arXiv subjects

R. Flukiger

Publications and source records attributed to R. Flukiger.

18 recordsLinked to original sources

arXiv2014

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.

physics.acc-ph↗

arXiv2009

Strong enhancement of Jc in binary and alloyed in-situ MgB2 wires by a new approach: Cold high pressure densification

Cold high pressure densification (CHPD) is presented as a new way to substantially enhance the critical current density of in situ MgB2 wires at 4.2 and 20 K at fields between 5 and 14 T. The results on two binary MgB2 wires and an alloyed wire with 10 wt.% B4C are presented The strongest enhancement was measured at 20K, where cold densification at 1.85 GPa on a binary Fe/MgB2 wire raised both Jcpara and Jcperp by more than 300% at 5T, while Birr was enhanced by 0.7 T. At 4.2K, the enhancement of Jc was smaller, but still reached 53% at 10 T. After applying pressures up to 6.5 GPa, the mass density dm of the unreacted (B+Mg) mixture inside the filaments reached 96% of the theoretical density. After reaction under atmospheric pressure, this corresponds to a highest mass density df in the MgB2 filaments of 73%. After reaction, the electrical resistance of wires submitted to cold densification was found to decrease, reflecting an improved connectivity. A quantitative correlation between filament mass density and the physical properties was established. Monofilamentary rectangular wires with aspect ratios a/b < 1.25 based on low energy ball milled powders exhibited very low anisotropy ratios, Gamma = Jcpara/Jcperp being < 1.4 at 4.2 K and 10T. The present results can be generalized to alloyed MgB2 wires, as demonstrated on a wire with B4C additives. Based on the present data, it follows that cold densification has the potential of further improving the highest Jcpara and Jcperp values reported so far for in situ MgB2 tapes and wires with SiC and C additives. Investigations are under work in our laboratory to determine whether the densification method CHPD can be applied to longer wire or tape lengths.

cond-mat.mtrl-sci↗

arXiv2008

Upper critical fields well above 100 T for the superconductor SmFeAsO$_{0.85}$F$_{0.15}$ with T$_c$ = 46 K

We report specific heat measurements at magnetic fields up to 20 T on the recently discovered superconductor SmFeAsO$_{0.85}$F$_{0.15}$. The B-T diagram of a polycrystalline SmFeAsO$_{0.85}$F$_{0.15}$ sample with T$_c$ = 46 K was investigated. The temperature dependence of B$_{c2}$ was extracted from the specific heat curves, the corresponding B$_{c2}$(T = 0) value derived from the Werthamer-Helfand-Hohenberg formula being 150 T. Based on magnetization measurements up to 9 T, a first estimation of the field dependence of the inductive critical current J$_c$ is given. Evidence for granularity is found. The presence of a peak effect is reported, suggesting a crossover in the vortex dynamics, in analogy to the behaviour observed in high T$_c$ cuprates.

cond-mat.supr-con↗

arXiv2007

New aspects in the Bragg Glass-Disordered phase transition: an analysis based on the 3rd harmonics of the AC magnetic susceptibility

We analyse the phase transition between the Bragg Glass and the Disordered phase in the vortex lattice in type-II superconductors, both by analytical computations and experimental investigations. It is known that if the Peak Effect can be detected, a Bragg Glass/Disordered phase transition takes place. We show that, in some conditions, this transition can occur without the observation of the Peak Effect Phenomenon. We introduce a method based on the 3rd harmonics of the AC magnetic susceptibility to detect the transition also in these cases. Using this method, we obtain an experimental confirmation of the theoretical predictions on sphere shaped V3Si single crystals, in the high fields/low temperatures range too, where previous experimental studies failed to detect the Bragg/Disordered phase transition.

cond-mat.supr-con↗

arXiv2007

Mechanism of enhancement of 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.

cond-mat.supr-con↗

arXiv2006

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.

cond-mat.supr-con↗

arXiv2004

New apparatus for DTA at 2000 bar: thermodynamic studies on Au, Ag, Al and HTSC oxides

A new DTA (Differential Thermal Analysis) device was designed and installed in a Hot Isostatic Pressure (HIP) furnace in order to perform high-pressure thermodynamic investigations up to 2 kbar and 1200C. Thermal analysis can be carried out in inert or oxidising atmosphere up to p(O2) = 400 bar. The calibration of the DTA apparatus under pressure was successfully performed using the melting temperature (Tm) of pure metals (Au, Ag and Al) as standard calibration references. The thermal properties of these metals have been studied under pressure. The values of DV (volume variation between liquid and solid at Tm), ROsm (density of the solid at Tm) and ALPHAm (linear thermal expansion coefficient at Tm) have been extracted. A very good agreement was found with the existing literature and new data were added. This HP-DTA apparatus is very useful for studying the thermodynamics of those systems where one or more volatile elements are present, such as high TC superconducting oxides. DTA measurements have been performed on Bi,Pb(2223) tapes up to 2 kbar under reduced oxygen partial pressure (p(O2) = 0.07 bar). The reaction leading to the formation of the 2223 phase was found to occur at higher temperatures when applying pressure: the reaction DTA peak shifted by 49C at 2 kbar compared to the reaction at 1 bar. This temperature shift is due to the higher stability of the Pb-rich precursor phases under pressure, as the high isostatic pressure prevents Pb from evaporating.

