Christine Vollinger

Steering field quality in the main dipole magnets of the Large Hadron Collider

More than 10% of the collared coils of the main LHC dipoles have been produced. In this paper, we compare the measured field quality to beam dynamics targets using correlations to measurements at 1.9 K. The present status of field quality is given and corrective actions carried out to center field quality on optimal values are presented. Differences among the three manufacturers are analyzed, and the main results that concern correlation between cold and warm measurements are outlined. Present trends in the production and open points are discussed.

Identification of Assembly Faults Through the Detection of Magnetic Field Anomalies in the Production of the LHC Dipoles

Magnetic measurements at room temperature have been used to monitor the production of the superconducting coils of the Large Hadron Collider main dipoles. They have made it possible to identify several assembly errors, e.g. cases of bad gluing of the coil layers, bad conductor positioning, missing pole shims and other problems related to faulty procedures. This paper reviews the experience accumulated so far considering almost 1000 dipoles. After a short outline of the method used to pin out field anomalies and deduce realistic deformation of the coil, an exhaustive list of the cases met during the production is given. A discussion follows on the findings after decollaring as compared to the predictions, including the still open cases

Production follow-up of the LHC main dipoles through magnetic measurements at room temperature

In this paper, we review the tools used for controlling the production of the LHC main dipoles through warm magnetic measurements. For the collared coil measurements, control limits are based on the statistics relative to the pre-series production. For the cold mass, the difference between collared coil and cold mass is considered, allowing a very stringent test. In both cases, measurements are split in straight part average, variations and coil ends contributions. Two different alarm levels exist in case the measured field is out of limits. The analysis can be carried out at the manufacturer and allows detection of anomalies in the measured magnetic field. These can be either due to wrong measurements or caused by assembly defects. Techniques used to work out information on the magnet assembly from the field harmonics are outlined. We summarize the experience gathered on about 180 collared coils and 120 cold masses, pointing out the bad cases and investigating the reliability of the measurements.

Magnetic field calculations including the impact of persistent currents in superconducting filaments

The magnetic field in the coils of superconducting magnets induces so-called persistent currents in the filaments. Persistent currents are bipolar screening currents that do not decay due to the lack of resistivity. The NbTi-filaments are type II superconductors and can be described by the critical state model. This paper presents an analytical hysteresis model of the filament magnetization due to persistent currents which takes into account the changing magnetic induction inside the filament. This model is combined with numerical field computation methods, taking local saturation effects in the ferromagnetic yoke into consideration.

Comparison of 2-D Magnetic Designs of Selected Coil Configurations for the Next European Dipole (NED)

The next European dipole (NED) activity is developing a high-performance Nb3Sn wire (aiming at a non-copper critical current density of 1500 A/mm2 at 4.2 K and 15 T), within the framework of the Coordinated Accelerator Research in Europe (CARE) project. This activity is expected to lead to the fabrication of a large aperture, high field dipole magnet. In preparation for this phase, a working group on magnet design and optimization (MDO) has been established to propose an optimal design. Other parallel work packages are concentrating on relevant topics, such as quench propagation simulation, innovative insulation techniques, and heat transfer measurements. In a first stage, the MDO working group has selected a number of coil configurations to be studied, together with salient parameters and features to be considered during the evaluation: the field quality, the superconductor efficiency, the conductor peak field, the stored magnetic energy, the Lorentz forces and the fabrication difficulties. 2-D magnetic calculations have been performed, and the results of this comparison between the different topologies are presented in this paper. The 2-D mechanical computations are ongoing and the final stage will be 3-D magnetic and mechanical studies.

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

A large aperture superconducting dipole for beta beams to minimize heat deposition in the coil

The aim of beta beams in a decay ring is to produce highly energetic pure electron neutrino and anti-neutrino beams coming from b-decay of 18Ne10+ and 6He2+ ion beams. The decay products, having different magnetic rigidities than the main ion beam, are deviated inside the dipole. The aperture and the length of the magnet have to be optimized to avoid that the decay products hit the coil. The decay products are intercepted by absorber blocks inside the beam pipe between the dipoles to protect the following dipole. A first design of a 6T arc dipole using a cosine theta layout of the coil with an aperture of 80 mm fulfils the optics requirements. Heat deposition in the coil has been calculated using different absorber materials to find a solution to efficiently protect the coil. Aspects of impedance minimization for the case of having the absorbers inside the beam pipe have also been addressed.

The Effect of 2-Directional Magnetic Biasing Used for Tuning of a Ferrite-Loaded Re-entrant Cavity

Cavities that are partially filled with ferrite material provide a tunable resonance frequency by making use of the changing μ-characteristics of ferrites when exposed to an external magnetic bias field. The concept of using either parallel or perpendicular magnetic biasing to reach a certain resonance frequency of a cavity has been known for many years. However, a cavity based on superposition of perpendicular and parallel magnetic fields to obtain improved ferrite characteristics was suggested in W. R. Smythe “Reducing ferrite tuner power loss by bias field rotation,” IEEE Trans. Nucl. Sci., vol. 30, no. 4, pp. 273-275, 1983, but to our knowledge was neither tested nor built. Such a 2-directional biasing is expected to provide a reduction in RF losses for an identical tuning range as compared with the classical 1-directional magnetic bias. We have successfully tested this theory with a measurement set-up consisting of a ferrite-filled cavity, exposed to external biases that allow the clear separation of the two orientations of superposed magnetic bias fields. The outcome is an enlargement of tuning range with high cavity Q and the possibility of fast tuning. In this paper, we describe the measurement set-up and present the tuning ranges that we attained by applying different bias schemes.

Trends in cable magnetization and persistent currents during the production of the main dipoles of the Large Hadron Collider

The production of more than 60% of superconducting cables for the main dipoles of the Large Hadron Collider has been completed. The results of the measurements of cable magnetization and the dependence on the manufacturers are presented. The strand magnetization produces field errors that have been measured in a large number of dipoles (approximately 100 to date) tested in cold conditions. We examine here the correlation between the available magnetic measurements and the large database of cable magnetization. The analysis is based on models documented elsewhere in the literature. Finally, a forecast of the persistent current effects to be expected in the LHC main dipoles is presented, and the more critical parameters for beam dynamics are singled out.

Parametric quadrilateral meshes for the design and optimization of superconducting magnets

The program package ROXIE has been developed at CERN for the design and optimization of accelerator magnets. The necessity of extremely uniform fields in the superconducting accelerator magnets for LHC requires very accurate methods of field computation. For this purpose the coupled boundary-element/finite-element technique (BEM-FEM) is used. Quadrilateral higher order finite-element meshes are generated for the discretization of the iron domain (yoke) and stainless steel collars. A new mesh generator using geometrically optimized domain decomposition which was developed at the University of Stuttgart, Germany has been implemented into the ROXIE program providing fully automatic and user friendly mesh generation. The structure of the magnet cross-section can be modeled using parametric objects such as holes of different forms, elliptic, parabolic or hyperbolic arcs, notches, slots, .... For sensitivity analysis and parametric studies, point based morphing algorithms are applied to guarantee smooth adaptation of the mesh to geometry changes. For modeling three-dimensional iron structures, the 2-D meshes can be extruded into the third dimension. This paper discusses the use of the mesh generator with examples of the computation of the LHC dipoles.