Field dependence of the critical current and its relation to the anisotropy of BSCCO conductors and coils

The design of HTS magnets is often based on the properties of a number of short samples that are presumed to be representative of the conductor to be used. Variability in conductor properties and inhomogeneity in the magnetic field distribution within the magnets, coupled with conductor anisotropy, provide a significant challenge to accurately predict the field dependence of the magnet critical current. This work is based on measured superconducting properties of Bi-2212 and Bi-2223 conductors at 4.2 K in parallel and perpendicular magnetic fields up to 33 T. Properties of double pancake units and stacks, from the same or similar conductor batches, are presented, based on measurements at self-field and in applied co-axial background magnetic fields up to 19 T. Modeling of this data is based on short sample properties in perpendicular field; the average grain misalignment is used as the parameter to quantify the anisotropy. Correlations and discrepancies between the measured data and models based on short sample data are discussed for Bi-2212 and Bi-2223 conductors

The reduction of the critical current in Nb3Sn cables under transverse loads

The degradation of the critical current of impregnated Rutherford type Nb3Sn cables was investigated as a function of the applied transverse load and magnetic field. The cable is made of modified jelly-roll-type strand material and has a keystone angle of 1.0°. The voltage-current characteristics were determined for the magnetic field ranging from 2 to 11 T and transverse pressure up to 250 MPa on the cable surface. It was found that the 48-strand cable, made of strands with six elements in the matrix, showed a larger critical current degradation than the 26-strand cable with 36 elements per strand. The global degradation of the 48-strand cable was 63% at 150 MPa, and 40% at 150 MPa for the 26-strand cable. Microanalysis of the cross-section before and after compression is presented, showing significant permanent damage to the superconducting strands

Bending strain effect on critical current of Bi-2223 superconductor tapes-report of international round-robin-test

An inter-comparison program (round-robin-test) aimed at the establishment of a standard test method for the bending strain effect on the critical current in Ag alloy-sheathed Bi-2223 tapes was implemented with participation of 12 laboratories around the world. Samples were bent at room temperature using bending devices that were specially designed for this RRT. The critical current was measured at 77 K and in the self-field. After critical current measurement of an unbent sample, bending of the sample and subsequent critical current measurements were done from 0.2% to 1.0% in 0.2% steps of strains on a single sample. Reported data from participants showed that the critical current values start to decrease from 0.4% strain and continue to decrease to 1.0% strain. Moreover, the data scatter starts to increase stepwise at 0.4% strain, being correspondent to the variation in the critical current values with bending strain, and it continues to increase gradually to 1.0% strain. The coefficient of variation values are 2.5%, 5.1% and 6.4% for 0%, 0.4% and 1.0% bending strain, respectively. Results were also compared in terms of the irreversible bending strain. Possible sources of error are discussed

Traditional Cardiovascular Risk Factors Are Stronger Related to Carotid Intima-Media Thickness Than to Presence of Carotid Plaques in People Living With HIV

BACKGROUND: Cardiovascular disease is a major cause of morbidity and mortality in people living with HIV, who are at higher risk than the general population. We assessed, in a large cohort of people living with HIV, which cardiovascular, HIV-specific, and lipoproteomic markers were associated with carotid intima-media thickness (cIMT) and carotid plaque presence. We also studied guideline adherence on lipid-lowering medication in individuals with high and very high risk for cardiovascular disease. METHODS AND RESULTS: In 1814 individuals with a median (interquartile range) age of 53 (44–60) years, we found a carotid plaque in 909 (50.1%) and a median (interquartile range) intima-media thickness of 0.66 (0.57–0.76) mm. Ultrasonography was used for the assessment of cIMT and plaque presence. Univariable and multivariable regression models were used for associations with cIMT and presence of plaques. Age, Black race, body mass index, type 2 diabetes, and smoking (pack years) were all positively associated with higher cIMT. Levels of high-density lipoprotein cholesterol, specifically medium and large high-density lipoprotein subclasses, were negatively associated with higher cIMT. Only age and prior myocardial infarction were positively related to the presence of a carotid plaque. Lipid-lowering treatment was prescribed in one-third of people living with HIV, who are at high and very high risk for cardiovascular disease. CONCLUSIONS: Traditional cardiovascular risk factors were significantly associated with higher cIMT but not with carotid plaques, except for age. HIV-specific factors were not associated with both ultrasound measurements. Future studies are needed to elucidate which factors contribute to plaque formation. Improvement of guideline adherence on prescription of lipid-lowering treatment in high-and very high-risk patients for cardiovascular disease is recommended.</p

Bi-based HTS insert coils at high stress levels

Bi-2212 and Bi-2223 conductors from commercial sources are wound into double pancakes using the react and wind approach. One of the Bi-2223 conductors as supplied is reinforced with stainless steel strips. I–V characterization at 4.2 K until failure of the pancakes, in a 19 T–0.17 m cold bore magnet assembly, is performed. This gives insight in the operational limits of these conductors in conditions representative of insert magnets. The results are compared to the linear stress–strain properties of the conductors

Bi–Sr–Ca–Cu–O conductors and magnets at high stress–strain levels

We focus on a react and wind Bi-2223 conductor with a pure silver matrix. The stress–strain–critical current relation is determined for coils with a uniform stress distribution, at 4.2 K and 19 T. Somewhat surprisingly, the coils fail at a Lorentz force induced strain value close to the value for uni-axially loaded straight samples. This implies that bending strain is only partially retained due to the plastic nature of the pure silver matrix. Also, a double-pancake coil with a 15 mm winding thickness, resulting in a non-uniform stress distribution, is tested at 4.2 K and 19 T. The coil current is cycled with increasing peak values and the onset and evolution of critical current degradation in the coil is shown