Effect of Strand Damage in Nb₃Sn Rutherford Cables on the Quench Propagation in Accelerator Magnets

Accelerator magnets employing Nb$_3$Sn Rutherford cables are more susceptible to conductor degradation than Nb-Ti magnets. Recent measurements on a Nb$_3$Sn accelerator magnet have revealed unexpected behaviour such as decaying voltages at constant current plateaus of V-I measurements, inverse ramp rate and temperature dependence of quench currents, and anomalous quench propagation measured by so-called quench antennas. Numerical modelling has shown that these anomalies can be explained by an inhomogeneous degradation in the Rutherford cable, in which a subset of strands is fully or partially degraded. In this paper, we study how this type of degradation can affect the early stages of quench propagation. With the aid of a network model, we show how quench antenna signals can be used to diagnose inhomogeneous conductor degradation in the Rutherford cable

Effect of a DC transport current on the AC loss in no-insulation ReBCO racetrack coils exposed to AC parallel magnetic field at 77 K and 4.2 K

ReBCO coils are developed as DC field coils in linear motor systems to increase the force density, in favor of permanent magnets. Such coils have to sustain a relatively large heat load stemming from the AC magnetic field environment in which they operate. The use of no or partial turn-to-turn insulation can make them more stable against the effects of local heating. Conversely, the radial electrical connections in no-insulation (NI) coils allow for large coupling currents, causing additional AC loss on top of the already significant heat load. Here we report on the AC loss in sub-scale NI, 4 mm wide single-tape, ReBCO racetrack coils exposed to parallel-to-the-tape magnetic field in the frequency range of 10−4 to 1 Hz at 77 K and 4.2 K, while carrying a DC transport current. AC loss is measured magnetically and electrically. The main goal of these experiments is to validate our 2D numerical model, which provides more insight into the origin of the AC loss. At low frequencies, inter-turn coupling currents are spread more or less homogeneously throughout the winding pack. Whereas at high frequencies, the skin effect causes shielding of the interior of the coil and large induced currents only occupy the coil’s outer surface.</p

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.Comment: Presented at ICMC 2003, 25-28 May 200

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

Calculation and measurement of coupling loss in a no-insulation ReBCO racetrack coil exposed to AC magnetic field

No-insulation coils are self-protecting and can therefore generally be operated at higher current densities. However, the electrical turn-to-turn connections may cause additional AC loss when charging the coil or when it is exposed to a time-dependent magnetic field. In this work, we study the case of a no-insulation ReBCO tape racetrack coil exposed to a uniform AC field applied parallel to the tape surface. We show that an anisotropic continuum model allows to formulate analytical approximations for coupling loss in the low- and high-frequency limits. For intermediate frequencies, the continuum model needs to be evaluated numerically. The model was validated with representative measurements of AC loss in the coils, measured calorimetrically as well as magnetically using pick-up coils. The validation experiment confirms the predicted frequency dependence of the coupling loss, which is P ∝ f 2 at low frequencies and P ∝ f at high frequencies, due to the skin effect. The transition between low- and high-frequency regimes occurs around a characteristic frequency f c that is directly related to the characteristic time constant τ = 1 / 2 π f c associated with the current decay in (dis)charge experiments.</p

Cryogenics for an HTS degaussing system demonstrator

This paper describes the design, construction and test results of a high temperature superconducting (HTS) degaussing demonstrator system. Such a system compensates the local disturbance in the earth's magnetic field caused by the ferromagnetic hulls of ships, to prevent detection by active or passive magnetic field sensors. This is done by placing coils around the ship, creating a magnetic field opposing the effect of the earth's magnetic field. Degaussing systems for large naval vessels typically need currents of up to 1 or 2 kAturns, which gives rise to sizeable ohmic losses in conventional copper coils. These losses can be reduced if high temperature superconductors are used, since they have no electrical resistance when cooled down to temperatures below 90 K. For the demonstrator, 3 coils able to generate fields in 2 directions were realized both with HTS and copper to get a representative degaussing performance. A dedicatedly designed cooling system maintains the superconductors at a temperature of 77-85K using (subcooled) liquid nitrogen. Due to the relatively small laboratory scale that this first 1:5m long demonstrator system which was produced, the copper degaussing system is still more efficient than the HTS system because of the cooling power needed. A large fraction of this cooling power is needed to cool away parasitic heat loads, that hardly increases if the size of the system increases. Thereafter the performance of both systems was compared to evaluate on what scale HTS degaussing systems become more efficient than copper degaussing systems

Adiabatic normal zone development in MgB2 superconductors

A-priori knowledge of the normal zone development in MgB/sub 2/ conductors is essential for quench protection of applications. Therefore the normal zone propagation in a monofilament MgB/sub 2//Fe conductor under near-adiabatic conditions at 4.2 K has been measured and simulated. The results show normal zone propagation velocities up to several meters per second. In addition, by including the voltage-current relation into the computational model, the influence of the n-value on the normal zone propagation is determined. The simulations show that lower n-values suppress the normal zone propagation velocity due to lower heat generation in the MgB/sub 2/ filaments

The Effect of Ta and Ti Additions on the Strain Sensitivity of Bulk Niobium-Tin

The effect of tantalum and titanium additions on the composition, the superconducting properties, and their sensitivity to strain of bulk Nb3Sn is investigated. Using heat capacity analysis and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX), it is found that the binary Nb3Sn bulk and Nb3Sn bulk with added titanium and tantalum consist of stoichiometric Nb3Sn and niobium(-oxide). Furthermore, it is found that the niobium-to-tin ratio decreases in the presence of tantalum and increases in the presence of titanium, which suggests that tantalum is replacing niobium and titanium is replacing tin in the A15 crystal structure. Using a 10% resistivity criterion, it is observed that the critical magnetic field of unstrained binary bulk is 26.7 T, while the presence of tantalum and titanium raises the critical magnetic field to 29.3 and 30.1 T, respectively. The curves of normalized critical magnetic field as function of strain of all three samples nearly overlap, a strong indication that the variation in strain sensitivity observed in wires is not caused by the titanium and tantalum additions. Understanding the effect of additions on the composition, superconducting properties, and strain sensitivity of Nb3Sn is important for optimizing Nb3Sn conductor technolog

Two-gap superconductivity in MgB2_{2}: clean or dirty?

A large number of experimental facts and theoretical arguments favor a two-gap model for superconductivity in MgB$_{2}$. However, this model predicts strong suppression of the critical temperature by interband impurity scattering and, presumably, a strong correlation between the critical temperature and the residual resistivity. No such correlation has been observed. We argue that this fact can be understood if the band disparity of the electronic structure is taken into account, not only in the superconducting state, but also in normal transport