Modelling boundary-induced coupling currents in Rutherford-type cables

In this paper it is shown that spatial distributions in the field-sweep rate and in the contact resistances along the length of Rutherford-type cables provoke a non-uniform current distribution during and after a field sweep. This process is described by means of Boundary-Induced Coupling Currents (BICCs) flowing through the strands over lengths far larger than the cable pitch. The dependence of the BICCs on the cable parameters (geometry, contact resistances etc.) is investigated by modelling the cable by means of a comprehensive network model. Working formulas are presented that give a first estimate of the characteristic time, the amplitude, and the characteristic length of the BICCs in any kind of magnet wound from a Rutherford-type cable. The results of these calculations show that BICCs can attain large values in multistrand cables, and hence play an important role in the ramp-rate limitation and field quality of high-field accelerator magnets even if the field-sweep rate is small

Critical Current Measurements of the Main LHC Superconducting Cables

For the main dipole and quadrupole magnets of the LHC, CERN has ordered from industry about 7000 km of superconducting Nb-Ti Rutherford type cables, delivered between 1999 and 2005. The strands of these cables are produced by six different companies, and cabled on five different machines. In the framework of the US contribution to the LHC, BNL has been testing and analyzing the electrical properties of samples of these cables. The main purpose of these tests was to qualify the critical current of the entire cable production in the frame of the quality assurance program implemented by CERN to assure the overall strand and cable performances. In total more than 2100 cable samples have been evaluated at 4.3 K in terms of critical current $I_{C}$, n-value and the residual resistance ratio, RRR. This paper will present an overview of the results, and show the correlations of the critical current and n-value between virgin strands, extracted strands, and cables. Also described are correlations of $I_{C}$ measured at BNL and those made at the FRESCA facility in CERN. Furthermore a few trends and anomalies of the cable production that were detected from testing cables are highlighted

Linking bayesian belief networks and GIS to assess the ecosystem integrity in the brazilian Amazon.

Deforestation and climate change heavily impact the ecosystem of the Amazon rainforest threatening its resilience and the sustainability of many human activities. Land protection may prevent ecosystems and their services to deteriorate from the pressures of agricultural expansion, population growth and wood harvesting. In the Brazilian Amazon land protection occurs in several forms such as environmental conservation, setting biodiversity priority areas and the delineation of indigenous lands. Still, the effects are not clear as understanding of the ecosystems is incomplete and responses to human actions are highly uncertain. Bayesian Belief Networks (BBN) are models that probabilistically represent correlative and causal relationships among variables. BBNs have been successfully applied to natural resource management to address environmental management problems and to assess the impact of alternative management measures. By training the probabilistic relationships using field data, Remote Sensing data and GIS data the BBN can provide information on the ecosystems: the ecosystem integrity and their likely response to climate change or alternative management actions. An increasing number of studies train and apply BBNs with evidence originating from GIS data; a cumbersome and error prone soft-linking method requiring manual conversion of data files between the BBN and GIS software systems. This paper presents the full integration of a BBN software system within an existing GIS based Discussion Support System (DSS) illustrated by the case of the ecosystem integrity of the Brazilian amazon. The full integration speeds up the processing and thereby allows doing multiple runs within a short period of time such as a stakeholder workshop. Each consecutive run is based upon insights from a previous one. Furthermore, the DSS provides the management of different options, visualize spatial summaries and trade-offs between different impact indicators and see regional differences

Boundary-induced coupling currents in a 1.3 m Rutherford-type cable due to a locally applied field change

In this paper the existence of so called Boundary-Induced Coupling Currents (BICCs) is experimentally demonstrated in a 1.3 m long Rutherford-type cable. These BICCs are induced by applying a field change locally onto the cable and can be represented by a non-uniform current distribution between the strands of the cable during and after the field sweep. In order to better understand the characteristic time, amplitude and characteristic length of these coupling currents and the parameters by which they are influenced, a special set-up has been built. With this set-up it is possible to scan the field induced by the BICCs along the full length of a Rutherford-type cable. Special attention is paid on the influence of the contact resistance between crossing strands on the characteristics of the BICCs, and results are presented where parts of the cable are soldered, simulating the joints of a coil

