Digital Integrator for Fast Accurate Measurement of Magnetic Flux by Rotating Coils

A fast digital integrator (FDI) with dynamic accuracy and a trigger frequency higher than those of a portable digital integrator (PDI), which is a state-of-the-art instrument for magnetic measurements based on rotating coils, was developed for analyzing superconducting magnets in particle accelerators. Results of static and dynamic metrological characterization show how the FDI prototype is already capable of overcoming the dynamic performance of PDI as well as covering operating regions that used to be inaccessibl

A Fast Digital Integrator for magnetic measurements

In this work, the Fast Digital Integrator (FDI), conceived for characterizing dynamic features of superconducting magnets and measuring fast transients of magnetic fields at the European Organization for Nuclear Research (CERN) and other high-energy physics research centres, is presented. FDI development was carried out inside a framework of cooperation between the group of Magnet Tests and Measurements of CERN and the Department of Engineering of the University of Sannio. Drawbacks related to measurement time decrease of main high-performance analog-to-digital architectures, such as Sigma-Delta and integrators, are overcome by founding the design on (i) a new generation of successive-approximation converters, for high resolution (18-bit) at high rate (500 kS/s), (ii) a digital signal processor, for on-line down-sampling by integrating the input signal, (iii) a custom time base, based on a Universal Time Counter, for reducing time-domain uncertainty, and (iv) a PXI board, for high bus transfer rate, as well as noise and heat immunity. A metrological analysis, aimed at verifying the effect of main uncertainty sources, systematic errors, and design parameters on the instrument performance is presented. In particular, results of an analytical study, a preliminary numerical analysis, and a comprehensive multi-factor analysis carried out to confirm the instrument design, are reported. Then, the selection of physical components and the FDI implementation on a PXI board according to the above described conceptual architecture are highlighted. The on-line integration algorithm, developed on the DSP in order to achieve a real-time Nyquist bandwidth of 125 kHz on the flux, is described. C++ classes for remote control of FDI, developed as a part of a new software framework, the Flexible Framework for Magnetic Measurements, conceived for managing a wide spectrum of magnetic measurements techniques, are described. Experimental results of metrological and throughput characterization of FDI are reported. In particular, in metrological characterization, FDI working as a digitizer and as an integrator, was assessed by means of static, dynamic, and time base tests. Typical values of static integral nonlinearity of ±7 ppm, ±3 ppm of 24-h stability, and 108 dB of signal-to-noise-anddistortion ratio at 10 Hz on Nyquist bandwidth of 125 kHz, were surveyed during the integrator working. The actual throughput rate was measured by a specific procedure of PXI bus analysis, by highlighting typical values of 1 MB/s. Finally, the experimental campaign, carried out at CERN facilities of superconducting magnet testing for on-field qualification of FDI, is illustrated. In particular, the FDI was included in a measurement station using also the new generation of fast transducers. The performance of such a station was compared with the one of the previous standard station used in series tests for qualifying LHC magnets. All the results highlight the FDI full capability of acting as the new de-facto standard for high-performance magnetic measurements at CERN and in other high-energy physics research centres

Metrological Characterization of an Improved DSP-Based On-line Integrator for Magnetic Measurements at CERN

An improved on-line version of the self-calibrating digital instrument for flux measurements on superconductive magnets for particle accelerators, prototyped at the European Organization for Nuclear Research (CERN) in cooperation with the University of Sannio, is proposed. The instrument acquires voltage arising from rotating coils transducers. Then, the samples are online integrated and suitably processed in order to achieve flux analysis time down to 2.0 ìs, with resolution of 50 ns. Details about hardware and firmware conception, on-line measurement principle, and preliminary results of metrological characterization of the prototype are provided

Advanced measurement systems based on digital processing techniques for superconducting LHC magnets

The Large Hadron Collider (LHC), a particle accelerator aimed at exploring deeper into matter than ever before, is currently being constructed at CERN. Beam optics of the LHC, requires stringent control of the field quality of about 8400 superconducting magnets, including 1232 main dipoles and 360 main quadrupoles to assure the correct machine operation. The measurement challenges are various: accuracy on the field strength measurement up to 50 ppm, harmonics in the ppm range, measurement equipment robustness, low measurement times to characterize fast field phenomena. New magnetic measurement systems, principally based on analog solutions, have been developed at CERN to achieve these goals. This work proposes the introduction of digital technologies to improve measurement performance of three systems, aimed at different measurement target and characterized by different accuracy levels. The high accuracy measurement systems, based on rotating coils, exhibit high performance in static magnetic field. With varying magnetic field the system accuracy gets worse, independently from coil speed, due to the limited resolution of the digital integrator currently used, and the restrictions of the standard analysis. A new integrator based on ADC conversion and numerical integration is proposed. The experimental concept validation by emulating the proposed approach on a PXI platform is detailed along with the improvements with respect to the old integrators. Two new analysis algorithms to reduce the errors in dynamic measurements are presented. The first combines quadrature detection and short time Fourier transform (STFT) of the acquired magnetic flux samples; the second approach is based on the extrapolation of the magnetic flux samples. Unlike other algorithms presented in the literature, both the proposals do not require the information about the magnet current and are able to work in real time so, can be easily implemented in firmware on DSP. The performance of the new proposals are assessed in simulation. As far as medium accuracy systems are concerned, at CERN was originally developed a probe to measure the sextupolar and decapolar field harmonics of the superconducting dipoles using a suitable Hall plates arrangement for the bucking of the main dipolar field, which is, 4 orders of magnitude higher than the measurement target. The output signals of each Hall plate belonging to the same measurement ring are mixed using analog cards. The resultant signal is proportional to the field harmonic to measure. A complete metrological characterization of this sensor was carried out, showing the limitation of a fully analog solution. The main problems found were the instability of the analog compensation cards and the impossibility to correct the non linearity effects beyond the first order. An automatic calibration procedure implemented in the new instrument software is presented to guarantee measurement repeatability. In alternative a digital bucking solution, namely the compensation of the main field after the sampling of each hall plate signal by means of numerical sum, is proposed. An implementation of this approach, based on 18 bit ADC converter, over-sampling and dithering techniques as well as compensation of the Hall plates non linearity in real time is analyzed. Finally, as far as the low accuracy measurement systems are concerned, the design of an instrument based on a rotating Hall plate to check the polarity of all LHC magnets is presented. Even if this architecture is characterized by low accuracy in the measurement of field strength and phase, the results are sufficient to identify main harmonic order, type and polarity with practically no errors, thanks to an accurate definition of the measurement algorithm. A complete metrological characterization of the prototype developed and a correction of all the systematic measurement errors was carried out. This instrument, integrated in a test bench developed ad hoc, is become the standard at CERN for the polarity test of all the magnets will compose the machine

