Minimum Propagation Current Research Articles

Numerical and experimental investigation on NbTi superconducting liquid helium level sensor based on a resistance heating

A resistance-heated NbTi superconducting liquid helium level sensor based on the principle of thermal conduction is proposed. A heat transfer model for the sensor was established, and its mathematical model was characterized. The effects of temperature, current, heating resistance value, and superconducting wire diameter on the sensor performance were investigated. It indicates that the temperature of the sensor in the vapor phase is decreased linearly from high to low, and the voltage is decreased with the increase of the liquid level. And the 45 μm diameter superconducting wire has a minimum propagation current, a minimal heat input to the system, and a small measurement error. The variation of the sensor output voltage with a level height was studied experimentally. The linear correlation coefficients R2 of the 1 Ω, 5.1 Ω, and 10 Ω sensors are all greater than 0.999, which are almost identical to the simulation results. The sensor’s response time is all less than 1 s, the sensitivity is 8.7070 mV/mm, 8.0110 mV/mm, and 6.9660 mV/mm, respectively, and the accuracy is 0.8226%, 0.8611%, and 0.9030%.

The experimental research and analysis on the quench propagation of YBCO coated conductor and coil

Quench propagation characteristics of YBCO tapes and coils were studied in this paper. Quench propagation in each test was initiated by a NiCr wire heater mounted on the sample tape and coil. Quench propagation velocity was measured through voltage taps and temperature sensors across different zones of the tapes and coil. Voltage taps and temperature sensors are arranged in longitudinal and radial direction of the coil. The thermal stability margin, the longitudinal quench propagation velocity for the tape and coil, radial quench propagation velocity for coil and the minimum propagation current were measured. In this study, we investigated the relationship between heat inputs and normal zone propagation velocities and the impact of operating currents on normal zone propagation velocities. We also compared the longitudinal quench propagation velocity and the radial quench propagation velocity. Furthermore, we performed numerical simulations by using a computer program based on a one-dimensional heat flow equation model for YBCO tape. The temperature longitudinal distributions of YBCO tape were simulated. The simulation results also prove the relation between heat input and normal zone propagation velocities and agree with the experimental results.

Performance of the Superconducting Helical Coils of LHD

Performance of the superconducting helical coils of the Large Helical Device (LHD) during the past 12 cooling cycles is reviewed. The pair of helical coils are pool cooled by liquid helium and wound with aluminum-stabilized NbTi/Cu composite-type superconductors. Intensive efforts have been made to reliably carry out excitations, as more than 20 temporary normal transitions were observed. It was found that the minimum propagation current was about 10% lower than the nominal operation current. To improve the cryogenic stability, subcooled liquid helium has been supplied since 2006 using cold compressors, and the inlet temperature is lowered to be 3.2 K. The toroidal magnetic field has been raised by 5% and the plasma parameters are being enhanced. Pulse-height analysis is successfully applied on the balance voltage and acoustic emission signals to investigate the mechanical properties of the windings and their changes in years of operation. Short-duration normal transitions are automatically detected using a sophisticated monitoring system and careful operations are continued.

Study on Minimum Propagation Current of Bi-2223/Ag Superconducting Multifilament Tape

Minimum propagation current of Bi-2223/Ag superconducting multifilament tape was studied in detail, including experimental investigation and numerical calculation using finite element method (FEM). A set of experimental system has been established, quench propagation velocities with various transport current have been measured and minimum propagation current has been obtained in the condition of N2 gas cooling. The effect of parallel background magnetic field on minimum propagation current has also been studied experimentally. In addition, minimum propagation current at different operating temperature has been gained by the simulated calculation using FEM.It has been found that the high temperature superconductor is stable if only transport current is smaller than minimum propagation current of the time no matter what condition it is in. Minimum propagation current can be considered as an important stability criterion of high temperature superconductor.

