Dependence of Magnetic Field Quality on Collar Supplier and Dimensions in the Main LHC Dipole

C. Santoni, coll. Atlas, to be published in the proceeding of the conferenceIn order to keep the electro-magnetic forces and to minimize conductor movements, the superconducting coils of the main Large Hadron Collider dipoles are held in place by means of austenitic steel collars. Two suppliers provide the collars necessary for the whole LHC production, which has now reached more than 800 collared coils. In this paper we first assess if the different collar suppliers origin a noticeable difference in the magnetic field quality measured at room temperature. We then analyze the measurements of the collar dimensions carried out at the manufacturers, comparing them to the geometrical tolerances. Finally we use a magneto-static model to evaluate the expected spread in the field components induced by the actual collar dimensions. These spreads are compared to the magnetic measurements at room temperature over the magnet production in order to identify if the collars, rather than other components or assembly process, can account for the measured magnetic field effects. It has been found that in one over the three Cold Mass Assemblers the driving mechanism of the magnetic field harmonics b2 and a3 is the collar shape

Correlation between magnetic field quality and mechanical components of the Large Hadron Collider main dipoles

N° d'ordre : DU 1722, EDSF : 521The 1234 superconducting dipoles of the Large Hadron Collider, working at a cryogenic temperature of 1.9 K, must guarantee a high quality magnetic field to steer the particles inside the beam pipe. Magnetic field measurements are a powerful way to detect assembly faults that could limit magnet performances. The aim of the thesis is the analysis of these measurements performed at room temperature during the production of the dipoles. In a large scale production the ideal situation is that all the magnets produced were identical. However all the components constituting a magnet are produced with certain tolerance and the assembly procedures are optimized during the production; due to these the reality drifts away from the ideal situation. We recollected geometrical data of the main components and coupling them with “ad hoc” electro-magnetic models we reconstructed a multipolar field representation of the LHC dipoles defining their critical components and assembling procedures.La production des dipôles supraconducteurs de la machine LHC du CERN s'est terminée en automne 2006. Les aimants fonctionnent à la température cryogénique de 1.9 K et doivent produire un champ magnétique très uniforme permettant de conduire les protons dans la machine. Le champ magnétique a été contrôlé avec beaucoup de rigueur et différentes mesures ont été réalisées pendant les différentes phases de l'assemblage des dipôles. Le but a été la découverte de défauts de production et d'assemblage qui prouvent limiter les performances des aimants. Dans le travail de thèse les effets de la variation de la géométrie des composantes mécaniques comme les câbles supraconducteur, les cales et les colliers de soutien des bobines sur l'uniformité du champ magnétique ont été étudiés. Une méthode pratique pour identifier et corriger les problèmes d'usinage a été développée et utilisée dans la phase de production. La thèse consiste en trois parties principales :-Analyse de la production des principales composantes mécaniques des dipôles et étude de l'influence de la géométrie et des procédures d'assemblage sur la qualité du champ magnétique.-Utilisation pratique des mesures effectuées sur les dipôles dans l'atelier d'assemblage pour résoudre les problèmes de production et comprendre le comportement de la géométrie des bobines pendant les phases d'assemblage.-Etude théorique des composantes aléatoires des harmoniques du champ magnétique pour qualifier la production des dipôles

Corrélation entre la qualité du champ magnétique et les composants mécaniques des aimants dipolaires du Large Hadron Collider

