Nb3Sn Magnets Research Articles

Perspectives of<tex>$rm Nb_3rm Sn$</tex>in Future Accelerator Magnets

Upgrades presently being planned for high-energy hadron colliders require magnet performance beyond the capabilities of NbTi superconducting wire. Recent improvements in the performance of Nb3 Sn material and in the capability to use it in practical magnets make it the logical choice for the next generation of accelerator magnets. We discuss recent results of R&D programs, some remaining issues and possible solutions. Recent progress in understanding and in technical details of design and assembly have made Nb3Sn magnets easier to fabricate and more reliable in performance

Operation of the Levitated Dipole Experiment Floating Coil

The Levitated Dipole Experiment (LDX) is an innovative facility to study plasma confinement in a dipole magnetic field, created by a superconducting solenoid (floating coil), which is magnetically levitated in the center of a 5 m diameter by 3 m tall vacuum chamber. This persistent mode, floating coil (F-coil) consists of a Nb3Sn magnet installed inside a high-pressure vessel filled with 12.5 MPa helium gas at room temperature. It is surrounded by a high heat capacity fiberglass-lead composite radiation shield and by a toroidal vacuum shell. The built-in tube heat exchanger serves to cool the magnet, the helium vessel, and the thermal shield. When positioned at the lower part of the vacuum chamber the F-coil is cooled by retractable cryogenic transfer lines to about 4.5 K and it is charged inductively by the charging coil installed outside of the vacuum chamber. Then the helium flow is interrupted, the heat exchanger is pumped out, retractable lines are disengaged from F-coil ports, the ports are plugged, and the F-coil is lifted to the middle of the chamber to initiate and study plasmas. After several hours and just before the F-coil warms up to about 10 K it is lowered down for the next re-cooling or for discharging. This paper describes the F-coil cooling system and inductive charging system operation and performance

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 Nb 3Sn 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.

Manufacturing of lightweight low-current Nb 3Sn ADR magnets operating at 10 K

Adiabatic demagnetization refrigerators (ADRs) using superconducting magnets are under development for cooling many NASA instruments. Due to higher efficiency of cryocoolers at 10 K, Nb 3Sn magnets operating at 10 K are favored for space ADRs. Further, magnets need to be as light as possible and have low operating currents. This paper discusses technologies for the manufacture of lightweight, low-current Nb 3Sn magnets and reports on testing of a 35 mm bore by 60 mm long magnet. This magnet weighed less than 1 kg and successfully produced 3 T at 11.5 K with an operating current of 8 A.

A lightweight low-current 10 K magnet for space-flight ADRs

Future space missions will include detectors and other components cooled to cryogenic temperatures by adiabatic demagnetization refrigerators (ADRs) coupled with mechanical cryocoolers. In such systems the ADRs require lightweight, low-current superconducting magnets. At least one of an ADR’s magnets must operate at the cryocooler’s coldest stage temperature. This temperature should be as high as possible in order to improve operating efficiency and design flexibility. Until now all space-flight compatible magnets have been made with NbTi wire, which has limited their operating temperatures to below about 5 K. We have developed a lightweight (1 Kg) low-current (8 A) Nb 3Sn magnet which produces a 3 T central field at 10 K. We explain the choice of this magnet’s specifications and describe its performance testing.

Voltage Spike Detection in High Field Superconducting Accelerator Magnets

A measurement system for the detection of small magnetic flux changes in superconducting magnets, which are due to either mechanical motion of the conductor or flux jump, has been developed at Fermilab. These flux changes are detected as small amplitude, short duration voltage spikes, which are /spl sim/15 mV in magnitude and lasts for /spl sim/30 /spl mu/sec. The detection system combines an analog circuit for the signal conditioning of two coil segments and a fast data acquisition system for digitizing the results, performing threshold detection, and storing the resultant data. The design of the spike detection system along with the modeling results and noise analysis will be presented. Data from tests of high field Nb3Sn magnets at currents up to /spl sim/20 KA will also be shown.

A 10 K magnet for space-flight ADRs

Future NASA missions will include detectors cooled by adiabatic demagnetization refrigerators (ADRs) coupled with mechanical cryocoolers. A lightweight, low-current 10 K magnet would allow the interface between these devices to be at temperatures as high as 10 K, adding flexibility to the instrument design. We report on the testing of a standard-technology Nb 3Sn magnet and the development of a lightweight, low-current 10 K magnet. We also discuss the outlook for flying a 10 K magnet as part of an ADR system.

