Advanced measurement systems based on digital processing techniques for superconducting LHC magnets
Creators: name: Masi, Alessandro
Publisher: name: Università degli Studi di Napoli Federico II
Year: 2006

description: 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.