Superconducting magnet for K-500 cyclotron at VECC, Kolkata

K-500 superconducting cyclotron is in the advanced stage of commissioning at VECC, Kolkata. Superconducting magnet is one of the major and critical component of the cyclotron. It has been successfully fabricated, installed, cooled down to 4.2 K by interfacing with LHe plant and energized to its rated current on 30th April, 2005 producing magnetic field of 4.8 T at median plane of cyclotron. The superconducting magnet (stored energy of 22MJ) consists of two coils (α and β), which were wound on a sophisticated coil winding machine set-up at VECC. The superconducting cable used for winding the coils is multi filamentary composite superconducting wire (1.29 mm diameter) having 500 filaments of 40 μm diameter Nb-Ti in copper matrix which is embedded in OFHC grade copper channel (2.794 mm × 4.978 mm) for cryogenic stability. The basic structure of coil consists of layer type helical winding on a SS bobbin of 1475 mm ID × 1930 mm OD × 1170 mm height. The bobbin was afterwards closed by SS sheet to form the LHe chamber. The total weight of the coil with bobbin was about 6 tonne and the total length of the superconducting cable wound was about 35 km. Winding was done at very high tension (2000 PSI) and close tolerance to restrict the movement of conductor and coil during energization. After coil winding, all four coils (two each on upper and lower half of median plane of cyclotron) were banded by aluminium strip (2.7 mm × 5 mm) at higher tension (20,000 PSI) to give more compressive force after cool down to 4.2 K for restricting the movement of coil while energizing and thereby eliminating the chances of quench during ramping of current.After completion of coil winding by October, 2003, cryostat assembly was taken up in house. The assembly of cryostat (13 tonne) with support links (9 Nos.) refrigeration port, instrumentation port, helium vapour cooled current loads, etc. was completed by June, 2004. Meanwhile assembly of magnet frame was taken up and the cryostat was positioned in the magnet frame with proper alignment by August, 2004. After installation of cryostat on magnet, the cryostat was connected to the helium refrigerator/liquefier, having refrigeration capacity of 200 W and 100 l/h in liquefier mode with LN2 pre-cooling. The cryogenic delivery system supplying the liquid helium and liquid nitrogen to the superconducting magnet was successfully commissioned in November, 2004. The cool down of the cryostat to 10 K took around 8 days following which the LHe was filled in the cryostat (300 l) on 15th January, 2005. Subsequently the superconducting coils (α and β) were energized by two DC current regulated power supplies (20 V, 1000 A, 10 ppm stability) with slow and fast dump resistors connected externally across the superconducting coils for protection of coils at the time of power failure and quench.The paper describes the intricacies involved in coil winding, winding set-up, assembly of cryostat, cooling down the superconducting coils, filling by LHe and energization to rated current. The paper also highlights the operating experience of superconducting magnet and related test results.     

Detachable leads for superconducting magnets

A simple design of detachable leads has been incorporated in superconducting solenoids. Preliminary tests have shown very satisfactory behaviour of the leads up to 100 A to produce a magnetic field of 5.0 T in a bore of 22 mm. Such leads will be useful for energizing superconducting magnets in small liquid helium dewars of 1–2 ? capacity.     

Sweeping a persisting superconducting magnet with a transformer

A method for sweeping a persisting superconducting magnet is described. The field sweep is achieved by including in the superconducting loop of the magnet a coil which acts as the secondary coil of a transformer. Variation of the current in the primary coil of the transformer, controlled from outside the cryostat, causes the field-sweeping action through flux-linking with the superconducting loop. Compared by the ratio of the magnet's inductance to the transformer's inductance. The advantages of using an all-metal vacuum-tight superconducting feedthrough are discussed.     

Wide Variety of Experiments Using a Cryogen-Free 27.5 T Hybrid Magnet and a Cryogen-Free 18.1 T Superconducting Magnet

