129 research outputs found
The behaviour of copper in view of radiation damage in the LHC luminosity upgrade
In view of the safe operation of the quadrupoles in the luminosity upgrade of
the LHC accelerator, the response of the copper stabilizer at low temperatures
to the various high energy radiation sources is of primary importance. The
present study takes into account the expected high energy spectrum of the
simultaneous radiation by neutrons, protons, pions, electrons and photons,
calculated using the FLUKA code by F. Cerutti (CERN) as well as on literature
values. It was found that proton irradiation causes a considerably higher
damage than neutron irradiation: in spite of a 3.8% proton fraction, the
measured damage is of the order of 20%, which fits with the calculations of N.
Mokhov (Fermilab) on the contribution of protons to the dpa. The same
calculations indicate that the total effect of protons, pions and electrons is
at least as high as that of neutrons. Since recent neutron experiments of
Nakamoto et al. show that the RRR of Cu is reduced from 200 to 50-120 for a
fluence of 10^{21} n/cm^{2}, it follows that the inclusion of all high energy
sources would lead to RRR values well below 50, thus endangering stability and
protection. This result confirms the necessity of including a tungsten shield
inside the quadrupoles.Comment: 7 pages, Contribution to WAMSDO 2013: Workshop on Accelerator Magnet,
Superconductor, Design and Optimization; 15 - 16 Jan 2013, CERN, Geneva,
Switzerlan
Improved critical current densities in B4C doped MgB2 based wires
An improvement of the transport critical current density, Jc, of MgB2 wires
was obtained after addition of 10 wt.% B4C powders, after reaction at 800 C: Jc
values of 1.10^4 A/cm2 at 4.2 K and 9T were obtained for wires of 1.11 mm
diameter in a Fe matrix. The starting mixture of Mg and B was doped with
sub-micrometric B4C, the ratio being Mg : B : B4C = 1: 2 : 0.08, corresponding
to 10 wt.% B4C. For T > 800 C, a decrease of Jc was found, due to the reaction
with the Fe sheath. In order to investigate the origin of the improvement of
the transport properties for heat treatments up to 800 C, X-ray diffraction
measurements were performed. A decrease of the lattice constant a from 0.30854
nm to 0.30797 nm was found, thus suggesting an effect of the substitution of
Carbon on the properties of the wires. A comparison with the literature data
shows that the addition of B4C powders leads to the second highest improvement
of Jc reported so far after SiC, thus constituting an alternative for future
applications.Comment: 18 pages, 8 figures, 1 tabl
Observation of coherent Josephson response in the non-linear ab-plane microwave impedance of single crystals
We report novel non-linear phenomena in the -plane microwave impedance of
single crystals. The vs. data are well
described by the non-linear RSJ model : . The entire crystal behaves like a single Josephson junction. The
extraordinary coherence of the data suggests an intrinsic mechanism.Comment: 2 pages,1 figure, Submitted to Proc. of M^2SHTSC-V (Beijing), also
available at http://sagar.physics.neu.edu/preprint
Mechanism of Enhancement in Electromagnetic Properties of MgB2 by Nano SiC Doping
A comparative study of pure, SiC, and C doped MgB2 wires has revealed that the SiC doping allowed C substitution and MgB2 formation to take place simultaneously at low temperatures. C substitution enhances Hc2, while the defects, small grain size, and nanoinclusions induced by C incorporation and low-temperature processing are responsible for the improvement in Jc. The irreversibility field (Hirr) for the SiC doped sample reached the benchmarking value of 10 T at 20 K, exceeding that of NbTi at 4.2 K. This dual reaction model also enables us to predict desirable dopants for enhancing the performance properties of MgB2
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