Current Redistribution around the Superconducting-to-normal Transition in Superconducting Nb-Ti Rutherford Cables

Sufficient thermal-electromagnetic stability against external heat sources is an essential design criterion for superconducting Rutherford cables, especially if operated close to the critical current. Due to the complex phenomena contributing to stability such as helium cooling, inter-strand current and heat transfer, its level is difficult to quantify. In order to improve our understanding, many stability tests were performed on different cable samples, each incorporating several point-like heaters. The current redistribution around the heat front is measured after inducing a local normal zone in one strand of the cable. By using voltage taps, expansion of the normal zone is monitored in the initially quenched strand as well as in adjacent strands. An array of Hall probes positioned at the cable edge is used to scan the selffield generated by the cable by which it becomes possible to estimate the inter-strand current transfer. In this paper it is demonstrated that two different stability regimes can be distinguished depending on the local conditions for local normal zone recovery through heat and current transfer to adjacent strands. It is shown that in the first regime every normal zone will lead to a quench, while in the second regime a normal zone in one strand can recover. Combining the predictions developed using a novel version of the numerical network model CUDI and new measurement results, it is possible to derive char acteristic quench decision times as well to calculate and predict the influence of a change in cable parameters

Determination of interstrand contact resistance from loss and field measurements in LHC dipole prototypes and correlation with measurements on cable samples

Loss and field errors due to ramping in LHC accelerator dipole magnets are mainly determined by the contact resistance between the strands of the magnet cable. It is therefore important to develop cables having sufficiently high contact resistance in the magnets in order to ease operation of the future LHC collider during ramping. In this paper the contact resistance Rc and its distribution in the magnet windings are determined for several dipole prototypes using both the measured loss and field errors during ramping of the magnet. We compare these results with interstrand contact resistance measurements made on short samples of the cables used in these magnets

1.9 K Test Facility for the Reception of the Superconducting Cables for the LHC

A new test facility (called FRESCA) is under construction at CERN to measure the electrical properties of the LHC superconducting cables. Its main features compared to existing test facilities are: a) independently cooled background magnet, b) test currents up to 32 kA, c) temperature between 1.8 and 4.5 K, d) long measurement length of 60 cm, e) field perpendicular or parallel to the cable face, f) 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 current through the magnet 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 superconducting cable samples. 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 cable samples are connected through self-cooled leads to an external 32 kA power supply. The lower bath of the inner cryostat, containing the sample holder, is separated by means of a so called lambda-plate from the upper bath and can 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

Stability of Nb-Ti Rutherford Cables Exhibiting Different Contact Resistances

Dipole magnets for the so-called SIS-300 heavy-ion synchrotron at GSI are designed to generate 6 T with a field sweep rate of 1 T/s. It is foreseen to wind the magnets with a 36 strands Nb-Ti Rutherford cable. An important issue in the cable design is sufficiently low AC loss and stability as well. In order to keep the AC loss at low level, the contact resistance between crossing strands Rc is kept high by putting a stainless steel core in the cable. The contact resistance between adjacent strands Ra is controlled by oxidation of the Sn-Ag coating of the strands, like in the LHC. In order to investigate the effect of Ra on the stability of the cable, we prepared four samples with different Ra by varying the heat treatment and applying a soldering technique, resulting in values between 1 mW to 9 mW. The stability of each sample against transient point-like heat pulses was measured. The results of the stability experiments and a comparison with calculations using the network model CUDI are presented. It is concluded that variation of Ra has a strong influence on cable stability and that optimization of Ra is mandatory to properly design the cable for the SIS-300 magnets, or likewise for similar magnets that might be used at CERN for a possible LHC injector upgrade