257 research outputs found
Synchrotron Tomography for the Study of Void Formation in Internal Tin NbSn Superconductors
Synchrotron absorption tomography has been applied for the study of voids formed during the thermal treatment of internal tin NbSn strands. Possible void formation mechanisms and in particular the effect of Sn phase transformations and melting are discussed based on a quantitative void description. Sn melting changes mainly the shape and volume of the individual voids but does not increase the total void volume in the strand
Thermomechanical Behavior of the HL-LHC 11 Tesla Nb3Sn Magnet Coil Constituents during Reaction Heat Treatment
The knowledge of the temperature induced changes of the superconductor
volume, and of the thermo-mechanical behaviour of the different coil and
tooling materials is required for predicting the coil geometry and the stress
distribution in the coil after the Nb3Sn reaction heat treatment. In the
present study we have measured the Young's and shear moduli of the HL-LHC 11 T
Nb3Sn dipole magnet coil and reaction tool constituents during in situ heat
cycles with the dynamic resonance method. The thermal expansion behaviours of
the coil components and of a free standing Nb3Sn wire were compared based on
dilation experiments.Comment: 6 pages, 12 figures, presented at MT25 conferenc
Electron stimulated carbon adsorption in ultra high vacuum monitored by Auger Electron Spectroscopy (AES)
Electron stimulated carbon adsorption at room temperature (RT) has been studied in the context of radiation induced surface modifications in the vacuum system of particle accelerators. The stimulated carbon adsorption was monitored by AES during continuous irradiation by 2.5 keV electrons and simultaneous exposure of the sample surface to CO, CO2 or CH4. The amount of adsorbed carbon was estimated by measuring the carbon Auger peak intensity as a function of the electron irradiation time. Investigated substrate materials are technical OFE copper and TiZrV non-evaporable getter (NEG) thin film coatings, which are saturated either in air or by CO exposure inside the Auger electron spectrometer. On the copper substrate electron induced carbon adsorption from gas phase CO and CO2 is below the detection limit of AES. During electron irradiation of the non-activated TiZrV getter thin films, electron stimulated carbon adsorption from gas phase molecules is detected when either CO or CO2 is injected, whereas the CH4 partial pressure has no influence on the C-KLL intensity evolution
Phase transformations during the reaction heat treatment of Powder-In-Tube NbSn superconductors
The phase transformations prior to the superconducting A15 phase nucleation can influence the critical properties of fully reacted NbSn superconductors. We describe for the first time the phase transformations occurring during the heat treatment of state-of-the-art NbSn Powder-in-Tube strands, based on in-situ synchrotron diffraction measurements. All pure Sn present in the non-heat treated PIT strand is transformed into Cu6Sn5 by solid state diffusion. Subsequently NbSn2 and Cu6Sn5 are partly transformed into a Sn-rich ternary Cu-Nb-Sn phase. When Sn starts to diffuse into the Nb tubes at about 520°C all Sn-containing phases are transformed into NbSn2 and subsequently into Nb6Sn5
In-Situ Neutron Diffraction Under Tensile Loading of Powder-in-Tube Cu/NbSn Composite Wires: Effect of Reaction Heat Treatment on Texture, Internal Stress State and Load Transfer
The strain induced degradation of NbSn superconductors can hamper the performance of high field magnets. We report elastic strain measurements in the different phases of entire non-heat treated and fully reacted NbSn composite strands as a function of uniaxial stress during in-situ deformation under neutron beam. After the reaction heat treatment the Cu matrix loses entirely its load carrying capability and the applied stress is transferred to the remaining Nb-Ta alloy and to the brittle (Nb-Ta)3Sn phase, which exhibits a preferential grain orientation parallel to the strand axis
The Influence of Air Exposures and Thermal Treatments on the Secondary Electron Yield of Copper
The variation of the secondary electron yield (SEY) of sputter-cleaned OFHC-copper has been studied as a function of air exposure duration at room temperature. After short air exposures of some seconds the maximum SEY (deltaMAX) of clean copper is reduced from 1.3 to less than 1.2, due to the oxidation of the copper surface. Prolonged air exposure increases the SEY steadily until, after about 8 days of atmospheric exposure, deltaMAX is higher than 2.Air exposures at higher temperatures have been found to be effective in reducing the SEY of technical copper surfaces. A 5-minute air exposure of copper at 350°C followed by a 350°C bake-out under vacuum reduces deltaMAX to about 1.05, which is lower than the value of pure copper and that of Cu2O
Effects of Neutron Irradiation on Pinning Force Scaling in State-of-the-Art Nb3Sn Wires
We present an extensive irradiation study involving five state-of-the-art
Nb3Sn wires which were subjected to sequential neutron irradiation up to a fast
neutron fluence of 1.6 * 10^22 m^-2 (E > 0.1 MeV). The volume pinning force of
short wire samples was assessed in the temperature range from 4.2 to 15 K in
applied fields of up to 7 T by means of SQUID magnetometry in the unirradiated
state and after each irradiation step. Pinning force scaling computations
revealed that the exponents in the pinning force function differ significantly
from those expected for pure grain boundary pinning, and that fast neutron
irradiation causes a substantial change in the functional dependence of the
volume pinning force. A model is presented, which describes the pinning force
function of irradiated wires using a two-component ansatz involving a
point-pinning contribution stemming from radiation induced pinning centers. The
dependence of this point-pinning contribution on fast neutron fluence appears
to be a universal function for all examined wire types.Comment: 8 page
The secondary electron yield of air exposed metal surfaces at the example of niobium
The secondary electron yield (SEY) variation of atomically clean metal surfaces due to air exposures and during subsequent heat treatments is described with the example of a sputter-deposited Nb thin film. Corresponding variations of the surface chemical composition have been monitored using AES and SSIMS. On the basis of these results and of previously obtained SEY results on metals and metal oxides the origin of the SEY variations is discussed. The SEY increase, which is generally observed during long lasting air exposures of clean metals, is mainly caused by the adsorption of an airborne carbonaceous contamination layer. The estimated value of about 3 for the maximum SEY of this layer is higher than that of all pure metals. Only in some cases the air-formed oxide can contribute to the air exposure induced SEY increase while many oxides have a lower SEY than their parent metals. From the experimental data it can also be excluded that the SEY increase during air exposures is mainly due to an increased secondary electron escape probability
On the formation of voids in internal tin NbSn superconductors
In this article we describe three void growth mechanisms in NbSn strands of the internal tin design on the basis of combined synchrotron micro-tomography and x-ray diffraction measurements during in-situ heating cycles. Initially void growth is driven by a reduction of void surface area by void agglomeration. The main void volume increase is caused by density changes during the formation of Cu3Sn in the strand. Subsequent transformation of Cu-Sn intermetallics into the lower density a-bronze reduces the void volume again. Long lasting temperature ramps and isothermal holding steps can neither reduce the void volume nor improve the chemical strand homogeneity prior to the superconducting A15 phase nucleation and growth
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