In this work a detailed analysis of the influence of the mechanical components and the assembly procedures of the LHC main dipole on the field shape at room and cryogenic temperatures has been done. The analysis of the productions of cables, copper wedges and austenitic steel collars showed that the dimensions are inside the tolerance limits with some exceptions for the early productions of the wedges and for few dimensions of the collars. Moreover, the productions of the six suppliers of cables and of the two of collars are very homogeneous. Some differences (~15%) have been found only in the magnetization of the two suppliers of the inner layer cable of about 15%. Coupling magneto-static models and the geometrical measurements of the mechanical components the influence of the dimensions on the field quality of the dipoles has been investigated; in particular, the main results are: Superconducting cables: * The simulations show that the cable dimension variations could account for most of the specified random components of a2 and a4 and it is negligible for the other multipoles. * For high order allowed harmonics (b5 and b7) measured at 1.9 K there is a difference between magnets that can be traced back to the difference in magnetization between inner cable manufacturers. Copper wedges: * A relevant systematic effect on the b3 (1.5 units) of the first produced wedges is visible in the collared coil magnetic measurements at room temperature. This explains part of the upward trend observed in b3 in the first 25 collared coils. * It has been shown that the advices given at the beginning of the wedge production brought to a more careful control on the manufacturing and as a result the total influence of the copper wedge dimensions on the collared coil magnetic field is not relevant Austenitic steel collars: * The collar shape is the driving mechanism of field harmonics only for the even normal and odd skew in particular for b2 and a3 in Firm3, where collars of the supplier S2 are used. Two independent observations support this fact: firstly, we have strong correlations between apertures of the same magnet as expected from the assembly procedure. Secondly, the 134 expected values based on the measured dimension of the collars agree with magnetic measurements both for the average and for the standard deviation. A more general study on the random components of the field harmonics has been worked out in order to evaluate the uncertainty in the coil position in the transverse cross-section generated by mechanical tolerances. This is one of the main sources of random components of the field harmonics, limiting the possibility of obtaining a perfect field quality. We reviewed the data of the production of dipoles relative to four accelerators to analyze the agreement of the Monte-Carlo estimates with the measured values. The above quoted Monte-Carlo method, widely used in the past, gives similar estimates for normal and skew harmonics of the same order. However, already in the Tevatron production it has been observed that random components of normal and skew harmonics of the same order can differ of a factor 4 to 6. We proposed to associate different amplitudes to generate normal and skew harmonics, in order to better fit the experimental data. The final result of the analysis is an improved phenomenological model based on the acquired experience of the four large scale dipole productions to describe and forecast the random errors in a superconducting dipole. With these studies we found that there is an improvement of the degree of precision in positioning the cable block: for the first dipole production, Tevatron, the order of magnitude of geometric random components is compatible with a random movement of the blocks of ~ 65 μm whilst for the more recent productions (LHC and RHIC) e the lowest values is recorded (52 - 54 μm). In order to better estimate the field errors the four classes of harmonics are separately considered and four displacements are calculated. For RHIC and LHC dipole productions random movements of the blocks of ~50 μm r.m.s. are needed for the odd normal multipoles, ∼30 μm for the even skew, and 5 to 20 μm for the even normal and for the odd skew. Such parameters allow estimating the random geometric errors with an average error of ∼20%. In the last chapter a method based on magnetic measurements at room temperature to locate electrical shorts in the coil of the main LHC dipole has been presented. The approach is reliable since the field anomalies generated by the short are, in general, very large compared to the natural spread in field quality induced by tolerances and assembly procedures. We have shown that using an electromagnetic code, one can forecast the effect of shorts between adjacent cables on field quality, and that the comparison to experimental data gives a location of the short. The method is very sensitive, also allowing to detecting if the short is perfect or only partial. Along the LHC main dipole production, 18 coils presenting electrical shorts have been analyzed and rescued using this procedure.La production des dipôles supraconducteurs de la machine LHC du CERN est terminée en automne 2006. Les aimants fonctionnent à la température cryogénique de 1.9 K et doivent un champ magnétique très uniforme permettant de conduire les protons dans la machine. Le but a été la découverte de défauts de production et d'assemblage qui peuvent limiter les performances des aimants. Dans le travail de thèse les effets de la variation de la géométrie des composantes mécaniques comme les câbles supraconducteurs, les cales et les colliers de soutien des bobines sur l'uniformité du champ magnétique ont été étudiés. Une méthode pratique pour identifier et corriger les problèmes d'usinage a été développée et utilisée dans la phase de production

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

Current Center Line Integration in the Manufacturing Process of the ITER Toroidal Field Coils

The first ITER & x0027;s European Toroidal Field Coil (TFC) is going to be assembled in 2019. The TFC is composed mainly by the superconducting Winding Pack (WP - manufactured in Europe), and the Coil Cases (TFCC - manufactured in Japan), which provide structural integrity to the magnet and offer interface connections with the rest of the machine. Dimensional measurements and other manufacturing data are taken during the WP manufacture and are used to reconstruct the Current Centre Line (CCL), which is defined as the barycentre of the as-built conductors inside the WP. The CCL is useful to characterize the magnetic field generated by the magnet, and its monitoring and control can minimize the Error Field during ITER operation. Fusion For Energy (F4E) developed a method to calculate the CCL using manufacturing data. Since the CCL represents how different the WP is manufactured from its nominal conductor layout, this deviation can be corrected where there is need of a more controlled magnetic field shape, namely in the straight inboard area of the D-shaped tokamak. That is why each WP position inside its respective TFCCs is optimized, compensating the deviations detected and maintaining an allowable gap between components for the subsequent welding and resin filling operation. This paper presents the strategy followed by F4E to calculate the CCL and optimize the WP position inside the TFCC, by means of extensive CAD and CAE modelling activity. It explains also the data management process developed and followed to ensure configuration control of all the data inputs and outputs, coming from different sources and formats, and details the successful insertion operation on the first ever ITER TFC coil manufactured in Europe, and how in the future the updated CCL coil position will be used to define the final TFC machining, after the welding and gap filling operations

Commissioning of the LHC Magnet Powering System in 2009

On 19th September 2008 the Large Hadron Collider (LHC) experienced a serious incident, caused by a defective electrical joint, which stopped beam operation just a few days after its beginning. During the following 14 months the damage was repaired, additional protection systems were installed and the measures to avoid a similar incident were taken, i.e. new layer of the Magnet Quench Protection System (nQPS) and more efficient He release valves. As a consequence, a large number of powering tests had to be repeated or carried out for the first time. The re-commissioning of the already existing systems as well as the commissioning of the new ones was carefully studied, then performed taking into account the history of each of the eight LHC sectors (either warmed-up or left at floating temperature). Moreover, a campaign of measurements of the bus-bar splice resistances as well as the ones internal to the cold masses was carried out with the original and the nQPS in order to spot out non conformities, thus assessing the risk of the LHC operation for the initial energy level. This paper discusses how the guidelines for the LHC 2009 re-commissioning were defined, providing a general principle to be used for the future re-commissionin