, and a quench heater that has open channels to help extract the steady state heat load. This paper presents results from tests at room temperature and 1.8 K for the initial model magnet. We report magnet training, transfer function and field quality measurements, quench heater performance, and heat extraction studies using imbedded heaters to simulate the deposited beam heating profile.

Steering the Field Quality in the Production of the Main Quadrupoles of the Large Hadron Collider

The main issues concerning the field quality in the main quadrupoles of the Large Hadron Collider are presented. We show the trend plots for the focusing strength and multipoles at room temperature covering more than 2/3 of the production. We describe the correction of the coil layout to improve b6 at injection field level. A non-negligible fraction of the quadrupoles has been manufactured with collars featuring a magnetic permeability somewhat higher than the specified limits. We show plots for this anomaly. Field quality correlations to measurements in operational conditions are discussed. The dependence of field quality on cable manufacturer is analysed

Magnetic Performance of the Main Superconducting Magnets for the LHC

The field strength and homogeneity of all the LHC superconducting magnets were measured as a part of the production control and qualification process that has taken place during the past four years. In addition to field measurements at room temperature performed on the integral of the production, a significant part of the magnets has been subjected to extensive magnetic measurements at cold. The measurements at cryogenic temperatures, generally performed up to excitation currents of 12 kA corresponding to the ultimate LHC energy of 7.6 TeV, were mainly based on static and dynamic field integral and harmonic measurements. This allowed us to study in detail the DC effects from persistent current magnetization and long-term decay during constant current excitation. These effects are all expected to be of relevance for the field setting and error compensation in the LHC. This paper reports the main results obtained during these tests executed at operating conditions. The integrated field quality is discussed in terms of distribution (average and spread) of the field strength and low-order harmonics as obtained for all the main ring magnet families (dipoles, main and matching quadrupoles). The dependence of field quality on coil geometry, magnet and cable manufacturer is analyzed. A projection of the field quality expected for the critical components in the machine is presented.

submitter : Training Behavior of the Main Dipoles in the Large Hadron Collider

In 2015, the 1232 Nb-Ti dipole magnets in the Large Hadron Collider (LHC) have been commissioned to 7.8 T operational field, with 172 quenches. More than 80% of these quenches occurred in the magnets of one of the three cold mass assemblers (3000 series), confirming what was already observed in 2008. In this paper, the recent analysis carried out on the quench performance of the Large Hadron Collider dipole magnets is reported, including the individual reception tests and the 2008 and 2015 commissioning campaigns, to better understand the above-mentioned anomaly and give an outlook for future operation and possible increase of the operational field. The lower part of the quench probability spectrum is compatible with Gaussian distributions; therefore, the training curve can be fit through error functions. An essential ingredient in this analysis is the estimate of the error to be associated with the training data due to sampling of rare events, allowing to test different hypothesis. Using this approach, an estimate of the number of quenches required to reach 8.3 T (corresponding to the 7 TeV nominal energy) is given, and we propose to have two LHC sectors trained toward this target before the next warm up of the LHC.

Retraining of the 1232 Main Dipole Magnets in the LHC

The Large Hadron Collider (LHC) contains eight main dipole circuits, each of them with 154 dipole magnets powered in series. These 15-m-long magnets are wound from Nb-Ti superconducting Rutherford cables, and have active quench detection triggering heaters to quickly force the transition of the coil to the normal conducting state in case of a quench, and hence reduce the hot spot temperature. During the reception tests in 2002-2007, all these magnets have been trained up to at least 12 kA, corresponding to a beam energy of 7.1 TeV. After installation in the accelerator, the circuits have been operated at reduced currents of up to 6.8 kA, from 2010 to 2013, corresponding to a beam energy of 4 TeV. After the first long shutdown of 2013-2014, the LHC runs at 6.5 TeV, requiring a dipole magnet current of 11.0 kA. A significant number of training quenches were needed to bring the 1232 magnets up to this current. In this paper, the circuit behavior in case of a quench is presented, as well as the quench training as compared to the initial training during the reception tests of the individual magnets.

Geometric field errors of Short Models for MQXF, the Nb3Sn low-beta Quadrupole for the High Luminosity LHC

In the framework of the high-luminosity upgrade of the large hadron collider, the U.S. LARP collaboration and CERN are jointly developing a 150-mm aperture Nb3Sn quadrupole for the Large Hadron Collider (LHC) interaction regions. Due to the large beam size and orbit displacement in the final focusing triplet, MQXF has challenging targets for field quality at nominal operation conditions. Three short model magnets have been tested and around 30 coils have been built, allowing a first analysis of the reproducibility of the coil size and turns positioning. The impact of the coil shimming on field quality is evaluated, with special emphasis on the warm magnetic measurements and the correlation to field measurements at cold and nominal field. The variability of the field harmonics along the magnet axis is studied by means of a Monte-Carlo analysis and the effects of the corrective actions implemented to suppress the low-order unallowed multipoles are discussed.

Performance of CERN LHC main dipole magnets on the test bench from 2008 to 2016

Throughout 2015 and 2016, the LHC is operated with a current in the main dipoles of 10980 A, equivalent to a proton–proton collision energy of 13 TeV in the center of mass. A total of 175 training quenches were needed in 2014 in the 1232 main dipole magnets installed in the LHC at CERN to reach operational conditions. Since 2008, a number of dipole magnets have been removed from the LHC and were, sometimes after repairs of nonconformities, retested in the CERN based SM18 magnet test facility up to ultimate current. Other magnets have been retested after long storage. The results confirm earlier findings that some magnets series are more prone to quenching than others after thermal cycle. The correlation between a short and long thermal cycle is under investigation. Special cases with many thermal cycles will be highlighted and a new magnet series, fully produced at CERN is introduced. Results of a quench heater fatigue test, assessing the long-term reliability of the quench heaters, will be given. The results of repairs following high internal splice resistances are discussed.