The interstrand contact resistance (R c ) in Rutherford type cables for fast cycling superconducting magnets must be sufficiently high in order to limit eddy current losses. The required value for R ...c depends on the cable and magnet geometries and on the foreseen cycling rate, but is typically of the order of one mOmega. Such values can be reached with a dedicated strand coating or with a resistive internal cable barrier. As a possible candidate Al strand coatings have been tested. For a Rutherford type inner conductor cable of the Large Hadron Collider (LHC) made of Al coated strands i?c values higher than 500 muOmega are achieved. The native Al 2 O 3 oxide layer formed at ambient temperature in air is sufficient to reach this high contact resistance. A 6 h-200degC oxidation heat treatment in air with 100% relative humidity further increases R C to values above 600 muOmega. Due to the high thermal and mechanical stability of Al 2 O 3 only a relatively moderate R c drop of about 40% is obtained during a 190degC heat treatment under 50 MPa pressure (the so-called curing cycle of the coil insulation) subsequent to the 6 h-200degC oxidation heat treatment.
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.
A study of the critical current of a PIT strand under transverse compressive and axial tensile loads is presented. The conductor is not reinforced and has a proof strength, , of 142 MPa. Although the ...tensile strain at the maximum of the critical current, , is only 0.14% (low thermal precompression) the strand behaves as expected. However it is particularly sensitive to transverse compressive forces. Even relatively small forces yield to a reduction of the critical current. Qualitatively this degradation may be explained by a non-uniform deformation of filaments, yielding to irreversible micro cracks, and by a reduced of intact filaments. The recovery of upon unloading of the transverse compressive force has also been investigated.
The main purpose of Next European Dipole (NED) project is to design and to build an Nb3Sn ~ 15 T dipole magnet. Due to budget constraints, NED is mainly focused on superconducting cable development ...and production. In this work, an update is given on the NED conductor development by Alstom-MSA and SMI, which uses, respectively, Internal-Tin-Diffusion and Powder-In-Tube methods, with the aim of reaching a non-copper critical current density of ~ 3000 A/mm2 at 12 T and 4.2 K. Characterization results, including critical current and magnetization data, are presented and discussed, as well, for conductors already developed by both companies for this project. SMI succeeded to produce a strand with 50 μm diameter filaments and with a critical current of ~ 1400 A at 4.2 K and 12 T, corresponding to a non-copper critical current density of ~ 2500 A/mm2. Cabling trials with this strand were successfully carried out at LBNL.
Critical Current Studies on Nb-Ti Deformed-Strands Previtali, V.; Boutboul, T.; Le Naour, S. ...
IEEE transactions on applied superconductivity,
06/2006, Letnik:
16, Številka:
2
Journal Article, Conference Proceeding
Recenzirano
The Nb-Ti hard conductors used in LHC dipole and quadrupole magnets are Rutherford cables composed of several tens of strands. During the cabling process, the strands are severely compacted ...especially at the thin edge of the cable. In order to assess, on the whole wire length, the deformation effect on the transport current of the wires, LHC-type Nb-Ti superconducting strands of various types were flattened by means of rollers. The critical current was then measured as a function of deformation and applied magnetic field at both 4.3 K and 1.9 K. The measurements were performed for both orientations (flat face perpendicular or parallel to magnetic field). The critical current density anisotropy of such deformed strands and the correlation with magnetization effects are discussed. This study permits to better understand and to quantify the critical current degradation of few percent observed in strands due to cabling. Comparisons with wires extracted from Rutherford cables are presented
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.
For the LHC upgrade, CERN has launched a large program to develop next generation accelerator magnets based on high- Jc Nb3 Sn Rutherford cables. These magnets are characterized by a magnetic field ...and/or an aperture significantly larger than that of current Nb-Ti LHC magnets. The increased field/aperture will require coil pre-stresses much larger than 100 MPa. Since Nb3Sn cables are extremely sensitive to strain, critical current measurements under traverse compression are essential to estimate the transport current properties of the conductor within the magnet. To this purpose CERN has developed a sample holder (to be used in the FRESCA test station) that allows testing Rutherford cables under a transverse force of up to 2 MN/m. The new holder can house cable samples up to 1.8 m long and 20 mm wide. The large transverse force is only applied over the sample high field region, which is 70 cm long and over which the FRESCA dipole magnet generates a homogeneous fields of up to 10 T. Recently the critical current of the first cable sample has been measured at different transversal loads ranging from 90 MPa to 155 MPa. The measurement was carried out at 4.3 K on a 10-mm-wide Rutherford cable based on eighteen Powder In Tube (PIT) wires with a diameter of 1.0 mm. In this paper, the results of the test are reported, discussed and compared with recently measured data of the same single wire (1.0-mm PIT) tested under transverse loads.
One of the main issues for the operation of the LHC accelerator at CERN is the field errors generated by persistent and coupling currents in the main dipoles at injection conditions, i.e., 0.54 T ...dipole field. For this reason we are conducting systematic magnetic field measurements to quantify the above effects and compare them to the expected values from measurement on strands and cables. We discuss the results in terms of DC effects from persistent current magnetization, AC effects with short time constant from strand and cable coupling currents, and long-term decay during constant current excitation. Average and spread of the measured field errors over the population of magnets tested are as expected or smaller. Field decay at injection, and subsequent snap-back, show for the moment the largest variation from magnet to magnet, with weak correlation to parameters that can be controlled during production. For this reason these effects are likely to result in the largest spread of field errors over the whole dipole production.
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