A. V. Gavrilin

Design of a Superconducting 32 T Magnet With REBCO High Field Coils

The design and fabrication of a 32 T, 32 mm cold bore superconducting magnet with high field REBCO inner coils is underway at the NHMFL. In support of the design, conductor characterization measurements have been made including critical current as a function of field, field orientation, temperature, and strain on conductors and joints. Various conductor and turn insulation systems were examined. The selected coil fabrication method for the 32 T magnet is pancake wind, dry wind coils with sol-gel insulation on a stainless steel co-wind. Quench protection of the REBCO coils by distributed heaters is under development. Small REBCO coils have been made and tested in a 20 T background field to demonstrate performance of the technology. The design of the 32 T magnet is described, including coil configuration and conductor lengths, fraction of critical current, selection of conductor copper content for protection, and stress in the windings.

Progress in the Development of a Superconducting 32 T Magnet With REBCO High Field Coils

The design and development of a 32 T, 32 mm cold bore superconducting magnet with high field REBCO inner coils are underway at the NHMFL. The two nested REBCO coils that form the high field section are dry wound, with uninsulated conductor and insulated stainless steel cowind reinforcement. Active quench protection uses distributed protection heaters. As part of the development activity, prototype coils of the two REBCO coils with full scale radial dimensions and final design features, but with reduced axial length are being constructed. The first of these prototype coils was tested in a 15 T resistive background field magnet. The coil has inner and outer winding diameters of 40 mm and 140 mm, respectively, and consists of six double pancakes with a total conductor length of roughly 900 m. The construction of this prototype coil is described, including the protection heaters. Coil test results are reported including coil critical current, coil ramping characteristics, thermal stability, joint, and terminal resistance with field cycling. The corresponding operating stress in the windings is calculated. Importantly, the performance characteristics of the protection heaters will be measured including activation time.

Field Angular Dependence of Hysteresis Losses of Coated Conductor for High Field Magnets

Hysteresis loss (Qhyst) of REBCO coated conductors strongly depends on magnetic field orientation. This is mostly due to the shape and critical current (Ic) anisotropy of the coated conductors. For example, while Qhyst in a field parallel to the ab plane is negligibly small, Qhyst in a field perpendicular to the ab plane is much greater than what is typical in a multifilamentary LTS wire. Therefore, during a field ramp of a high-field magnet made of coated conductor tape, at locations where the radial field component is significant, the heat generated by Qhyst will significantly affect the design temperature margin, and increase the operation cost associated the liquid helium consumption. Therefore, it is very important to characterize and understand the field angular dependence of Qhyst. In this paper, we use a vibrating sample magnetometer to measure the field angular dependence of Qhyst of a coated conductor made by SuperPower Inc. Most of the measurements were performed at 4.2 K in ±9 T magnetic field cycle with different field orientations. Our results of Qhyst angular dependence and the correlation between Qhyst and the Ic angular dependence are presented. The factors affecting measurement accuracy such as sample size, field angle uncertainty due to the magnetic torque will be discussed. Based on our experimental data, a practical formulation calculating Qhyst(B, θ, T, I) is proposed.

Quench propagation and detection in the superconducting bus-bars of the ATLAS magnets

The ATLAS superconducting magnet system comprising Barrel (BT) and End-Cap Toroids (ECT) and also Central Solenoid (CS) will store more than 1.5 GJ of magnetic energy. The magnet system will have many superconducting busbars, a few meters long each, running from the current leads to Central Solenoid and Toroids as well as between the coils of each Toroid. Quench development in the busbars, i.e., the normal zone propagation process along the busbar superconductors, is slow and exhibits very low voltages. Therefore, its timely and appropriate detection represents a real challenge. The temperature evolution in the busbars under quench is of primary importance. Conservative calculations of the temperature were performed for all the magnets. Also, a simple and effective method to detect a normal zone in a busbar is presented. A thin superconducting wire, whose normal resistance can be easily detected, is placed in a good thermal contact to busbar. Thus, the wire can operate as straightforward and low-noise quench-detector.

Protection Heater Development for REBCO Coils

Normal zone propagation rates are widely reported to be low in coils containing REBCO coated conductor, creating difficulties for quench protection of high field REBCO insert solenoids. A method of active quench protection using densely distributed heaters in high field REBCO coils has been proposed. When heaters are used to quench a sufficiently large fraction of the coil windings, the hot-spot temperature can be limited to acceptable values. The feasibility of protection of REBCO coils by distributed heaters depends on the amount of power to quench the required fraction of the coil volume, and the ability of the heaters to quench REBCO coils quickly. The high critical temperature of REBCO compared to low-temperature superconductor implies that the windings must be elevated significantly higher in temperature to initiate a normal zone. A number of measurements have been made of the performance of protection heaters on REBCO high field test coils. The results provide information on the response of coils to active protection heaters. Typically, there is a delay between the activation of the heater and the onset of quench in a coil. Measurements give the relation between the heater power and the quench delay time.