Thermal stability analysis of high temperature superconducting magnet coils under overcurrent pulses

This paper presents some numerical simulation results of thermal behavior on the high temperature superconducting (HTS) magnet coils wounded by Bi2223/Ag multifilamentary tapes subjected to an overcurrent pulse using the finite element method (FEM). The analytical results show that the time-varying spatial temperature distribution of the HTS magnet coils under the condition of different overcurrent pulses has an important influence on its stability estimation, and the temperature dependence of minimum propagation current plays a key role in the stability analysis of the HTS magnet. Through the FEM analysis, the permitted duration time of 5-multiple overcurrent pulse has been obtained for the HTS magnet under the various operating conditions and cooling conditions. In this paper, an analytical method for determining the stability of the HTS magnet coils subjected to an overcurrent pulse has been proposed and a stability criterion has been presented. The critical current and minimum propagation current can be chosen as the restrictive condition of operating current in the HTS magnet coils subjected to an overcurrent pulse.

Effect of Surface Oxidation on Stability of LHD Conductor Immersed in Pressurized<tex>$rm He$</tex>II

Stability tests were performed on two small test coils wound with the Large Helical Device (LHD) conductors with different surface conditions. One used the conductor with a chemically oxidized copper surface, which is the same as that used for the LHD helical coils. Another used the same structural conductor with a polished copper surface. The stability tests were performed for the magnetic flux densities from 3 T to 7 T and at the bulk liquid helium temperatures from 2.0 K to 4.2 K at atmospheric pressure. Stability limit current for the LHD test coil was defined as a minimum dynamic propagation current, beyond which a short normal zone initiated by a thermal disturbance dynamically propagated to both sides along the conductor. There was a slight difference in the stability limit currents for the two test coils in case of He I cooling. However, the stability limit current for the polished coil was significantly improved as compared with that for the oxidized one in case of He II cooling. The Kapitza conductance on an oxidized copper surface is known to be quite smaller than that on a polished copper surface. One of the main factors that govern the stability of the LHD conductor cooled by He II is the Kapitza conductance

Minimum quench energy and normal zone propagation velocity in MgB 2 superconducting tape

The quench characteristics of Cu–Ni sheathed MgB 2 superconducting monofilament tape were studied experimentally. We have investigated the stabilities i.e. minimum quench energy (MQE), normal zone propagation velocity v p at 4.2 K. Sample tape was triggered a local normal zone with a heater. By measuring the MQE and v p of MgB 2 tape, we derived the dependence of MQE and v p on transport current and magnetic field. In addition, minimum propagation current was also obtained from the transport current dependence of v p.

Measurements of temperature dependence of the stability and quench propagation of a 20-cm-long RABiTS Y-Ba-Cu-O tape

Thermal stability and quench propagation in a composite tape made of YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) superconductor were studied experimentally. Quench propagation in each test was initiated by applying a sequence of a short overcurrent pulse followed by a longer pulse at a typical operating current for the tape. The resulting change in resistivity due to internal heating was measured through voltage taps across different zones of the tape. Measurements were performed as a function of both initial overcurrent and operating current for several operating temperatures between 45 and 80 K. These experimental results provided the thermal stability margin, the minimum propagation current, and the quench propagation velocity for the tape. Experimentally obtained temperature dependence of normal zone propagation velocity was compared with the adiabatic theory taking into account minimum propagation current. It was noted that the measured normal zone propagation velocity compared favorably with the theory at each operating temperature.

Normal-zone propagation velocities in Bi-2223/Ag superconducting multifilament tape

Normal-zone propagation velocity is a very important parameter for designing superconducting magnets. This paper presents some results of experiments about quench propagation properties in Bi-2223/Ag superconducting multifilament tapes. The experiments were carried out under the condition of nitrogen atmosphere cooling system. The results are different from those in the case of liquid nitrogen or adiabatic cooling environment. Sample tape was triggered a local normal-zone with a heater. The voltage and temperature properties corresponding to normal-state transition were measured by voltage taps and thermocouples attached to the sample tape. We calculated longitudinal-direction quench propagation velocities from the voltage and temperature traces with different transport current. Minimum propagation current was also derived from the transport current dependence of propagation velocity. In addition, quench propagation velocities of sample tape were also influenced by background magnetic field.

