Direct measurement of the AC loss of an ITER relevant coil

In the ENEA Frascati Laboratory, an ITER relevant coil is being tested in pulsed regime. One item of the testing program is the search for possible ramp rate limitations. The range explored (up to 3.2 T/s) covers amply the ITER Central Solenoid operating regime (0.5–1 T/s). An interesting outcome of these tests is that repeated run sets, performed in similar conditions, show an increasing value of the ramp rate that produces a quench. This behaviour is attributed to a continuous decrease of the value of the AC loss constant nτ, due to the increase of the cable transversal resistivity. An explanation on the basis of a ‘classical' training was also considered, but for a number of reasons reported in the paper, it was discarded. Direct measurements of the AC loss on the whole coil confirm that the original value of nτ (140 ms), as measured on a ‘virgin' piece of conductor, has gone below 40 ms. The measurements are made difficult by the presence of a heavy mechanical structure and of a background coil. The techniques applied and the results are reported in detail.     

AC loss measurements on model Bi-2223 conductors

Measurements of the AC loss in applied magnetic fields at 77 K have been made on model composite Bi-2223 conductors. A vibrating sample magnetometer (VSM) and a dual Hall sensor magnetometer (HSM) were used to cover the frequency range from below 0.01 Hz to over 250 Hz at AC fields up to 0.05 T rms. The VSM was limited to the frequency range below 0.2 Hz. A comparison of the two measurement techniques was possible at intermediate frequencies. The samples consisted of vertical stacks of well separated flat filaments of superconductor in Ag and Ag-alloy matrix, allowing a range of filament coupling conditions to be explored.     

Transient stability of AC multi-strand superconducting cables

AC application, it is necessary to estimate the stability of multi-strand superconducting cable. Therefore, we have been studying the transient stability of non-insulated multi-strand cable when one strand in a cable turns into the normal state locally. In the quench process, local temperature rise produced by current redistribution among strands is not desirable for stability. In a previous work, we discussed the effect of Cu matrix allocated to each strand on the transient stability and showed that the Cu matrix allocation can improve the stability of non-insulated multi-strand cable through mainly numerical simulations. In this paper, we carried out experiments on three kinds of non-insulated three-strand cables; one consists of NbTi/CuNi strands and the others consist of NbTi/Cu/CuNi strands having different cross-sectional arrangement. These sample strands have almost the same diameter, the same matrix to superconductor ratio and the same B–J characteristics to evaluate the effect of Cu allocation quantitatively. We choose to define the transient stability in terms of the minimum quench energy (MQE) at each DC transport current. We also investigated the transient stability of sample cables when quench is initiated in two or three (all) strands simultaneously.     

AC power loss for superconducting strips of arbitrary thickness carrying a transport current

The magnetic field and current distributions are computed numerically for a superconducting strip in the critical state with infinite length, but finite thickness and width. The width to thickness ratio is varied from 1 to 100, covering the range from square bars to thick films. For large aspect ratios (>10) the current and field distributions differ significantly from the 2D solution, because the Norris solution does not take into account (and thus does not shield) the transverse field parallel to the strip. Power loss exponents are calculated and found to vary from roughly 3 in a square bar to 4 in a highly aspected thin strip. In particular in the thin strip case the power loss exponents are current dependent due to the contribution of the loss caused by the transverse magnetic field. The loss in a multifilamentary conductor (or cable) is also estimated, taking into account the loss in the filaments. It is found that the overall loss (without coupling) is enhanced by a factor 1+1/n, where n is the number of filaments.     

AC losses in multifilamentary HTS-composite tapes based on BiPbSrCaCuO

Some results of AC loss measurements are presented for 19, 61, 127-filamentary Bi-2223/Ag tapes prepared by the ‘powder-in-tube' method. All measurements have been made at T=77 K under sinusoidal transport current with frequency in the range of 30–600 Hz and the current amplitude up to 30 A. The measurements have been carried out both in self field conditions and at the external magnetic field applied to the tape at the different angles. The dependencies of the AC losses on current amplitude and frequency have been obtained. It is found that for all tapes the current amplitude dependencies of the AC losses show good agreement with the Norris prediction for an elliptical or strip geometry. The AC loss dependencies on frequency were linear. The measurements of AC losses in external magnetic field show that the change of AC losses is only through the change of the critical current. So the transport AC losses in the tapes are the ‘saturation losses' that is they are different from classic hysteresis losses.     

