Performance of five and six block coil geometries in short superconducting dipole models for the LHC

Abstract

A series of similar one meter long superconducting dipole models for the LHC is being manufactured and tested since 1995 for exploring design variants and assembly parameters. Until the end of 1997 all magnets of this series were based on a coil geometry subdividing the conductors in five distinctive winding blocks. In order to cope with new requirements of magnetic field distribution and coil design flexibility, one additional block has been added in the beginning of 1998. A significant number of models of both types have been built and tested, some of them re-built in a different version, adding up in more than 40 models tested so far. The paper reviews the performance of these two different coil designs in terms of manufacture, training behaviour and temperature margins as well as mechanical behaviour and magnetic field quality. THE MODELSThe regular CERN in-house model program for the development of the LHC dipoles was started in 1995 with the fabrication of 1m-long single-aperture magnets, socalled MBSMS. The design of the MBSMS models, presented in previous conference papers [1] [2], is based on circular collars of 197 mm outer diameter placed inside a vertically split yoke, held together by a bolted shrinking cylinder for easy re-assembly of the structure. COIL GEOMETRY:DESIGN FEATURESThe LHC dipole coils consist of two superposed layers, an internal layer and an external one. Each layer is subdivided in blocks of conductors separated by copper spacers. The two coil geometries tested on the models are shown in Fig.1 below.The first 13 magnets have been built with the 5-block version of the Yellow Book [3], consisting in 3 blocks of conductors in the inner layer and two in the outer layer ( fig.1a). As from MBSMS15, the coil design has undergone a substantial evolution, being replaced by a 6-block one, consisting in four blocks of conductors in the inner layer and two in the outer layer (fig.1b). The last 6 magnets have been built according to this 6-block geometry. Magnetic designThe original 5-block coil cross-section was optimized using deterministic techniques with the CERN program ROXIE [4]. First a preliminary geometric modelling was done with a given choice on cable dimensions and conductor blocks, thereafter the model was iterated to find an acceptable solution for field quality and peak field/main field ratio. This coil design however did not ensure sufficient tunability and flexibility for later field adjustments like compensating the persistent current multipoles at injection and fine-tuning of field quality. In particular a further compensation of the b3 term, if required, would have been impossible because the copper wedges would become too small at the inner edge. Moreover the performance of the magnets with 5-block coils and laboratory tests indicate that the stress distribution in the inner block of the inner layer is not favourable. With the implementation of genetic algorithms in ROXIE in 1997 [5] it was possible to make an extended the search for more appropriate designs consider...

Training quench performance and quench location of the short superconducting dipole models for the LHC

“…These models allowed to select the series-design features among several variants for the coil cross section [2], the cable insulation, the material of the collars and of the coil end-spacers and the coil pre-stress [3], [4], and the relation between field harmonics and coil size [5]. The recent double-aperture models were dedicated to explore the quench performance in different manufacturing configurations continuing the work reported in [6].…”

LHC arc dipole status report

“…A 6-block coil cross-section, optimised thanks to the recent availability of genetic algorithms [9] allowing to consider discrete and continuous parameters at the same time, is chosen [10]. With regard to the previous 5-block coil design, it features a wider tuning range, a more stable mechanical behaviour because of a more favourable geometry and distribution of electromagnetic forces, a lower field at the inner layer turn which is submitted to the S-bend transition to the outer layer.…”