cond-mat.mtrl-sci↗

arXiv2003

Third Harmonic Susceptibility and the Irreversibility Line of Fe/MgB2 Tapes

We report a study of the pinning properties of Fe/MgB2 tapes performed by means of ac magnetic susceptibility measurements. In particular, the third harmonic response has been measured as a function of the dc magnetic field up to 9 T, at different temperatures, for various frequencies and amplitudes of the ac field. The irreversibility line has been determined using the third harmonic onset criterion for samples made with different ball-milled MgB2 powders. The study of the harmonic response allows us to probe the nature of the vortex pinning and to analyse its influence on the enhancement of the irreversibility field in strongly ball-milled samples. We have also investigated the dynamical regimes governing the vortex motion in the tapes by comparing the experimental curves with numerical simulations of the non-linear diffusion equation for the magnetic field.

cond-mat.supr-con↗

arXiv2003

Transport Properties and Exponential n-values of Fe/MgB2 Tapes With Various MgB2 Particle Sizes

Fe/MgB2 tapes have been prepared starting with pre-reacted binary MgB2 powders. As shown by resistive and inductive measurements, the reduction of particle size to a few microns by ball milling has little influence on Bc2, while the superconducting properties of the individual MgB2 grains are essentially unchanged. Reducing the particle size causes an enhancement of Birr from 14 to 16 T, while Jc has considerably increased at high fields, its slope Jc(B) being reduced. At 4.2K, values of 5.3*10^4 and 1.2*10^3 A/cm^2 were measured at 3.5 and 10 T, respectively, suggesting a dominant role of the conditions at the grain interfaces. A systematic variation of these conditions at the interfaces is undertaken in order to determine the limit of transport properties for Fe/MgB2 tapes. The addition of 5% Mg to MgB2 powder was found to affect neither Jc nor Bc2. For the tapes with the highest Jc values, very high exponential n factors were measured: n = 148, 89 and 17 at 3.5, 5 and 10T, respectively and measurements of critical current versus applied strain have been performed. The mechanism leading to high transport critical current densities of filamentary Fe/MgB2 tapes based on MgB2 particles is discussed.

cond-mat.supr-con↗

arXiv2003

Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements

In a type-II superconductor the generation of higher harmonics in the magnetic response to an alternating magnetic field is a consequence of the non-linearity in the I-V relationship. The shape of the current-voltage (I-V) curve is determined by the current dependence of the thermal activation energy U(J) and is thus related to the dynamical regimes governing the vortex motion. In order to investigate the vortex dynamics in MgB2 bulk superconductors we have studied the fundamental (chi1) and third (chi3) harmonics of the ac magnetic susceptibility. Measurements have been performed as a function of the temperature and the dc magnetic field, up to 9 T, for various frequencies and amplitudes of the ac field. We show that the analysis of the behaviour in frequency of chi3(T) and chi3(B) curves can provide clear information about the non-linearity in different regions of the I-V characteristic. By comparing the experimental curves with numerical simulations of the non-linear diffusion equation for the magnetic field we are able to resolve the crossover between a dissipative regime governed by flux creep and one dominated by flux flow phenomena.

cond-mat.supr-con↗

arXiv2002

Transport critical current densities and n factors in mono- and multifilamentary MgB2/Fe tapes and wires using fine powders

Mono- and multifilamentary MgB2/Fe tapes and wires with high transport critical current densities have been prepared using the powder-in-tube (PIT) process. The fabrication details are described. The effect of powder grain sizes and recrystallization temperature on jc has been investigated. At 25K and 1 T, jc values close to 105 A/cm2 were measured, while jc of 106 A/cm2 were extrapolated for 4.2K/0T in our monofilamentary tape. MgB2/Fe tapes exhibit high exponential n factors for the resistive transition: n ~ 80 and 40 were found at 5 T and 7 T, respectively. The highest transport jc values obtained so far in MgB2/Fe wires with 7 filaments were 1.1 * 105 A/cm2 at 4.2 K and in a field of 2 T, which is still lower than for monofilamentary tapes. Improved deformation and recovering processing is expected to lead to higher jc values.

cond-mat.supr-con↗

arXiv2002

AC losses in Bi,Pb(2223) multifilamentary wires with square cross-section

We have fabricated prototype Bi,Pb(2223)/Ag superconduting multifilamentary wires with a square cross-section. The AC losses of these wires were measured, compared with those of tapes and also compared with the theory. Wires show largely reduced AC magnetic field losses at 47 Hz, compared to those of tapes in a perpendicular magnetic field. The effects of the twist pitch and of the use of the high resisitive barriers were also investigated. The results show that the use of a twist pitch of about 10 mm is sufficient for decoupling the filaments, and that the use of very short twist pitches does not further reduce the absolute value of the AC losses. The losses in the wires are still higher than those of tapes in a parallel magnetic field, but the results show that the square or round configurations are interesting for applications where a perpendicular component of the magnetic field cannot be avoided, or for applications in rotating magnetic fields.

