Measurement of minimum quench energy in a superconducting winding using a YAG laser

Laser heating technology has been applied to evaluate the minimum energy to induce quenching in a superconducting winding. After calibration of the actual energy absorbed by the conductor, it was irradiated with YAG laser light while increasing the power step by step. The minimum quench energy (MQE) determined by an average of the absorption energies in and just before quenching was in good agreement with the calculated one. Further, reproducibility within 5% was obtained. It was concluded that the laser heating method is valid to measure MQE of the winding.     

Guidelines for LTS magnet design based on transient stability

Stabilities of low critical temperature superconducting (LTS) magnets and their designs are studied and discussed. There are two contradictory necessities; those are low cost and high performance, in the other words, high magnetic field and large current density. Especially, the maximum magnetic fields of the latest high performance Nb₃Sn magnets are around 20 T. Mentioned necessities result in the small stability margins. Needless to say, the superconducting magnet must produce its nominal field reliably. Therefore, maintaining adequate stability margin, the magnet design to draw out the high potential of the superconductor is required. The transient stability of the superconducting magnet is determined by the relationship between mechanical disturbance energy and stability margin. The minimum quench energy (MQE) is one of the index of stability margin and it is defined as the minimum energy to trigger quenching of a superconductor. MQE should be beyond any possible disturbance energy during the operation. It is difficult to identify the mechanical disturbance energy quantitatively. On the contrary, MQE had been evaluated precisely by means of our developed resistive carbon paste heater (CPH). At the same time, we can predict MQE by numerical simulations. Because the magnet comes to quench if the mechanical disturbance exceeds the MQE, the disturbance energies are suspected to be equivalent to MQEs during the magnet-training. When we achieved somewhat larger MQE, we may exclude numbers of training quenches.In this paper, we discuss the guidelines of LTS magnet design from the standpoint of MQE. We represent some case studies for various superconducting magnets and/or some different winding methods.     

Normal zone propagation characteristics of the HTS wires by heat energy applied

With the successful commercialization of Bi-2223 powder-in-tube wire, various attempts in the R&D of the high-T_c superconducting (HTS) magnets for high magnetic field applications are being implemented actively. Operating temperature of HTS magnet has to be maintained at the designed level but the magnetic energy and mechanical disturbance can cause unstable operational temperature of HTS magnet. Especially, the generated heat energy of inner HTS winding is apt to be accumulated, so the normal region appears in HTS winding. This paper deals with the quenching characteristics of three kinds of selected Bi-2223 wires: the high current density wire (HC-A) and the high strength wire (HS-A) made by AMSC and HTS wire (HW-I) made by Innost. The Innost wire has the highest minimum quench energy (MQE). The high current density wire has the highest normal zone propagation velocity (NZPV).     

Stability of superconductors against localized disturbances of limited magnitude

Steady state equilibrium theory has been used to predict the size of the minimum energy disturbance needed to quench a cryostable superconductor; experimental results are in reasonable agreement. The size of the disturbance from which a superconductor can recover decreases rapidly with increasing normal state ohmic heat generation in the conductor. Nevertheless, for disturbances of moderate size, stable recovery can occur in conductors with normal state generation higher than the normally accepted values of ～ 3 kW m^?2. It should therefore be possible to design cryostable magnets to operate at higher current densities than hitherto, provided the size of the maximum disturbance can be predicted.     

Experimental simulation of helium pressure rise during a quench of a superconducting coil cooled by a superfluid helium bath

Experimental and numerical studies have been conducted with the aim of modeling pressure rises which occur in the helium, during quenches of the 11.7-T superconducting magnet named Iseult. Iseult is based on a double-pancake winding internally cooled by superfluid helium channels opening into a pressurized He II bath at 1.8 K. A scale mock-up has been built of 10 copper equivalent pancake slices and 7 helium channels per pancake. The heat produced by a quench of the Iseult magnet is simulated by electrical heaters put inside each copper plate. Cryogenic pressure and temperature sensors have been fitted in the helium channels and in the bath. Bath pressure measurements are given for various heating powers, various numbers of heated plates and various bath volumes. Comparisons with a simple numerical model permit to identify the main physical mechanisms which drive the pressure rise during a quench.     

