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The work presented in this PhD thesis concerns the metrological performance improvement of a superconducting cable test station based on superconducting transformers. The main cable’s parameter to be assessed –as a function of temperature and magnetic field– is the critical current, i.e. beyond this limit the phase transition to the normal state occurs. Ramping the current at levels in the order of the tens of kA can be achieved with superconducting transformers at moderate capital and operational cost. But, issues such as (i) accurate/precise measurements and (ii) monitoring of the secondary current during the device operation have to be addressed. In this regard, the goals of the thesis are the design, prototyping, and validation of a new cryogenic current transducer and effective monitoring system for test stations transformer-based. Among the available transducers for current sensing at room temperature, the DC current transformer (DCCT) provides measurement accuracy in the order of the hundreds of ppm (and below) with high repeatability. Thus, one of the aims of this work is to develop a DCCT capable of measuring currents at liquid helium temperature improving critical current tests capabilities. On the other hands, resistive losses in the secondary circuit of superconducting transformers require to be compensated; a suitable control of the secondary current is mandatory. Therefore, further enhancement follows the implementation of a fully digital system for monitoring the transformer operation. After illustrating the basic idea and the design steps for the cryogenic DCCT transducer, the detailed design of the DCCT sensing element is reported. To overcome state-of-the-art drawbacks a cryogenic permalloy is used for the DCCT cores and an optimized design is carried out for a rated current of 100 kA. Moreover, the concept of combining ferromagnetic and superconducting materials to increase the rejection of external disturbance field, up to 1.0 T, is exploited. In particular, the design of a cylindrical superconducting shield made of MgB2 composite is proposed. A fully digital monitoring system provides the basic tools for a proper operation of a superconducting transformer. The high-performance digital measurement and control systems are described. Two compensation mechanisms of the secondary current are proposed. The former is based on a standard Proportional-Integral algorithm and the latter implements a smart control strategy. The design of the cryogenic DCCT is validate via both numerical simulation and on field characterization of a lab-scale prototype. Finally, the monitoring system is validated carrying out critical current assessment of a superconducting cable.

One of the main requirements for the operation of the Large Hadron Collider at CERN is the correction of the dynamic multipole errors produced in the main magnets*. In particular, integrated sextupole errors in the main dipoles must be kept well below 0.1 units to ensure acceptable chromaticity. The feed-forward control of the LHC is based on the Field Description for the LHC (FiDel), a semi-empirical mathematical model capable of forecasting the magnet’s behaviour in order to generate suitable corrector current waveforms. Measurement campaigns were recently undertaken to validate the model making use of a novel fast rotating-coil magnetic measurement system (FAME)**, able to detect superconductor decay and snapback transients with unprecedented accuracy and temporal resolution. In this paper we discuss the test setup and the results obtained both on the test bench and in the actual operation of the accelerator., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan

A new cryogenic dc current transformer (Cryo-DCCT) has recently been designed and assembled at CERN. The device, whose design is based on that of a high-accuracy 600 A market solution suitable for room temperature applications, is optimized for measuring currents of up to 80 kA and for operation at 4.2 K. The CryoDCCT has been conceived with the objective of preserving the metrological performance of the original commercial device in the new extended range of operation. For reducing the effect of interfering magnetic fields arising from test conditions, it incorporates ferromagnetic and MgB 2 superconducting shields. In this paper, the design of the CryoDCCT and the results of the commissioning of the device at CERN are reported. The effectiveness of the current transducer is analysed and discussed. This new device will be used for measuring the secondary current of a 80 kA superconducting transformer feeding a sample of NbSn 3 cable at the Facility for Research on Superconducting Cables (FRESCA) at CERN.

A swarm intelligence-based procedure to detect critical conditions of a patient, affected by a specific disease, at an early stage in absence of clinician, is proposed. The procedure is to be integrated inside a remote health care system for patients at home, where some physiological parameters related to a specific disease are being monitored. A significant variation in the monitored parameters can lead the patient to a critical state, thus the proposed method is aimed at predicting a possible future bad condition of the patient on the basis of past measurements. Moreover, different physiological parameters contribute to diverse degrees in dissimilar diseases; consequently, a swarm intelligence-based method is proposed for optimizing the weight of each parameter for a more accurate diagnosis. The proposed approach has been validated experimentally under the framework of the industrial research project Patient Diagnosis and Monitoring at Domicile (PADIAMOND: co-funded by EU and the company Filia srl, Caserta, Italy).

A method for designing cylindrical hollow superconducting shields for cryogenic measurement devices operating in background fields of 1 T is proposed. The shield design is based on MgB2 composite, manufactured by the reactive Mg liquid infiltration process [1]. The MgB2 composite allows low-cost shields with good mechanical resistance to be realized easily. The geometrical design is benchmarked by the experimental characterization at 4.2 K. A design case study for the shield of a cryogenic DC current transformer is reported. Design results show a shielding efficiency of 70% for both the axial and radial components, with prospective measurement accuracy up to 10 ppm on 100 kA.

A swarm intelligence-based approach to multiple-fault diagnostics for industrial applications is proposed. Drawbacks of swarm-based algorithms in heuristic search strategy related to mutual dependence of solutions are overcome by a likelihood-based trail intensity modification of ant-colony optimization. Numerical results of a comprehensive characterization through statistical experiment design on high-dimension multiple-faults diagnosis applications are shown. Experimental results under the framework of an industrial research project committed to industrial remote monitoring of operating machines are discussed. Numerical and experimental results show excellent performance, outperforming genetic algorithms, especially in high-dimension problems, and easiness in algorithm configuration.

A system for the active compensation of magnetic field errors within ±2 ppm relative to the main field in superconducting magnets is proposed. A high-performance rotating coils system is exploited in order to combine high time resolution and integral field measurements. The compensation is based on an enhanced characterization of the corrector magnets and the measured field: this allows the system to be used as off-line reference in accelerator facilities. As an experimental case study, the measurement system and the procedure for compensating field errors of dipole magnets of the Large Hadron Collider at the European Organization for Nuclear Research (CERN) are illustrated.

An accurate and flexible system for magnetic permeability measurements is described. In particular, effects such as eddy currents and signal drift are minimized by exploiting the main system features: (i) high-performance digital integration by means of a digital integrator, with a theoretical resolution of 10 ppt using an 18-Bit A/D Converter at 800 kS/s, specifically developed for magnetic measurements, and (ii) flexibility of the measurement procedure for overcoming magnetic field non-uniformity errors through a procedure specifically generated by means of a flexible software framework. In particular, the theoretical background underlying the measurement, the procedure, and the system architecture are illustrated, as well as experimental results from testing a nonoriented steel are described. © 2010 IEEE.