Study of the effects of high temperatures during quenches on the performance of a smallNb3Sn racetrack magnet

Several high field Nb3Sn magnets of different design are under development for future particle accelerators. The highlevels of stored energy in these magnets and the high current densities in theconductor can cause high peak temperatures during a quench. The thermal gradientsgenerated in the epoxy-impregnated magnet coils during the fast temperaturerise can result in high thermo-mechanical stresses. Considering the sensitivity ofNb3Sn to strain and epoxy cracks, it is important to define a maximum acceptable temperature inthe coils during a quench which does not cause degradation of the magnet performance. Aprogram was launched at Fermilab to study the effects of thermo-mechanical stress inNb3Sn coils, supported by experiments and by analysis. In collaboration with LBNL,a sub-scaled magnet was built and instrumented to measure the effect of thethermo-mechanical shock during magnet quenches. The magnet consisted of tworacetrack coils, assembled in a common coil configuration with a small gap inbetween. During the test, the magnet reached the maximum field of T at the short sample current of 9100 A. Temperature excursions up to 400 K did notdiminish the magnet quench performance; only after temperature excursions over 430 Kdid the magnet show detraining effects, which occasionally reduced the quenchcurrent to about 6%. Signs of irreversible degradation (reducing the maximumcurrent of about 3%) appeared only after temperature excursions over 550 K.

A compact, continuous adiabatic demagnetization refrigerator with high heat sink temperature

In the continuous adiabatic demagnetization refrigerator (ADR), the existence of a constant temperature stage attached to the load breaks the link between the requirements of the load (usually a detector array) and the operation of the ADR. This allows the ADR to be cycled much faster, which yields more than an order of magnitude improvement in cooling power density over single-shot ADRs. Recent effort has focused on developing compact, efficient higher temperature stages. An important part of this work has been the development of passive gas-gap heat switches that transition (from conductive to insulating) at temperatures around 1 and 4K without the use of an actively heated getter. We have found that by carefully adjusting available surface area and the number of 3He monolayers, gas-gap switches can be made to operate passively. Passive operation greatly reduces switching time and eliminates an important parasitic heat load. The current four stage ADR provides 6μW of cooling at 50mK (21μW at 100mK) and weighs less than 8kg. It operates from a 4.2K heat sink, which can be provided by an unpumped He bath or many commercially available mechanical cryocoolers. Reduction in critical current with temperature in our fourth stage NbTi magnet presently limits the maximum temperature of our system to ∼5K. We are developing compact, low-current Nb3Sn magnets that will raise the maximum heat sink temperature to over 10K.

Quench in a conduction-cooled Nb3Sn SMES magnet

Due to the rapid development of cryocoolers, conduction-cooled Nb3Sn devices are nowadays enabled. A 0.2 MJ conduction-cooled Nb3Sn SMES system has been designed and constructed. The nominal current of the coil was 275 A at 10 K. The quench tests have been performed and in this paper the experimental data are compared to the computational one. Due to a slow normal zone propagation, Nb3Sn magnets are not necessarily self-protective. In conduction-cooled coils, a thermal interface provides a protection method known as a quench back. The temperature rise in the coil during a quench was measured with a sensor located on the inner radius of the coil. The current decay was also monitored. The measured temperature increased for approximately 15 s after the current had already decayed. This temperature rise is due to the heat conduction from the hot spot. Thus, the measured temperature does not represent the hot-spot temperature. A computational quench model which takes into account quench back and heat conduction after the current decay was developed in order to understand the measured temperatures. According to the results, a quench back due to the eddy current induced heating of the thermal interface of an LTS coil was an adequate protection method.

Ceramic insulation for Nb 3Sn wires and magnets

The processing of In 2O 3–ZrO 2 and SnO 2–ZrO 2 coatings were evaluated on Nb 3Sn wires. Coatings were produced by using reel-to-reel, continuous sol–gel system from solutions of Zr, In, and Sn based organometallic compounds. Microstructure, phase composition and formation of the coatings were characterized by means of ESEM, SEM, EDS, XRD and DTA studies. Sol–gel zirconia based ceramic coatings, varying from submicron to several of microns in thickness, have been successfully applied to Nb 3Sn wires at the National High Magnetic Field Laboratory. Insulated and uninsulated Nb 3Sn conductors, that were heat treated together, showed about the same J c.

Large superconductors and joints for fusion magnets: From conceptual design to testing at full scale

A new kind of superconductor, using the cable-in-conduit concept, is emerging, mainly in the context of fusion activity. At present no large Nb3Sn magnet in the world is operating using this concept. The difficulty of this technology, which has now been studied for 20 years, is that it requires major advances in several interconnected new fields, such as handling a large number (1000) of superconducting strands, high current conductors (50 kA), forced flow cryogenics, Nb3Sn technology, low loss conductors in pulsed operation, high current connections and high voltage insulation (10 kV), as well as demonstration of its economical and industrial feasibility. CEA has been very much involved, during the past ten years, in this development, which took place in the framework of the NET and ITER technological programmes. One major milestone was reached in 1998-1999 with the successful tests by Euratom-CEA of three full size conductor and connection samples in the SULTAN facility in Switzerland.