Influence of high heat capacity substances doping on quench currents of fast ramped superconducting oval windings

The effect of high heat capacity substances doping on superconducting magnets ramp rate induced quench currents has been investigated for three oval windings. The windings were wound from Rutherford type cable (1.44 × 4.64 mm²), made of 10 multifilamentary NbTi strands 0.85 mm dia. Before application of electrical insulation and winding the cable was covered with epoxy resin with three different fillers: BN (boron nitride—a standard filler for preventing of epoxy cracking at low temperatures), and two rare-earth intermetallic compounds (HoCu₂ and CeCu₆). The specific heat of these compounds at liquid helium temperatures is extremely high. The volumetric fraction of these compounds was 2.9% of the total winding volume, corresponding to 4.5 times increase of averaged winding heat capacity for HoCu₂ and to 1.5 times increase for CeCu₆. At high ramp rates (∼4 kA/s, or ∼6.5 T/s) quench current of HoCu₂ doped winding was 35% higher than that for the BN doped one, while for CeCu₆ doped winding the quench current increase was 12%. Measured quench currents match values calculated with our theoretical model, in which whole doping substances enthalphy was taking into account. It indicates that the enhanced enthalphy is fully utilized in the used range of dB/dt.     

Quench detection system of the EURATOM coil for the Large Coil Task

A special quench detection system has been developed for the EURATOM Large Coil Task (LCT) coil. The system is based on a bridge circuit which uses a special ‘two in hand’ winding technique for the pancakes of the EURATOM LCT coil. The electronic circuit was designed in a fail safe way to prevent failure of the quench detector due to failure of one of its components. A method for quick balancing of the quench detection system in a large toroidal magnet system was applied. The quench detection system worked very reliably during the experimental phase of the LCT and was within the quench detection level setting of 50 mV, i.e. the system was not sensitive to poloidal field transients at or below this level. Non-electrical methods for quench detection were also investigated.     

Noise in a point-contact dc SQUID

Measurements of five variations of a toroidal point-contact dc superconducting quantum interference device (SQUID) are presented. The energy resolution and other parameters of these SQUIDs are examined and compared with the predictions of the Resistively Shunted Junction model. For these SQUIDs, the measured minimum energy resolution was approximately 2×10^–30 J/Hz. Excess noise in the point contacts was found to limit the energy resolution of the SQUIDs. A comparison between the typical junction parameters and noise obtained for our niobium-niobium point contacts and those of others is given. TheI–V characteristics of the junctions showed the effects of Joule heating. The white voltage noise spectral density was found to have an approximately parabolic dependence on the average voltage for bias currents larger than the critical current. While this parabolic dependence is consistent with heating effects in the junctions, the amplitude of the noise cannot be explained in terms of a heating model. The low-frequency noise of the point contacts has also been investigated.     

Experimental evidence of considerable stability increase in superconducting windings with extremely high specific heat substances

The influence of rare-earth based intermetallic compounds with very high heat capacities on the transient stability of NbTi superconducting windings was experimentally investigated. The intermetallic compounds CeCu₆ and HoCu₂ were introduced as filling powders into an epoxy resin based composite in a wet winding process. Doping of about 6 vol.% of such compounds increases minimum quench energies several times even for short (of order of 1 ms) electromagnetic disturbances.The prospects and most promising applications are briefly discussed.     

