D. Leroy

Status of the LHC superconducting cable mass production

Six contracts have been placed with industrial companies for the production of 1200 tons of the superconducting (SC) cables needed for the main dipoles and quadrupoles of the Large Hadron Collider (LHC). In addition, two contracts have been placed for the supply of 470 tons of NbTi and 26 tons of Nb sheets. The main characteristic of the specification is that it is functional. This means that the physical, mechanical and electrical properties of strands and cables are specified without defining the manufacturing processes. Facilities for the high precision measurements of the wire and cable properties have been implemented at CERN, such as strand and cable critical current, copper to superconductor ratio, interstrand resistance, magnetization, RRR at 4.2 K and 1.9 K. The production has started showing that the highly demanding specifications can be fulfilled. This paper reviews the organization of the contracts, the test facilities installed at CERN, the various types of measurements and the results of the main physical properties obtained on the first batches. The status of the deliveries is presented.

1.9 K test facility for the reception of the superconducting cables for the LHC

A new test facility (FRESCA-Facility, reception of superconducting cables) is under construction at CERN to measure the electrical properties of the LHC superconducting cables. Its main features are: independently cooled background magnet, test currents up to 32 kA, temperature between 1.8 and 4.5 K, long measurement length of 60 cm, field perpendicular or parallel to the cable face, measurement of the current distribution between the strands. The facility consists of an outer cryostat containing a superconducting NbTi dipole magnet with a bore of 56 mm and a maximum operating field of 9.5 T. The magnet current is supplied by an external 16 kA power supply and fed into the cryostat using self-cooled leads. The lower bath of the cryostat, separated by means of a so called lambda-plate from the upper bath, can be cooled down to 1.9 K using a subcooled superfluid refrigeration system. Within the outer cryostat, an inner cryostat is installed containing the sample insert. This approach makes it possible to change samples while keeping the background magnet cold, and thus decreasing the helium consumption and cool-down time of the samples. The lower bath of the inner cryostat, containing the sample holder with two superconducting cable samples, can as well be cooled down to 1.9 K. The samples can be rotated while remaining at liquid helium temperature, enabling measurements with the background field perpendicular or parallel to the broad face of the cable. Several arrays of Hall probes are installed next to the samples in order to estimate possible current imbalances between the strands of the cables.

DC measurement of electrical contacts between strands in superconducting cables for the LHC main magnets

In the LHC main magnets, using Rutherford type cable, the eddy current loss and dynamic magnetic field error depend largely on the electrical resistance between crossing (Rc) and adjacent (Ra) strands. Cables made of strands with pre-selected coatings have been studied at low temperature using a DC electrical method. The significance of the inter-strand contact is explained. The properties of resistive barriers, the DC method used for the resistance measurement on the cable, and sample preparation are described. Finally the resistances are presented under various conditions, and the effect is discussed that the cable treatment has on the contact resistance.

Quench observation in LHC superconducting one meter long dipole models by field perturbation measurements

A novel method to localize and characterize the origin of a quench in a superconducting dipole has been developed during the tests of the Large Hadron Collider (LHC) 1-m-long superconducting dipole models. It consists of an extended analysis of the voltage signals generated at the onset of a quench in pickup coils distributed along the inside of the magnet bore in conjunction with the pole voltage signals. The authors discuss the measurement method and the main results obtained during the training of these magnets, reaching a bore field of 10 T.<<ETX>>

Strand coating for the superconducting cables of the LHC main magnets

The electrical resistance of contacts between strands in the Rutherford type superconducting cables has a major effect on the eddy current loss in cables, and on the dynamic magnetic field error in the LHC main magnets. In order to guarantee the value and constancy of the contact resistance, various metallic coatings were studied from the electrical and mechanical points of view in the past. We report on the molten bath Sn/sub 95wt/Ag/sub 5wt/ coating, oxidized thermally in air after the cabling is completed, that we adopted for the cables of the LHC main magnets. The value of the contact resistance is determined by the strand coating and cabling procedures, oxidation heat treatment, and the magnet coil curing and handling. Chemical analysis helps to understand the evolution of the contacts. We also mention results on two electrolytic coatings resulting in higher contact resistance.

Quench location in the superconducting model magnets for the LHC by means of pick-up coils

High field superconducting dipole magnets were manufactured in industry or at CERN as model magnets for the future Large Hadron Collider (LHC) particle accelerator and tested in superfluid helium. The pick-up coil method is now in use to precisely locate the origin of training quenches and to monitor the propagation of the transition. The improvements made on this diagnostic method is reviewed. This experience allows the location of the onset of the quenches both axially and in the cross section of the winding even for magnets equipped with a minimum of voltage taps on the winding. The location of training quenches are now understood to be related to the structure of the superconducting coil.<<ETX>>

Review of the RaD and Supply of the LHC Superconducting Cables

The construction of the superconducting magnets for the LHC machine has required the supply of ~7350 km of superconducting cables. The delivery of cables which is completed at 97% has made use of a large part of the world wide production capacity. Ten contracts have been placed with firms in Europe, Japan, USA. The Nb-Ti and the Nb materials have been contracted by CERN. Before tendering and placing the contracts, a R&D program has combined studies at CERN and orders of finished cables of significant lengths to industry. The report will present the main results of the R&D program, the characteristics of the LHC cables, the encountered difficulties and the obtained successes during the long duration fabrication contracts of the highly sophisticated LHC superconducting cables

Test results on the long models and full scale prototypes of the second generation LHC arc dipoles

With the test of the first full scale prototype in June-July 1998, the R&D on the long superconducting dipoles based on the LHC design of 1993-95 has come to an end. This second generation of long magnets has a 56 mm coil aperture, is wound with 15 mm wide cable arranged in a 5 coil block layout. The series includes four 10 m long model dipoles, whose coils have been wound and collared in industry and the cold mass assembled and cryostated at CERN, as well as one 15 m long dipole prototype, manufactured totally in industry in the framework of a CERN-INFN collaboration for the LHC. After a brief description of particular features of the design and of the manufacturing, test results are reported and compared with the expectations. One magnet reached the record field for long model dipoles of 9.8 T but results have not been well reproducible from magnet to magnet. Guidelines for modifications that will appear in the next generation of long magnets, based on a six block coil design, are indicated in the conclusions.

Examination of contacts between strands by electrical measurement and topographical analysis

The contact resistance between crossing strands of Rutherford type superconducting be an essential parameter the main magnets in accelerators like the LHC. A strong development program to study the parameters fixing its value has been launched at CERN. The electrical contact resistance of individual strands with pre-selected coatings has been studied at 4.2 K under varying loading force by means of a 3 contacts DC method. The electro-mechanical properties of contacts have been studied on bare strands and strands with Sn, Ni, or Zn based coatings. The contact resistances measured on strands were compared to measurements made on cables. Contact resistance properties were analysed with respect to the requirements for the LHC main magnet cables.

Critical Current Density in Superconducting

The knowledge of the critical current density in a wide temperature and applied magnetic field range is a crucial issue for the design of a superconducting magnet, especially for determining both current and temperature margins. The critical current density of LHC-type Nb-Ti strands of 0.82 and 0.48 mm diameter was measured by means of critical current and magnetization measurements at both 4.2 K and 1.9 K and for a broad magnetic field range (up to 11 T). For the magnetic field range common to both measurement methods, critical current density values as extracted from transport current and from magnetization data are compared and found fairly consistent. Our experimental data are compared to other sets from literature and to scaling laws as well