cond-mat.supr-con↗

arXiv2002

High-pressure synthesis of pure and doped superconducting MgB2 compounds

Superconducting properties of bulk, dense, pure MgB2 and doped (Mg1-xAx)B2 samples with A = Na, Ca, Cu, Ag, Zn and Al were studied for compositional ranges 0 < x < 0.20. The effect on pinning properties and critical current were investigated, particularly for A = aluminium. The samples were sintered and/or synthesised at high pressure - high temperature in a cubic multi-anvils press (typically : 3.5-6 GPa, 900-1000C). They were characterised by x-ray diffraction, scanning electron microscopy and their superconducting properties were investigated by a.c. susceptibility, magnetisation (VSM and SQUID) and transport measurements under magnetic field. Only Al substitutes really on the Mg site. The other elements form secondary phases with B or Mg which do not act as pinning centres. No positive effect is observed on the superconducting properties of the bulk MgB2 samples added with these doping elements : Tc, jc critical current, Hirr and Hc2. For Al, the effect on Hc2 remains small, and the irreversibility line does not move, thus not improving the critical current of the Al-doped MgB2 samples.

cond-mat.supr-con↗

arXiv2002

Superconductivity in high-pressure synthesized pure and doped MgB2 compounds

Dense pure and doped (Mg1-xAx)B2 samples with A = Na, Ca, Cu, Ag, Zn and Al were synthesized at high pressure - high temperature in a multi anvils press (3.5-6 GPa, 900-1000C) for x up to 0.20. They were studied by x-ray diffraction, scanning electron microscopy and their superconducting properties were investigated by a.c. susceptibility, magnetization and transport measurements. Only Al is really substituted on the Mg site. The others elements form secondary phases with B or Mg. No large effect is observed on the superconducting properties Tc, jc critical current, Hirr and Hc2.

cond-mat.supr-con↗

arXiv2002

Transport critical current, anisotropy, irreversibility fields and exponential n factors in Fe sheathed MgB2 tapes

The influence of the initial MgB2 grain size on critical current density, upper critical fields and irreversibility has been studied on Fe sheathed monofilamentary MgB2 tapes prepared by the Powder-In-Tube technique. The effect of the reduction of MgB2 grain size by ball milling was mainly to enhance both the critical current density, jc, and the irreversibility field, while the upper critical field remained unchanged. The anisotropy ratio of the upper critical field between magnetic fields parallel and perpendicular the tape surface was determined to 1.3, reflecting a deformation induced texture. A good agreement has been found between resistive and inductive jc values, measured at various temperatures. At 25K and 1 T, jc values close to 105 A/cm2 were measured. The exponential n factor of the resistive transition was found to be quite high at low fields, and decrease linearly from 60 at 4T to 10 at 8.5T.

cond-mat.supr-con↗

arXiv2002

Fabrication and transport critical currents of multifilamentary MgB2/Fe wires and tapes

Multifilamentary MgB2/Fe wires and tapes with high transport critical current densities have been fabricated using a straightforward powder-in-tube (PIT) process. After annealing, we measured transport jc values up to 1.1 * 105 A/cm2 at 4.2 K and in a field of 2 T in a MgB2/Fe square wire with 7 filaments fabricated by two-axial rolling, and up to 5 * 104 A/cm2 at 4.2 K in 1 T in a MgB2/Fe tape with 7 filaments. For higher currents these multifilamentary wires and tapes quenched due to insufficient thermal stability of filaments. Both the processing routes and deformation methods were found to be important factors for fabricating multifilamentary MgB2 wires and tapes with high transport jc values.

cond-mat.supr-con↗

arXiv1997

Observation of coherent Josephson response in the non-linear ab-plane microwave impedance of $YBa_{2}Cu_{3}O_{6.95}$ single crystals

We report novel non-linear phenomena in the $ab$-plane microwave impedance of $YBaCu_{2}O_{7-δ}$ single crystals. The $R_s$ vs. $H_{rf}$ data are well described by the non-linear RSJ model : $\dotϕ+\sin ϕ=i_{rf}\cos ωt$. The entire crystal behaves like a single Josephson junction. The extraordinary coherence of the data suggests an intrinsic mechanism.

cond-mat.supr-con↗

arXiv1996

New features in the microwave response of YBCO crystals: Evidence for a multi-component order parameter

New features are reported in precision measurements of the complex microwave conductivity of high quality $YBa_{2}Cu_{3}O_{6.95}$ crystals grown in $BaZrO_{3}$ crucibles. A third peak in the normal conductivity, $\sigma_{1}(T)$, at around $80K$, and enhanced pair conductivity $\sigma_{2}(T)$ below $\sim 65K$ are observed. The data are inconsistent with a single order parameter, and instead are indicative of multi-component superconductivity. Overall, these results point to the presence of multiple pairing interactions in $YBa_{2}Cu_{3}O_{6.95}$ and also provide a natural explanation to account for the low temperature $35K$ conductivity peak observed in all $YBa_{2}Cu_{3}O_{6.95}$ crystals.

cond-mat.supr-con↗