A cryogen-free hybrid magnet without liquid helium for operation, generating 27.5 T in a 32 mm room temperature bore of an 8 MW water-cooled resistive insert magnet in an 8.5 T background field of a cryogen-free superconducting outsert magnet, is being operated for basic research at low temperatures down to 17 mK in combination with a dilution refrigerator. In addition, we are developing functional materials using a differential thermal analysis DTA at high temperatures up to 1473 K in high fields up to 27 T. This cryogen-free hybrid magnet will be upgraded to generate 29 T by improving the outer superconducting magnet. A cryogen-free 18.1 T superconducting magnet with a 52 mm room temperature experimental bore, consisting of a Bi₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) insert coil, has been developed using a GM-JT cryocooler. Recently, bronze-tape-laminated Bi2223 has revealed excellent irreversible stress tolerance of 250 MPa at 77 K. In addition, the critical current properties for recent Bi2223 tapes are largely improved from 200 to 400 A/cm-width at 77 K in a self-field. Therefore, the stainless steel reinforcement tape incorporated for the previous Bi2223 insert coil is no longer needed for a new Bi2223 one. A new Bi2223 insert coil with almost the same size as the existing insert coil can generate two times higher fields at the elevated operation current from 162 to 191 A. An upgraded cryogen-free superconducting magnet can offer a long-term experiment at the constant magnetic field of 20 T for an in-field heat-treatment investigation.     

Development of a 7 T Experiment System for Superconducting Switch Test

A 7 T experiment system, which was designed for a MRI superconducting switch test, has been developed at the Institute of High Energy Physics (IHEP) of China. The system mainly consists of a 7 T background superconducting coil, two pairs of 800 A vapor cooled current leads, a data acquisition system and more. The NbTi superconducting coil, with an operating current of 376 A, has a central field of 7 T and clear bore of 80 mm. The no-impregnation technique was adopted to fabricate this coil. The coil reached 96% of its short sample performance after three times quench. The system detailed experiments are presented in this paper.     

A 10 tesla cryomagnetic system for neutron scattering experiments

A cryomagnetic system is described in which the first main component is a cryostat providing variable temperatures and the second a superconducting coil. The cryostat enables the coil to operate at either 4.2 K or 2.16 K, and allows a sample of diameter 10 mm, height 10 mm, to be brought to temperatures varying from 1.5 K to 300 K.The magnet is an asymmetrical split coil with a vertical magnetic axis. Aluminium windows provide access vertically over 15°, horizontally over 340° to a bore of useful diameter 32 mm. The superconducting magnet is wound from multifilamentary NbTi and Nb₃Sn wires and provides a central field of 8.7 T at 4.2 K and 10 T at 2.16 K.     

Excitation characteristics of magnets impregnated with paraffin wax

The Superconducting Electromagnetic Iron Separator (SEIS) based on paraffin wax vacuum pressure impregnation (VPI) technology was developed to remove magnetic impurities from coal. The superconducting coil was wound by Cu/NbTi wires. The inner and outer diameter of the coil are 928 mm and 1022 mm respectively. The height of the coil is 355 mm. The magnet generates a 3 T central field at 165 A of operating current.There are three SEIS devices that have been manufactured and tested. All the design requirements are achieved by the first device. However, the others can’t be charged to the design current in 20 min, which is a requirement of coal handing port. This paper presents the details of the VPI process and the charging tests. The excitation characteristics of three SEIS devices are shown in the results.     

Design and operation of a protection system for transformers with superconducting windings

Power transformers with superconducting windings need a protection system to prevent damage to the low-loss superconducting winding by an abnormally high current. The generally accepted protection technique which uses auxiliary coils has been analysed using a network representation. The current distribution between main and auxiliary coil is expressed in terms of geometrical parameters. Experimental data on current transfer and main coil recovery in a test transformer are presented and a method of obtaining a very low auxiliary coil current is suggested.     

The 42+ T Hybrid Magnet Project at CNRS-LNCMI-Grenoble

Hybrid magnets, the combination of a resistive inner coil with a superconducting outer one, allow to generate the highest continuous magnetic fields for a given electrical power installation. A new superconducting coil outsert has been designed to be integrated in the existing infrastructure at LNCMI-Grenoble (GHMFL). Based on the specific development of a Nb–Ti Rutherford Cable On Conduit Conductor (RCOCC) cooled at 1.8 K by a bath of superfluid helium at atmospheric pressure, the superconducting coil aims to produce a continuous magnetic field of 8.5 T in a 1.1 m bore diameter. Combined with resistive insert coils, an overall continuous magnetic field of 42+ T will be produced in a 34 mm warm aperture. The main results of the conceptual study will be presented together with first developments and tests of the RCOCC.     

11 T liquid helium-free superconducting magnet

An 11 T liquid helium-free superconducting magnet designed at 6 K in vacuum using high temperature superconducting current leads was developed. The coil was conductively cooled down from room temperature to 4.1 K in 40 h by two 4 K GM-cryocoolers. In a performance test, the coil temperature rose to 6.8 K for the inner Nb₃Sn coil and 5.9 K for the outer NbTi coil, while sweeping the field at 5 A min⁻¹. A central field of 10.7 T in a 52 mm room temperature bore was generated at an operating current of 149 A. Holding the field at 10.5 T was achieved continuously for 24 h at a constant coil temperature of 4.8 K.     

