Holger Neumann

An approach for experimental investigation of the optimum sensor current of a superconducting level sensor is established and is then used for the investigation of the same of a NbTi and a MgB2/SS level sensors in liquid helium. On comparing the experimental results with that of the computational model established earlier, to simulate the operation of a superconducting level sensor, it was found that both agree closely. This shows the validity of the computational model for the characterization of a superconducting level sensor. Further on, the operation of these level sensors in a two-current method was studied. The results showed a higher heat load and level measurement time. However, it indicated an opportunity of retaining the linearity of the voltage versus liquid level curve even under high boil-off rate inside the cryostat. To scrutinize these findings, the NbTi level sensor operation in liquid helium was studied up to a boil-off rate of 5.77 g/s. It was observed that the sensor operation was not affected; however, the two-current method showed a more accurate level measurement compared to the one current method.

The SUPRAPOWER, an EU FP7 funded research project, is developing an innovative 10 MW class superconducting generator to provide an important breakthrough in the offshore wind industry. Benefiting from the modular concept, the key challenges of such innovative 10 MW generator e.g. the modular superconducting coil and associated cryogenic systems could be validated through a scale-down rotating magnetic validator. The cryogenic cooling system for the RMV consists two modular cryostats, a distributing cryostat and the corresponding rotating cryocooler system with a self-developed rotary joint. In order to achieve the SC coils working temperature of 20 K, a two-stage G-M cryocool-er will be applied and linked to the two modular cryostats by means of conductive copper connection. The distributing cryostat was developed to envelop the cold head of the cryocooler, the thermal link, and three binary current leads to feed electrically the coils. A rotary joint with Ferrofluid sealing was developed to transfer the helium gas between the rotating cold head and sta-tionary oil-lubricated compressor. In this paper, the design, manufacture and assembly of this cryogenic system for RMV will be presented in details.

An automated level control in cryogenic systems requires a precise and continuous level measurement. If properly characterized, superconducting level sensors are a viable option for the same. The operation of a superconducting level sensor is based on the difference in the heat transfer from the sensor wire to the liquid and the gaseous phases of the cryogen. The higher heat transfer in the liquid phase results in a superconductive status, whereas the lower heat transfer in the gaseous phase results in a normal conductive status with a voltage drop corresponding to the length of the wire in the gaseous phase. Hence, it demands a judicious selection of the sensor and the operational parameters such as the current input, for an accurate level measurement as well as the reduction of the liquid boil off. In this paper, to simulate the operation of a superconducting level sensor, a model has been developed by taking the steady-state heat balance along the sensor wire. It is then used for the investigation of the sensor current, voltage output at various liquid levels, and temperature profile of the sensor wires of NbTi and MgB₂ /SS level sensors in LHe. Furthermore, design guidelines have been proposed for an MgB₂/SS superconducting level sensor for LH₂ level measurement. The parameters considered for the analysis are the sensor wire diameter and the active length. In addition, the effect of an external resistor on the sensor operation is also studied.

The SUPerconducting Reliable lightweight And more POWERful offshore wind turbine (SUPRAPOWER), an EU FP7 funded research project, are under development for an innovative superconducting 10 MW class offshore wind turbine. Due to the requirements of handling, maintenance, reliability of long term and offshore operation, the cryostats are divided in two major parts: the modular cryostat able to accommodate a single coil and a thermal collector that links all the modules. The prototype modular cryostat was designed, manufactured and assembled in Karlsruhe Institute of Technology (KIT). The paper reports preliminary test results of prototype modular cryostat with a two-stage Gifford-McMahon (GM) cryocooler.

Superconducting generators show the potential to reduce the head mass of large offshore wind turbines. A 10 MW offshore superconducting wind turbine has been investigated in the SUPRAPOWER project. The superconducting coils based on MgB 2 tapes are supposed to work at cryogenic temperature of 20 K. In this paper, a novel modular rotating cryostat was presented for one single coil of the superconducting wind turbine. The modular concept and cryogen-free cooling method were proposed to fulfil the requirements of handling, maintenance, reliability of long term and offshore operations. Two stage Gifford-McMahon cryocoolers were used to provide cooling source. Supporting rods made of titanium alloy were selected as support structures of the cryostat in aim of reducing the heat load. The thermal performance in the modular cryostat was carefully investigated. The heat load applied to the cryocooler second stage was 2.17 W@20 K per coil. The corresponding temperature difference along the superconducting coil was only around 1 K.

Fiber Bragg grating (FBG) sensors were used to measure the torsional strain distribution in a second-generation (rare-earth) Ba 2 Cu 3 O y high-temperature superconducting sample tape (SST) at 77 K. Two SST samples were prepared. A slit was fabricated in each sample to allow an optical fiber to be embedded. In the first sample, a 10-mm-long single grating element was inserted into the slit. In the second sample, three grating elements of 10-mm length each were put into the slit. These grating elements were fabricated along a single-mode fiber and could be addressed individually using the wavelength division multiplexing (WDM) technique. Finally, the samples were recoated with a primary and secondary coating to form an embedded sensing system. When the high-temperature superconducting (HTS) tape is subjected to torsion, there is a strain induced in the FBG sensor, which varies the grating period accordingly. It was interrogated using a tunable laser in a WDM scheme. Based on the wavelength shift and the photoelastic properties of the FBG sensor embedded to the HTS tape, the torsional deformation of the HTS tape can be obtained. In this paper, the design and technology requirements to embed the single and WDM FBG sensors into the coated conductors are presented. Initial experiments to measure the torsional strain distribution at 77 K are reported.

Multipoint stress/strain measurement in a simple single layered Niobium Titanium wire wounded superconducting sample coil (SSC) was carried out using fiber Bragg grating sensors (FBGs) at low temperature (4.2 K) and at high background magnetic field (8 T) excited by power supply. Ten gratings with different spatial period were fabricated at various positions along a single mode fibre (SMF). The grating location in the SMF sensing array was then mounted at the measuring points on the SSC using stycast 2850 FT. The change in the strain in the SSC. This induced strain, changes the Bragg wavelength of the mounted FBG sensor, which was then recorded using a FS5100 Bragg meter. Experimental results showed that the FBG sensor array was highly reproducible with error of ±0.8 μm/m for high external magnetic field and high current. Also the Young's modulus of the SSC at 900 A, 8 T, and 4.2 K was calculated to be ~ 117 GPa, which is below to the average Young's modulus of the NbTi/Cu superconducting composite wire, which is in the range of 130 GPa at 4.2 K. The loading and unloading characteristic curves of the SSC exhibit reversible behavior without any hysteresis in the operated range. This ensures that the SSC operates linearly within the elastic limit for the applied current and magnetic field. In this paper, we report the behavior of each single turn of the single layered SSC and its corresponding strain due to Lorentz force. The FBG sensing principle is validated by executing various modes of SSC excitation.