No-insulation coils are in general self-protecting and can therefore generally be operated at higher current densities. However, the electrical turn-to-turn connections may cause additional AC loss when charging the coil or when it is exposed to a time-dependent magnetic field. In this work, we study the case of a no-insulation ReBCO tape racetrack coil exposed to a uniform AC field applied parallel to the tape surface. We show that an anisotropic continuum model allows to formulate efficient analytical approximations for coupling loss in the low- and high-frequency limits. For intermediate frequencies, the continuum model needs to be evaluated numerically. The model was validated with representative measurements of AC loss in the coils, measured calorimetrically as well as magnetically using pick-up coils. The validation experiment nicely confirms the predicted frequency dependence of the coupling loss, which is P ∝ f 2 at low frequencies and P ∝ √f at high frequencies, due to the skin effect. The transition between low- and high-frequency regimes occurs around a characteristic frequency f c that is directly related to the characteristic time constant τ = 1/2πf c associated with the current decay in straightforward (dis)charge experiments.

ReBCO racetrack coils may be used in high-dynamic superconducting linear motor systems, typically replacing either permanent- or electromagnets in the DC stator. Even so, in order to achieve a significant increase in force density, the superconductor needs to carry a high transport current while simultaneously experiencing the time-varying magnetic field from the copper mover coils. To aid with the design of such devices, a 2D numerical model has been developed that predicts the AC loss under motor-relevant conditions, i.e. under the combined influence of a stationary transport current and an alternating external magnetic field. The main aim of the experiments described in this paper is to validate this model with dedicated AC loss measurements. To this end, we constructed a set-up that simultaneously measures magnetization-, transport current- and overall AC loss. Two identical insulated sub-scale ReBCO racetrack coils were tested at 4.2 K while carrying a stationary transport current of up to 700 A in a sinusoidal, alternating magnetic field up to 1.5 T, applied perpendicular to the broad face of the windings. Just like with metallic superconductors, the transport current significantly increases the AC loss level and lowers the penetration field. The inductive, electric and calorimetric data were found to be consistent with each other, validating the experimental calibration methods involved. Furthermore, the numerical model accurately predicted all AC loss components in the coils without any fitting to the data and can thus reliably be used in the design of superconducting machines.

A few hundred temperature sensors are used to monitor the temperature behavior of the gigantic ATLAS toroid superconducting magnet system during cool down and normal operation. In order to guarantee good sensitivity of temperature measurements in the range from liquid helium to room temperature, two types of sensors are positioned at the same location: platinum resistance thermometers for the range 30-300 K and carbon composition resistors (Allen-Bradley) for the 4-30 K range. Both types are very well known for use in cryogenics and they have performed satisfactorily during 10 year of ATLAS operation. The sensors themselves are easily available and inexpensive and the main cost is for the many kilometers of cold and warm instrumentation cables, connectors, conditioners, and installation work. A reduction of the amount of measurement channels is an important issue and this motivated us to develop a new compact and robust sensor module covering the entire temperature range that would combine advantages of both platinum and carbon resistors. The solution is trivial, elegant, and simple. Two resistors with positive temperature derivative and two resistors with negative temperature derivative are connected in a full-bridge connection. We used two platinum and two carbon resistors. The output signal is a result of the subtraction of voltages across positive and negative temperature derivative resistors that makes this temperature bridge sensor very sensitive for the entire temperature range. Variable temperature characterization tests were performed in the helium gas environment in the CERN Cryogenic Laboratory. Our measurements have demonstrated that the bridge sensors have a full range sensitivity better than 0.1 mV/K at a supply current of 100 µA. In the meantime, a few other superconducting detector magnets in experiments operating at CERN are equipped with these new, simple, and robust temperature sensors.

ReBCO coils are developed as DC field coils in linear motor systems to increase the force density, in favor of permanent magnets. Such coils have to sustain a relatively large heat load stemming from the AC magnetic field environment in which they operate. The use of no or partial turn-to-turn insulation can make them more stable against the effects of local heating. Conversely, the radial electrical connections in no-insulation coils allow for large coupling currents, causing additional AC loss on top of the already significant heat load. Here we report on the AC loss in sub-scale no-insulation, 4 mm wide single-tape, ReBCO racetrack coils exposed to parallel-to-the-tape magnetic field in the frequency range of 10-4 to 1 Hz at 77 K and 4.2 K, while carrying a DC transport current. AC loss is measured magnetically and electrically. The main goal of these experiments is to validate our 2D numerical model, which provides more insight into the origin of the AC loss. At low frequencies, inter-turn coupling currents are spread more or less homogeneously throughout the winding pack. Whereas at high frequencies, the skin effect causes shielding of the interior of the coil and large induced currents only occupy the coil's outer surface.