Twist effect on hysteresis loss in Bi2223 multifilamentary wires exposed to an AC magnetic field

We fabricated Bi2223 multifilamentary sample wires with various twist pitches and investigated the electromagnetic properties experimentally. They showed monofilamentlike electromagnetic properties regardless of twisting due to the contacts among filaments and/or proximity effect. The observed AC losses in the non-twisted sample wire agreed roughly with the theoretical prediction for a homogeneous superconducting slab with the same thickness of the filamentary region on the basis of Irie–Yamafuji model. However the AC losses in the twisted wires deviated from the theoretical ones, especially for the amplitude around the theoretically predicted penetration field of the slab. We showed that the observed AC loss properties can be explained by both the twist effect for the macroscopic shielding current and the contribution of the local shielding current.     

Self-field AC losses of textured Bi2Sr2CaCu2O8+δ thin rods

Self-field AC losses of polycrystalline Bi-2212 thin rods textured by a Laser Floating Zone (LFZ) melting technique have been measured at 77 K. With the optimal processing parameters, these rods, of 1.6–2 mm diameter and 10 cm length, have a transport critical current density of 3 kA/cm² in the self-field which decreases to about 1.5 kA/cm² in fields of 0.02 T applied perpendicular to the rod axis. The self-field AC losses have been measured in DC magnetic fields up to 0.03 T. The measurements in zero field show that for a large current range the losses are dominated by hysteresis losses as described by the Critical State Model for a cylinder. For the measurements in DC fields the losses show an increasingly resistive-like dependence with current, while the hysteretic component expected from the CSM becomes less important. Measurements at different frequencies also indicated that the loss per cycle in fields is strongly frequency dependent.     

金属担载量对CuO/AC干法催化氧化苯酚的影响

采用程序升温氧化（TPO）技术和吸附 催化氧化循环实验,研究了金属担载量对CuO/AC催化-吸附剂干法催化氧化苯酚的影响。结果表明，随着金属担载量的增加，CuO/AC催化-吸附剂的苯酚催化氧化活性升高，并且苯酚与催化-吸附剂之间的相互作用增强，使吸附的苯酚不易脱附/降解脱附。同时，CuO/AC催化 吸附剂自身的烧蚀活性也随金属担载量的增加而升高。对于吸附-干法催化氧化法而言，CuO/AC存在一个较佳的金属担载量范围，该范围以Cu的质量分数计为3%～5%。     

Simple and accurate ac calorimeter for liquid crystals and solid samples

We have designed, built and calibrated a microcomputer-controller automated a.c. calorimeter for liquid crystals and solid samples, in the temperature range from 30 to 250°C, in which we enhance several features of previous ones reported in the literature. We have incorporate the lock-in amplifier model 5302 from EG&;G in our set-up, which permits to extend the frequency range to 1 mHz. This allows the performance of suitable frequency sweeps in order to choose the optimum operating frequency for low thermal conductivity materials. The calorimeter has been calibrated with the 8OCB liquid crystal compound. The resolution obtained forC _p values was better than 0.1% and the absolute erroer around 5%. As applications, we show the ferroelectric transition of the triglicerine sulphate (TGS), the Nematic-Smetic A transition of the 8OCB and a study in a binary system of ferroelectric liquid crystals, SCE9-SCE10.     

The effect of hydration/dehydration on the AC electrical conductivity of H4PVMo11O40 and some of its cesium salts used as catalysts

Changes in AC electrical conductance G of vanadium substituted 12-molybdophosphoric acid and of its (mono)-acid and neutral cesium salts were studied. Depending on the environment, the electrical conductivity is a combination of protonic and electronic conduction, the protonic one dominating at low temperature.