First experience with the new Coupling Loss Induced Quench system

New-generation high-field superconducting magnets pose a challenge relating to the protection of the coil winding pack in the case of a quench. The high stored energy per unit volume calls for a very efficient quench detection and fast quench propagation in order to avoid damage due to overheating.A new protection system called Coupling-Loss Induced Quench (CLIQ) was recently developed and tested at CERN. This method provokes a fast change in the magnet transport current by means of a capacitive discharge. The resulting change in the local magnetic field induces inter-filament and inter-strand coupling losses which heat up the superconductor and eventually initiate a quench in a large fraction of the coil winding pack.The method is extensively tested on a Nb–Ti single-wire test solenoid magnet in the CERN Cryogenic Laboratory in order to assess its performance, optimize its operating parameters, and study new electrical configurations. Each parameter is thoroughly analyzed and its impact on the quench efficiency highlighted.Furthermore, an alternative method is also considered, based on a CLIQ discharge through a resistive coil magnetically coupled with the solenoid but external to it. Due to the strong coupling between the external coil and the magnet, the oscillating current in the external coil changes the magnetic field in the solenoid strands and thus generates coupling losses in the strands. Although for a given charging voltage this configuration usually yields poorer quench performance than a standard CLIQ discharge, it has the advantage of being electrically insulated from the solenoid coil, and thus it can work with much higher voltage.     

Rapid thermomechanical tempering of iron–carbon martensite

Abstract

Tempering of martensite under simultaneous compressive stress has been studied within the temperature range of 20–400°C. Resistive heating was utilised to obtain rapid heating and cooling cycles of a few seconds. Material was obtained from a medium carbon pearlitic railway wheel steel, quench hardened to obtain martensitic structure. Greater than ～150°C dilatation effects where observed below the global yielding point of the material. Microstraining around dislocations in the body centred tetragonal crystallographic structure or viscous flow at higher temperatures was a probable explanation to this material behaviour. Hence, external stress may have an important influence on the tempering progression of martensitic steel. The trials also showed that tempering of martensite progresses fast, is near instantaneous and is independent of the presence of external stress or not.     

A critical current-margin design criterion for high performance magnet stability

A ‘critical-current-margin. (CCM) design criterion for superconducting magnet stability is presented. It is applicable to high-performance magnets that operate at high current densities; these magnets have cooled conductor surfaces, but their calculated heat fluxes are well beyond recovery values. For stability of a magnet based on this CCM design criterion, disturbances must be limited in size but need not be eliminated nor localized.The CCM criterion is compared with the MPZ/Cold-End Theory. It is shown that this criterion offers a much higher confidence level of stability than the MPZ/Cold-End Theory. Furthermore, and unique to this theory, stability is independent of current density in the stabilizer.The CCM criterion is based on two magnet winding elements: heat transfer and composite conductor. In this criterion heat transfer must always remain in the nucleate boiling region throughout the operation of a magnet. The design emphasis is on more superconductor and less stabilizer. It is also demonstrated that the CCM theory works particularly well with conductors such as cabled conductors that have high ratios of cooled perimeter to cross-section.Experimental results are presented supporting this CCM design criterion.     

Stabilization of large superconducting magnets: Experimental models

The four experiments reported here address the problem of stabilization in large superconducting magnets. The first two experiments concern frictional heating caused by conductor motion; the last two experiments measure minimum quench propagation currents in simulated cooling conditions.     

Heat conduction and thermal stabilization in YBCO tape

Yttrium barium copper oxide (YBCO) coated conductors are widely used in the conduction-cooled superconducting magnets with rapid development in refrigeration technologies at present. ‘Quench’ is a state that refers to the irreversible and uncontrolled superconductor to resistive transitions in the superconductor. The propagation of ‘quench’ or ‘normal zone’ has different characteristics in these high temperature superconductors (HTS) compared to low temperature superconductors. The superconductor to normal index, known as ‘n’ is much flatter in HTS. The hot spot emerging in local region due to quench and non-uniform critical current may cause permanent damage to whole HTS tape and hence the magnet winding pack. Thus it is necessary to determine the temperature profile along the length of HTS tape under a given energy (joule heating) such that propagation of the hot spot developed locally can be prevented early. In this study, a one dimensional, time dependent heat diffusion equation with appropriate boundary conditions are used to describe the consequences of the normal zone propagation resulting in the temperature diffusion in a HTS tape. The results demonstrate the necessity of adequate cooling of the edges of the flat HTS tapes to prevent irreversible normal zone transitions.     