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.     

Field enhancement in superconducting solenoids by holmium flux concentrators

The magnetic field strength in the bore of a 13 T superconducting solenoid was enhanced to 16 T in the 5.5 mm gap between two cylindrical holmium flux concentrators. This method is an economic way of extending the field region for measuring critical data of superconducting materials. The measured dependence of the gap field on coil current was reproduced in computer calculations by fitting the magnetization curve of holmium at 4.2 K. This curve may be used for optimizing the geometry of holmium flux concentrators at 4.2 K.     

Development of Bi-oxide superconducting tapes and coils

We have developed Bi-2212 and 2223 tapes. For Bi-2212, two double stacked pancake type coils were fabricated using Bi-2212/Ag tapes prepared by a combination of the continuous dip-coating process and melt-solidification. A small coil (13 mm inner bore, 46.5 mm outer diameter) was inserted in a conventional superconducting magnet system. In a bias field of 20.9 T, the generated field of the coil was 0.9 T, at an I_c of 310 A (criterion 10⁻¹³ Ωm) at 1.8 K. Thus, the superconducting magnet system achieved the generation of a field of 21.8 T in the full superconducting state. A large coil (20 mm inner bore, 94 mm outer diameter) generated a field of 2.6 T (I_c = 385 A (10⁻¹³ Ωm)) at 4.2 K and 1.53 T (I_c = 225 A (10⁻¹³Ωm)) at 20 K in self-field. For Bi-2223, tapes were prepared by the powder-in-tube technique using Ag-10% Cu-x%M (x = 0–1.0, M = Ti, Zr, Hf or Au) alloy sheaths. The high J_c values of 5–7 × 10⁴ A cm⁻² at 4.2 K and 14 T were obtained for the tapes doped with x = 0.03–0.1 at.% Ti, 0.1 at.% Zr, 0.1 at.% Hf or 0.3% Au. These tapes have a modified Bi-2223 grain structure at the sheath/core interface and also a dense and more aligned microstructure, resulting in higher J_c values.     

Proposal of rectifier type superconducting fault current limiter with non-inductive reactor (SFCL)

A rectifier type superconducting fault current limiter (SFCL) with non-inductive reactor has been proposed. The concept behind this SFCL is the appearance of high impedance during non-superconducting state of the coil. In a hybrid bridge circuit, two superconducting coils connected in anti-parallel: a trigger coil and a limiting coil. Both the coils are magnetically coupled with each other and have same number of turns. There is almost zero flux inside the core and therefore the total inductance is small during normal operation. At fault time when the trigger coil current reaches to a certain level, the trigger coil changes from superconducting state to normal state. This super-to-normal transition of the trigger coil changes the current ratio of the coils and therefore the flux inside the reactor is no longer zero. So, the equivalent impedance of both the coils increased thus limits the fault current. We have carried out computer simulation using EMTDC and observed the results. A preliminary experiment has already been performed using copper wired reactor with simulated super-to-normal transition resistance and magnetic switches. Both the simulation and preliminary experiment shows good results. The advantage of using hybrid bridge circuit is that the SFCL can also be used as circuit breaker. Two separate bridge circuit can be used for both trigger coil and the limiter coil. In such a case, the trigger coil can be shutdown immediately after the fault to reduce heat and thus reduce the recovery time. Again, at the end of fault when the SFCL needs to re-enter to the grid, turning off the trigger circuit in the two-bridge configuration the inrush current can be reduced. This is because the current only flows through the limiting coil. Another advantage of this type of SFCL is that no voltage sag will appear during load increasing time as long as the load current stays below the trigger current level.     

A multiple purpose 7TNbTi split coil superconducting magnet system for Mössbauer spectroscopy and susceptibility measurements at various temperatures

The design, construction and test results of a compact NbTi coil system are described. The dimensions are: 198 mm outside diameter, 290 mm total length, 52 mm vertical central bore, 20 mm horizontal bore. The system consists of two assistant coils in addition to the main split pair. A total of ten wire sections is involved. Each part of the system can be separately charged or several other modes of operation are possible. So the main split pair can be used alone. In the screening mode operation one or both of the assistant coils keep the magnetic induction below 0.1 T, within certain ranges on and near the axis. Finally, in the gradient mode operation, the assistant coils are charged in the opposite direction so that a linear gradient field is superposed onto the main field. The versatility of this coil system implies the use of three independent superconducting switches.The coil system is mounted in a stainless steel dewar which has horizontal and vertical access for Mössbauer spectroscopy at various temperatures. An additional insert allows susceptibility measurements in the temperature range from 2 K up to 300 K. Details on the cryostat (designed by L. Bogner, F. Parak and W. Wiedemann) will be published later.     

