W. D. Markiewicz

Recent Developments in 2G HTS Coil Technology

Recent developments in 2G HTS coil technology are presented highlighting the ability of 2G HTS wire to function under difficult operating conditions without degradation. The challenges of use in various coil constructions and applications are discussed. Several applications where the conductor is subjected to high stress levels include high field insert coils and rotating machinery. While these applications present different challenges, the ability of the conductor to operate under high stress levels has been demonstrated in both direct sample measurement and test coils. The high winding current density that is available with SuperPowers thin 2G HTS wire was utilized in a high field insert coil demonstration generating central fields in excess of 26.8 T . The ability of the wire to be tailored (stabilization, insulation, ac losses) to fit various operating parameters will also be discussed.

High Field Magnets With HTS Conductors

Development of high-field magnets using high temperature superconductors (HTS) is a core activity at the NHMFL. Magnet technology based on both YBCO-coated tape conductors and Bi-2212 round wires is being pursued. Two specific projects are underway. The first is a user magnet with a 17 T YBCO coil set which, inside an LTS outsert, will generate a combined field of 32 T. The second is a 7 T Bi2212 demonstration coil set to be operated in a large bore resistive magnet to generate a combined magnetic field of 25 T. Owing to the substantial technological differences of the two conductor types, each project faces different conductor and magnet technology challenges. Two small coils have been tested in a 38-mm cold bore cryostat inserted in a 31 T resistive magnet: a Bi2212 round-wire layer-wound insert coil that generated 1.1 T for a total of 32.1 T and a YBCO double-pancake insert that generated 2.8 T for a total central field of 33.8 T. Four larger layer-wound coils have been manufactured and tested in a 20 T, 186-mm cold bore resistive magnet: a sizeable Bi-2212 coil and three thin large-diameter YBCO coils. The test results are discussed. The current densities and stress levels that these coils tolerate underpin our conviction that >30 T all-superconducting magnets are viable.

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.

Lap Joint Resistance of YBCO Coated Conductors

The National High Magnetic Field Laboratory is constructing a 32 T all superconducting magnet. The high field insert coil, which produces about 17 T field, will be made of YBCO coated conductor. Due to the limited available coated conductor piece length, solder splice joints are necessary in the insert coil. In order for the magnet to achieve stable operation with reasonable liquid helium consumption, it is important to minimize the resistance of the joints. In this paper, we prepared 40 mm long lap joints made by 4 mm wide SuperPower coated conductors. Joints made from different conductor batches are soldered with Sn63Pb37 solder at different temperatures using different soldering method. The joint resistivity measurements were performed at both 77 K and 4.2 K in high magnetic fields with different field orientations. We found that the joint resistivity is strongly correlated with conductor batch number. The joint resistivity temperature, magnetic field and field angle dependence is not strong. Therefore the joint resistivity measured at 77 K self field can be used to approximate that at 4.2 K in magnetic field.

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.

Thermal Conductivity Test of YBCO Coated Conductor Tape Stacks Interleaved With Insulated Stainless Steel Tapes

A 32 Tesla, all-superconducting user magnet, which consists of two high temperature superconductor YBCO inner coils producing a field of 17 T in an low temperature superconductor Nb3Sn and NbTi outer magnet producing a background field of 15 T, is being developed at the National High Magnetic Field Laboratory. The YBCO inner coils are pancake-wound with YBCO coated conductor tapes with an interleaved insulation of sol-gel coated stainless steel tapes. The coils are to be cooled directly in liquid helium bath. Heat losses in the windings, such as ac losses during ramping and heat loss in the internal joints, are supposed to be transferred to the coil external surfaces through heat conduction. Thus, thermal conductivity of the coil structure is critical for the internal cooling of the coil and also quench propagation if any. Thermal conductivity measurements were carried out in the radial direction on stacks of alternating YBCO tapes and stainless steel tapes. This paper presents the test results that showed a very low thermal conductivity in the radial direction. For comparison purposes, calculated thermal conductivities in the axial and azimuthal direction are also presented.

Observations from the Analyses of Magnetic Field and AC Loss Distributions in the NHMFL 32 T All-Superconducting Magnet HTS Insert

A 17 T high-temperature superconducting two-coil magnet (insert) to be operated in a 15 T low-temperature superconducting multisection magnet (outsert) is the most demanding part of the National High Magnetic Field Laboratory all-superconducting 32 T magnet system. The HTS coils are of the pancake type and to be wound with REBCO coated conductors/tapes manufactured by SuperPower, Inc. The distribution of AC losses in the HTS windings during the magnet charging/discharging process are computed and analyzed with due regard for the AC loss density dependence on the magnetic field and the field angle. The calculations are based on the measured magnetization of a representative sample against magnetic field and field angle. The results enable determination of heat load on the magnet and its cryogenic system. Since the magnet is of the pool-cooled type, a related helium vapor bubble problem can develop owing to the high field and field gradients, and the diamagnetic susceptibility of helium.

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.

Closed-Loop Cryogenic Cooling for a 21 T FT-ICR Magnet System

A closed-loop cooling concept for 21 T Fourier transform ion cyclotron resonance (FT-ICR) superconducting magnets is presented. In the magnet system, low temperature superconducting coils are immersed in a subcooled 1.8 K bath, which is connected to the saturated helium reservoir through the weight load relief valve. Saturated liquid helium is refrigerated by a Joule-Thomson (JT) heat exchanger and flows through the JT valve, isenthalpically dropping its pressure to approximately 1.6 kPa, corresponding to a saturation temperature of 1.8 K. Helium gas exhausted from JT pump is liquefied by a two-stage cryocooler located after the vapor purify system. In the present paper, the amount of heat budget is determined and the structural design of cryostat is carried out by the relevant analyses. The position of a cryocooler in the magnet system is investigated, taking into account the requirement of magnetic field for normal performance. Helium liquefaction system, a key component of the closed-loop cooling system, is fabricated and tested in order to demonstrate the feasibility of our new cryogenic cooling for high field magnets.