Beam induced quench of superconducting solenoid

The beam induced quench of a superconducting solenoid was studied to estimate the effect of the energy deposition due to the high energy proton beam. The solenoid was directly exposed to 12 GeV proton beam with a typical beam duration time of 250 ms. The experimental results were compared with the MPZ/Cold-End energy calculation and a computer simulation solving the thermal equation. They can be well reproduced by the simulation result with reasonable parameters.     

Influence of self-magnetic field on a.c. quench current level of superconducting coils

The a.c. quench current level of a superconducting cable when formed into a coil is, in general, lower than that of a short sample. The current in the coil induces a self-magnetic field on the superconducting winding. It was found from our experiments that the transition from the superconducting state to the normal one in a superconducting coil originates in that part of the winding where the self-magnetic flux density is estimated to be the largest. It is concluded that degradation of the a.c. quench current level in the superconducting coil is mainly brought about by the influence of the self-magnetic field.     

Prediction of quench in cable-in-conduit conductors considering current-sharing effect of copper

A simple 0D analytical model for the prediction of the quench point has been presented by Anghel [Anghel A. A simple model for the prediction of quench point in cable-in-conduit conductors. Cryogenics 2003;43:225–32]. The model offered a straightforward approach to estimate the quench behavior of cable-in-conduit-conductor (CICC) and provided a clear view of the relationships between conductor parameters and its quench properties. However, this 0D model did not address the current-sharing effect of copper. In this paper, it was considered by approximation that superconductor and copper form two resistors in parallel, the redistribution of current due to the copper was taken into account. The result of the new model was compared with that of a previous model. The 0D model predicts the quench with a higher accuracy.     

Test of a model coil of "Tore Supra"

Inside the qualifying test programme, supporting the "Tore Supra" Tokamak design, a reduced scale model of coil was fabricated by an industrial firm and fully tested. This model coil is provided with the same features as those retained for the complete magnet and is built according to the same design; in particular the Nb-Ti mixed matrix monolithic conductor is cooled by a pressurized superfluid helium bath, supplied from a model of the envisaged complete cryogenic system. Three main objectives have been assigned to this test: operation of the cryogenic system, stability of the superconductor winding under high mechanical stresses, mainly shear, and simulation of coil quench conditions. For this purpose, the model coil (outside bore 0.8 m) is located inside a 4 T magnet, an hydraulic jack applies a 1 MN force along a coil diameter. Operation of the model coil has been found highly stable, under the conditions of applied field and forces, a coil transition can be induced by an electrical heater only when the superfluid bath temperature is close to Tλ. The 1.8 K cryogenic system provides a useful calorimetric measure of total losses induced inside the winding; its operation has been quite simple and reliable, permitting a sure extrapolation to a much larger size.     

Quench protection tests of a cryocooler cooled 6 T NbTi superconducting magnet by an active power method

When a quench occurs in a superconducting magnet, excessive joule heating may damage the magnet. We have presented the quench protection system based on an active power method. Our previous quench protection tests have been carried out for small superconducting magnets whose self inductances are less than several hundred mH to verify principles of our proposed system. In this paper, we present experimental results of quench protection tests of a cryocooler cooled 6 T NbTi superconducting magnet (self inductance 15.5 H), which is a commercial size magnet made by Tamakawa Co., Ltd. We confirmed that our proposed system could inhibit the maximum temperature of the superconducting magnet (initial temperature 4.3 K) after the quench to less than about 44 K at operation magnetic field 5.5 T. Experimental results suggest that our proposed system is useful for practical used superconducting magnets.