Recovery process of a transformer type superconducting fault current limiter

Recovery process of a transformer type superconducting fault current limiter (SCFCL) was investigated by experiments using three-phase test SCFCL designed and made. "Recovery time" decreases, as "fault time" is longer. It is pointed out that the FCL current at the limiting mode affects the cooling process of the secondary wire during the current limiting operation. A simple simulation of the SCFCL based on the heat equation was performed. The simulation results of the "recovery time" agree well with the experimental ones. The temperature of the normal zone was estimated by use of the simulation. It decreases even in the current limiting mode and its decreasing rate depends on the limiting current. After a long limiting operation (over 5 s), the temperature saturated at about 22 K, the secondary coil current is the minimum propagation current of the wire and the "recovery time" is quite short and independent of the limiting current.

Computer simulation for stability analysis using a one-dimensional model

A simulation program of normal-state propagation was built using a one-dimensional model. The results of the simulations were compared with those of the short-sample tests of the helical coil conductor for the LHD at the National Institute for Fusion Science. This paper deals with the magnetic field profile effect of the recovery currents and the initial temperature profiles of the minimum propagation current. The first purpose of the study is to determine the appropriate length of the short-sample experiment when estimating the properties of conductors in a uniform magnetic field. The second purpose is to examine the behavior of a superconductor when it is heated at several points at the same time instead of at only one point. The results of the simulation shows: (1) when the length of the typical short-sample experiment, which has a helical coil conductor 0.25 m long, is increased by at least four times, the same recovery currents are obtained in a uniform field; and (2) when the total input energy is less than 0.1 J, the minimum propagation current increases as the number of peaks in the initial temperature profile increases. The minimum propagation currents does not depend on the number of peaks when the total input energy is greater than 0.1 J.

Experimental results of thermally controlled superconducting switches for high frequency operation

As part of a study to develop thermally controlled switches for use in superconducting rectifiers operating at a few hertz and 1 kA, a theoretical model is presented of the thermal behavior of such a switch. The calculations are compared with experimental results of several switches having recovery times between 40 and 200 ms. A discussion is given of the maximum temperature T/sub N/ that occurs in the normal regions when the switch is in the resistive state. Once T/sub N/ is known, it is possible to predict the recovery time, activation energy, stationary dissipation and minimum propagation current. The calculated and measured results, in good agreement, show that T/sub N/ is approximately 12 K and largely independent of the thickness or material of the insulation layer. Mention is made of some problems, related to the room-temperature equipment which drives the rectifier, that so far have prevented the rectifiers from being used at their design specifications. >

Basic models for thermal stability and zone propagation calculations for a.c. superconductors

A.c. superconductor stability within the framework of four basic models is analysed, with each model illustrating specific features in a.c. superconductor behaviour. Expressions are obtained for the minimum zone existence current, minimum propagation current, the limiting current at high frequencies (large Ω parameter), and also the parameter α corresponding to a fully stabilized a.c. superconductor. A.c. superconductors are shown to have a larger stability factor than d.c. ones. The propagation of normal and resistive zones in a.c. superconductors is studied numerically within a wide frequency range. The current dependences of the zone propagation velocity for each model are obtained. It is shown that taking into account a variable magnetic field gives rise to a system in which the conductor within the cycle becomes fully superconducting.

Stabilization of Superconducting Wire and Minimum Propagation Current

Minimum propagation current (Ip) and V-I characteristics were investigated for composite Nb-Ti superconducting wire (10mil in dia) claded by two methods; electro plating and metallurgical bonding. The dependence of Ip and V-I characteristics on thickness of copper was measured. Experimental results were discussed by heat transfer between liq. He and composite superconducting wire.