Performance Analysis of a Fast Digital Integrator for Magnetic Field Measurements at CERN

A Fast Digital Integrator (FDI) has been designed at CERN for increasing performance of state-of-art instruments analyzing superconducting magnets in particle accelerators. In particular, in flux measurement, a bandwidth up to 50-100 kHz and an accuracy of 10 ppm has to be targeted. In this paper, basic concepts and architecture of the developed FDI are highlighted. Numerical metrological analysis of the instrument performance is shown, by focusing both on deterministic errors and on uncertainty in time and amplitude domains

Metrological Characterisation of a Fast Digital Integrator for Magnetic Measurements at CERN

A Fast Digital Integrator (FDI) was designed to satisfy new more demanding requirements of dynamic accuracy and trigger frequency in magnetic measurements based on rotating coil systems for analyzing superconducting magnets in particle accelerators. In particular, in flux measurement, a bandwidth up to 50-100 kHz and a dynamic accuracy of 10 ppm are targeted. In this paper, results of static and dynamic metrological characterization of the FDI prototype and of the Portable Digital Integrator (PDI), heavely used at CERN and in many sub-nuclear laboratories, are compared. Preliminary results show how the initial prototype of FDI is already capable of both overcoming dynamic performance of PDI and covering operating regions inaccessible before

Field Measurements

The measurement of the magnetic field is often the final verification of the complex design and fabrication process of a magnetic system. In several cases, when seeking high accuracy, the measurement technique and its realization can result in a considerable effort. This note describes most used measurement techniques, such as nuclear magnetic resonance, fluxmeters and Hall generators, and their typical range of application. In addition some of less commonly used techniques, such as magneto-optical, SQUIDs, or particle beams methods, are listed

Domain Specific Language for Magnetic Measurements at CERN

CERN, the European Organization for Nuclear Research, is one of the world’s largest and most respected centres for scientific research. Founded in 1954, the CERN Laboratory sits astride the Franco–Swiss border near Geneva. It was one of Europe’s first joint ventures and now has 20 Member States. Its main purpose is fundamental research in partcle physics, namely investigating what the Universe is made of and how it works. At CERN, the design and realization of the new particle accelerator, the Large Hadron Collider (LHC), has required a remarkable technological effort in many areas of engineering. In particular, the tests of LHC superconducting magnets disclosed new horizons to magnetic measurements. At CERN, the objectively large R&D effort of the Technolgy Department/Magnets, Superconductors and Cryostats (TE/MSC) group identified areas where further work is required in order to assist the LHC commissioning and start-up, to provide continuity in the instrumentation for the LHC magnets maintenance, and to achieve more accurate magnet models for the LHC exploitation. In view of future projects, a wide range of software requirements has been recently satisfied by the Flexible Framework for Magnetic Measurements (FFMM), designed also for integrating more performing flexible hardware. FFMM software applications control several devices, such as encoder boards, digital integrators, motor controllers, transducers. In addition, they synchronize and coordinate different measurement tasks and actions

Innovative calibration method for rotating-coil magnetometers

Rotating-coil magnetometers, which rely on induction-coil arrays, are commonly used for magnetic measurements of particle accelerator magnets. In order to characterize magnetic fields with high accuracy using the induced voltages, it is essential to calibrate the geometry of the coils. In this thesis we present an innovative method of calibration of the coil radius of rotation which is necessary for determination of the field gradient. The method, called rotating calibration method, is based on two well-known measurement techniques: single-stretched wire and rotating induction-coils. The flux measured with a rotating induction-coil over the entire length of a reference quadrupole magnet is cross-calibrated with a single-stretched wire measurement. In the experimental validation of the method, the radius of rotation of several coils has been calibrated with an accuracy of 10 µm on a radius of 30 mm. The advantage of this method is that the calibration is performed under the same conditions in which the rotating-coil magnetometers are used for measuring accelerator magnets. In the thesis also a proposal for calibration of other geometric factors of a coil by using higher order multipoles is presented