Progress in superconductivity

Superconductors are in a quantum state, extending over macroscopic distances. Consequently, they trap magnetic flux in multiples of the flux quantum. This point of view will be taken for a discussion of some recent developments related mainly to induction phenomena in superconductors. 1) In superconductors of the second kind magnetic flux penetrates at external magnetic field values within a certain interval (mixed state). This state violates the traditional characteristics of superconductivity. Not only does it fail to show the perfect Meissner diamagnetism, but it is also, in principle, a resistive state. The latter feature is correlated with the instability of the magnetic structure in the mixed state when a current is injected, the induced flux flow phenomenon being responsible for a variety of effects recently studied, including the Hall effect and a mode of magnetic coupling which makes a dc transformer feasible. 2) It is well known by now that superconductors of the second kind may obtain excellent current-carrying capacities if the mixed state magnetic pattern is stabilized, or pinned. This process is still not fully understood. The recent manufacture of some interesting and practicable wires and cables and the problems of coil construction and their stabilization will be briefly surveyed. 3) By proper manipulation of magnetic flux in superconducting systems, electromagnetic induction of heavy direct currents may be achieved. Fully superconducting dc generators or dynamos have recently been developed, the qualities of which will be discussed. The results of some of the author's recent investigations will be given, including those on multikiloampere dynamos for energizing supermagnets.     

Quench detection method without a central voltage tap by calculating active power

In this paper, we present an electric quench detection method without a central voltage tap which may cause the short-circuit of the lead-wires from the voltage taps in the quench detection of a large AC superconducting coil. In this method, an inductive voltage detection coil is used instead of the central voltage tap. The inductive voltage detection coil is electrically insulated from a superconducting coil and therefore the lead-wires do not break down. Through the quench detection tests for a Bi-2223/Ag HTS coil, we show the feasibility of the proposed method for detecting the quench of the large AC superconducting coil.     

Measurements of thermal conductivity of epoxy impregnated NbTi andNb3Sn windings

The effective thermal conductivity in the transverse direction is one of the less predictable coil parameters. In the framework of the construction of a high-field solenoid (18 T, 100 mm free bore at 4.2 K) at the LASA lab, an experimental apparatus was built for measurement of thermal conductivity of pure materials as well as of composites and coil blocks. The main aim of the experiment is the measurement of the thermal conductance of pieces of real NbTi and Nb₃Sn windings. The authors discuss how the thermal conductivity affects the quench properties of superconducting magnets. The main features of the experimental apparatus are described together with a calculation and a measurement of the thermal loss. Preliminary measurements on some pure materials as well as on a coil block are presented     

Superconducting magnet development in Japan

The present state of R & D works on the superconducting magnet and its applications in Japan are presented. On electrical rotating machines, 30 MVA superconducting synchronous rotary condenser (Mitsubishi and Fuji) and 50 MVA generator are under construction. Two ways of ship propulsion by superconducting magnets are developing. A superconducting magnetically levitated and linear motor propelled train "MAGLEV" has developed by the Japan National Railways (JNR). A large scale test track of 7 Km was constructed in Kyushu and the test vehicle reached its target speed of 517 Km/hr. The first manned test running was made by three-vehicles train on new U-shaped guideway. The superconducting magnet development for fusion is the most active field in Japan. The Cluster Test program has beer demonstrated on a 10 T Nb3Sn coil and the first coil of Large Coil Task in IEA collaboration has been constructed and the domestic test was completed in JAERI. These works are for the development of toroidal coils of the next generation tokamak machine. R & D works on superconducting ohmic heating coil are in progress in JAERI and ETL. The latter group has constructed 3.8 MJ pulsed coil. A high ramp rate of changing field in pulsed magnet, 200 T/s, has been tested successfully, for burning tokamak device project in IPP, by joint work of Nihon University, ETL, Mitsubishi and IPP. High Energy Physics Laboratory (KEK) are conducting active works. The superconducting μ meson channel and π meson channel have been constructed and are operating successfully. KEK has also a project of big accelerator named "TRISTAN", which is similar to ISABELLE project of BNL. Superconducting synchrotron magnets are developed for this project. The development of superconducting three thin wall solenoid has been started. One of them, CDF, is progressing under USA-Japan collaboration.