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 YBa2Cu3O6.95YBa_{2}Cu_{3}O_{6.95} single crystals

We report novel non-linear phenomena in the $ab$-plane microwave impedance of $YBaCu_{2}O_{7-\delta }$ single crystals. The $R_s$ vs. $H_{rf}$ data are well described by the non-linear RSJ model : $\dot{\phi}+\sin \phi =i_{rf}\cos \omega t$. 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