A Short-Period High-Field

With the intention to develop a technology suitable for a 2.4 m magnetic length, superconducting undulator with 0.8 T on the beam and a 15 mm period for the IXS beam line at the Advanced Photon Source at the Argonne National Lab, USA, a 0.6-m Nb3Sn undulator demonstration magnet is under development at the NHMFL. The intent is to investigate the applicability of Nb3Sn superconductor, in combination with soft magnetic materials, for a 15 mm period planar superconducting undulator. The demonstration requirements include a peak undulator magnetic field of at least 0.8 T and a vertical stay-clear aperture for the beam of at least 7 mm. In concept, the beam tube will be thermally isolated from the windings and operated at LN2 temperature to promote system reliability. Liquid helium and nitrogen reservoirs provide conduction cooling and the former also acts as a basic structural element. A brief description of a design is presented. A comparative study of fully and partly iron yokes and their effect on the required current density is performed. The results have implications for the yoke assembly procedure. Finally, a new yoke-winding geometry is presented

rm Nb_3rm Sn

The ATLAS superconducting magnet system consists of the Barrel Toroid, two End Cap Toroids and the Central Solenoid. However, the Toroids of eight coils each are magnetically separate systems to the Central Solenoid. The Toroids are electrically connected in series and energized by a single power supply. The quench protection system is based on the use of relatively small external dump resistances in combination with quench-heaters activated after a quench event detection to initiate the internal dump of stored energy in all the coils. A rather strong quench-back effect due to eddy-currents in the coil casings at the transport current decay is beneficial for the quench protection efficiency in the event of heater failures. The quench behaviour of the ATLAS Toroids was computer simulated for normal operation of the quench protection system and its complete non-operation (failure) mode.

Undulator Study

The National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida has designed and is now constructing two Series Connected Hybrid (SCH) magnets, each connecting a superconducting outsert coil and a resistive Florida Bitter insert coil electrically in series. The SCH to be installed at the NHMFL will produce 36 T and provide 1 ppm maximum field inhomogeneity over a 1 cm diameter spherical volume. The SCH to be installed at the Helmholtz Center Berlin (HZB) in combination with a neutron source will produce 25 T to 30 T depending on the resistive insert. The two magnets have a common design for their cable-in-conduit conductor (CICC) and superconducting outsert coils. The CICC outsert coil winding packs have an inner diameter of 0.6 m and contribute 13.1 T to the central field using three grades of CICC conductors. Each conductor grade carries 20 kA and employs the same type of Nb3Sn superconducting wire, but each grade contains different quantities of superconducting wires, different cabling patterns and different aspect ratios. The cryostats and resistive insert coils for the two magnets are different. This paper discusses the progress in CIC conductor and coil fabrication over the last year including specification, qualification and production activities for wire, cable, conductor and coil processing.

Quench propagation and protection analysis of the ATLAS Toroids

Details of the normal zone propagation and the temperature distribution in the coils of ATLAS toroids under quench are presented. A tailor-made mathematical model and corresponding computer code enable obtainment of computational results for the propagation process over the coils in transverse (turn-to-turn) and longitudinal directions. The slow electromagnetic diffusion into the pure aluminum stabilizer of the toroids conductor, as well as the essentially transient heat transfer through inter-turn insulation, is appropriately included in the model. The effect of nonuniform distribution of the magnetic field and the thermal links to the coil casing on the temperature gradients within the coils is analyzed in full.

Recent Progress of the Series-Connected Hybrid Magnet Projects

A pair of binary current leads has been constructed with a nitrogen vapor cooled resistive section and a stainless steel shunted HTS lower section. These leads are intended as replacement leads for the NHMFL 45 T Hybrid and also serve as half-current prototypes for the 20 kA leads foreseen for the Series Connected Hybrids that are under development at the NHMFL. An amount of liquid nitrogen is maintained in a reservoir at the bottom of the resistive section such that vapor flow is determined by self- demand. The liquid nitrogen delivery system and properties of the vapor cooled section of the leads are described. A comparison is made between the measured and calculated properties of the resistive section in steady state and transient conditions.