David K. Hilton

35.4 T field generated using a layer-wound superconducting coil made of (RE)Ba2Cu3O7−x (RE = rare earth) coated conductor

To explore the limits of layer wound (RE)Ba2Cu3O7-x (REBCO, RE = Rare Earth) coils in a high magnetic field environment > 30 T, a series of small insert coils have been built and characterized in background fields. One of the coils repeatedly reached 35.4 T using a single ~100 m length of REBCO tape wet wound with epoxy and nested in a 31 T background magnet. The coil was quenched safely several times without degradation. Contributing to the success of this coil was the introduction of a thin polyester film that surrounded the conductor. This approach introduces a weak circumferential plane in the coil pack that prevents conductor delamination that has caused degradation of several epoxy impregnated coils previously made by this and other groups.

Observed drag crisis on a sphere in flowing He I and He II

The pressure distribution on the surface of a sphere has been measured in flowing He I and He II as a function of Reynolds number. The drag coefficient was extracted by integrating the pressure distribution, using some assumptions about symmetry of the flow field. Drag coefficients are plotted against Reynolds number for both He I and He II against classical data for both smooth and nonsmooth spheres. Latest results in He II suggest that the drag crisis occurs at a Reynolds number of approximately 2×105, in fair agreement with classical data.

Study of second-generation high-temperature superconducting magnets: the self-field screening effect

Second-generation high-temperature superconductors (2G HTS) have high current density in very high magnetic fields. They are good candidates for high field magnets, especially when the magnetic field exceeds the critical fields of low-temperature superconductors. However, the thin and flat geometry of these conductors allows persistent screening currents (or shielding currents) to flow in the conductors. The screening currents caused by the ramping of applied current to the coil is identified as the self-field screening effect. The screening-current-induced magnetic field changes the magnetic field distribution of the magnet, and it also generates drift. This paper employs both experimental and numerical methods to study the mechanism of self-field screening currents for 2G HTS magnets. A 2G HTS magnet was constructed and tested, and a finite element model was built based on the magnet. The comparison between calculation and measurement is presented with detailed analysis. Current distributions inside the HTS magnet are calculated to illustrate the effects of screening. The screening-current-induced magnetic field is quantified by comparing the magnetic field distribution with a baseline copper model. The model is also used to explain the mechanism of the current sweep strategy, which can be used to effectively eliminate screening currents.

Practical fit functions for transport critical current versus field magnitude and angle data from (RE)BCO coated conductors at fixed low temperatures and in high magnetic fields

Applications of (RE = Y, Gd)BCO coated conductors for the generation of high magnetic fields are increasing sharply, this while (RE)BCO coated conductors themselves are evolving rapidly. This article describes and demonstrates recently developed and applied mathematical models that systematically and comprehensively characterize the transport critical current angular dependence of a batch of (RE)BCO coated conductor in high magnetic fields at fixed temperatures with an uncertainty of 10% or better. The model development was based on analysis of experimental data sets from various published sources and coated conductors with different microstructures. These derivations directly are applicable to the accurate prediction of the performance in high magnetic fields of coils wound with (RE)BCO coated conductors. In particular, a nonlinear fit is discussed in this article of transport critical current at T = 4.2 K versus field and angle data. This fit was used to estimate the hysteresis losses of (RE)BCO coated conductors in high magnetic fields, and to design the inserts wound with such conductors of the all-superconducting 32 T magnet being constructed at the NHMFL. A series of such fits, recently developed at several fixed temperatures, continues to be used to simulate the quench behavior of that magnet.

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.

Helium II level measurement techniques

In this paper, a survey of cryogenic liquid level measurement techniques applicable to superfluid helium (He II) is given. The survey includes both continuous and discrete measurement techniques. A number of different probes and controlling circuits for this purpose have been described in the literature. They fall into one of the following categories: capacitive liquid level gauges, superconducting wire liquid level gauges, thermodynamic (heat transfer-based) liquid level gauges, resistive gauges, ultrasound and transmission line-based level detectors. The present paper reviews these techniques and their suitability for He II service. In addition to these methods, techniques for measuring the total liquid volume and mass gauging are also discussed.

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.

A capacitive liquid helium level sensor instrument

The design, construction, and performance of a prototype capacitive liquid helium level sensor instrument with cold electronics is described.

Mechanical tests on the surface-coated Bi-2212 superconducting tapes

The strain dependence of the critical current of surface-coated Bi-2212 tapes is reported. Two methods are used in the measurements. The effect of bend strain on the critical current is obtained by a stepped cone apparatus. Peak stress or strain dependence of the critical current and the effect of the number of strain cycles on the critical current is also studied on multiple samples by employing a new in situ stress/strain device. The results show that the number of strain cycles does not significantly affect the critical current. Rather, the critical current of the tape depends only on the level of peak strain.

SUBCOOLED LIQUID OXYGEN CRYOSTAT FOR MAGNETO-ARCHIMEDES PARTICLE SEPARATION BY DENSITY

An instrument for the separation of particles by density (sorting) is being developed that uses the magneto-archimedes effect in liquid oxygen. With liquid oxygen strongly paramagnetic, the magneto-archimedes effect is an extension of diamagnetic levitation in the sense of increasing the effective buoyancy of a particle. The instrument will be able to separate ensembles of particles from 100 μm to 100 nm in size, and vertically map or mechanically deliver the separated particles. The instrument requires a column of liquid oxygen that is nearly isothermal, free of thermal convection, subcooled to prevent nucleate boiling, and supported against the strong magnetic field used. Thus, the unique cryostat design that meets these requirements is described in the present article. It consists in part of a column of liquid nitrogen below for cooling the liquid oxygen, with the liquid oxygen pressurized by helium gas to prevent nucleate boiling.