Cryogenic mechanical properties of an aged austenitic stainless steel

This study was carried out to clarify the effects of phosphorus and boron on cryogenic mechanical properties of an aged austenitic stainless steel strengthened by nitrogen because they are used for Nb 3Sn magnets and sometimes suffer aging before cryogenic use. These minor elements affected precipitation at grain boundaries during aging at about 700°C. CrNbN was mainly observed in both matrix and grain boundaries. Cr 2N and Nb(CN) were observed at grain boundaries too. Phosphorus promotes the precipitation of Fe 2Mo at the grain boundary. The 4 K tensile properties were not affected by aging. Fracture toughness at 4 K, however, was deteriorated by aging. The degradation seemed to be connected with Fe 2Mo at grain boundaries. This suggests that phosphorus leads to intergranular fracture at 4 K, consequently to the poor fracture toughness. But boron proved to suppress the phosphorus effect and to improve the fracture toughness.

Comparison of Nb3Sn and NbTi Superconductor Magnet ITER Devices

The TETRA tokamak systems code is used to compare designs for the International Thermonuclear Experimental Reactor (ITER) that use Nb3Sn and NbTi superconductor magnets. Similar minimum-cost device...

Current-carrying capacities of small V 3 Ga pancakes

As an intermediate step leading to the production of V 3Ga tapes long enough for magnets of practical size, we have produced by surface diffusion V 3Ga tapes ranging in length from 5 to 100 meters. These were wound into single pancakes and their current-carrying capacities were measured in external magnetic fields up to 180 kOe. Results indicate that it should be possible in the near future, to construct V 3Ga magnets capable of generating fields as high as 180 kOe, which exceeds by at least 30 kOe the maximum generated by an Nb 3Sn magnet. A further study in the area of stability, however, would be helpful.

Comparison of NbTi and Nb3Sn Material Tests with the Actual Performance of Coils

A 100-kG, 8.26-cm-bore, 40-kJ Nb3Sn magnet and a 75-kG, 10.2-cm-bore, 95-kJ twisted filament NbTi magnet have been constructed and tested. A number of other magnets of smaller size using both the Nb3Sn and twisted filament NbTi conductors will also be described. Material evaluation tests leading to these magnet designs will be reported. They include short sample tests with controlled surface cooling conditions and cusp coil tests in which an external field is applied perpendicular to the cusp coil axis. Finally, two successful stabilization techniques for Nb3Sn ribbon will be presented with coil performance data before and after the stabilization has been employed.

Pumped Helium Tests of a 51-cm-Bore Nb3Sn Magnet

Performance data are presented for tests of 51-cm-bore single coil and coil pair magnets operated at temperatures below 2°K. The magnet coils are wound of silver-plated, vapor-deposited Nb3Sn ribbon. Coil protection was provided by cross-layer shorting strips which also provide very limited stabilization. Each coil was designed to produce a 4.0-T center field. Results obtained at the low temperature are compared with those obtained at 4.2°K. System description and charging characteristics are included in the discussion.

Comparison Between Measured Critical Currents and a Model of Transiently Stabilized Nb3Sn Magnets

A previously proposed model used to predict transient stabilization in high-current superconductive solenoids is assessed in the light of results on actual test magnets made with a variety of fabrication parameters. The possibilities for the use of the model is discussed with reference to the missing elements and when considering special ``short-sample'' measurements on thermal propogation characteristics and magnetization characteristics. Results of recovery-current tests are presented.

A large bore, high-field modular Nb3Sn magnet

A large bore, high-field modular Nb₃Sn magnet

SUPERCONDUCTING ELECTROMAGNET OF Nb<sub>3</sub>Sn

摘要 本文报导了关于筒形Nb3Sn磁体的研究结果。观察到磁通跳跃现象。“冻结”场的分布和历史及激发场的分布紧密相关。“冻结”场可以不是中心对称的,这进一步证明了烧结的Nb3Sn材料是非均匀介质。我们描述了新的磁体结构,避免了磁通跳跃。除利用铁心来捕获磁通外,在激发和测量磁场的方法上均具有一定的特点。利用圆筒状Nb3Sn作为压挤磁通的活塞,在1.5°K时,曾获得的最高场强是28.5KG。 Abstract Results of study on tubular Nb3Sn magnets are presented. Magnetic flux jumping has been observed as usual. The "frozen" field distribution is closely related to the external field distribution and previous history. The distribution need not be symmetrical with respect to the mid-plane perpendicular to the cylindrical axis, as in the case of soft superconductors. This confirms the suggestion that the sintered Nb3Sn material is not a homogeneous medium.We describe a new magnet structure with which magnetic flux jumping can be avoided. With these magnets, besides the use of the iron core for trapping magnetic flux, there are also special features in both the excitation and the measurement of magnetic fields.Utilizing the tubular Nb3Sn as a piston, we have obtained a highest magnetic field of 28.5kG at 1.5°K. 作者及机构信息 管惟炎, 刘体汉, 郑国光 Authors and contacts KUAN WEI-YEN, LIU TI-HANG, CHENG GUO-KUANG 参考文献 [1] 施引文献