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The UA9 piezo-goniometer has been designed to guarantee micro-radians-accuracy angular positioning of a silicon crystal for a crystal collimation experiment in the LHC, and to minimize the impact on the LHC beam-coupling impedance. This paper presents a Finite Element Method (FEM) study of the device, in both parking and operational positions, to evaluate its impact on the LHC impedance budget. The study has been a progressive simulation work, started from the simplification of the original detailed design, and aimed at highlighting the effect of single details (e.g. objects in confining chambers) on the longitudinal and transverse components of beam-coupling impedance. In addition, the shielding contribution of the RF gaskets has been carefully evaluated, with the objective to assess the consequences for operation in case of their failure. Sensitivity analyses to simulation parameters are also performed, in order to test the FEM model robustness. A final word is drawn on the overall device impedance criticality., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

Resistive-wall impedance usually constitutes a significant percentage of the total beam-coupling impedance budget of many accelerator structures (e.g. for LHC, it can be more than 50%). Reduction techniques for resistive-wall components entail high electrical-conductivity coatings. This paper proposes the use of metamaterials, having negative values of magnetic permeability or dielectric permittivity (or both), for sensibly reducing or theoretically nearly cancelling the resistive-wall component of beam-coupling impedance. The proposed approach is developed by means of an equivalent transmission-line model, whose results show the potential reduction of both longitudinal and transverse impedance when using metamaterial insertions. The effects on the real and imaginary part have been singled out. The effectiveness of such materials is discussed both for negative-permittivity and for negative-permeability cases, which actually show different impacts and can be then target of proper engineering. This first-stage study opens the possibility of considering metamaterials for impedance mitigation or for setting up proper experimental setups., Proceedings of the 5th Int. Particle Accelerator Conf., IPAC2014, Dresden, Germany

Linear Variable Differential Transformers (LVDTs) are widely used for high-precision and high-accuracy linear position sensing in harsh environments, such as the LHC collimators at CERN. These sensors guarantee theoretically infinite resolution and long lifetimes thanks to contactless sensing. Furthermore, they offer very good robustness and ruggedness, as well as micrometer uncertainty over a range of centimeters when proper conditioning techniques are used (such as the three-parameter Sine-Fit algorithm). They can also be suitable for radioactive environments. Nevertheless, an external DC/slowly-varying magnetic field can seriously affect the LVDT reading, leading to position drifts of hundreds of micrometers, often unacceptable in high-accuracy applications. The effect is due to the presence of non-linear ferromagnetic materials in the sensor’s structure. A detailed Finite Element model of an LVDT is first proposed in order to study and characterize the phenomenon. The model itself becomes a powerful design tool for possible countermeasures to the interference effect. In particular, a combination of magnetic shielding and DC polarization is proposed to reduce the drift due to the external field. Nevertheless, such solutions cannot lead to complete immunity, given the unavoidable presence of magnetic materials in the sensor. Taking the CERN application as a starting point, this thesis aims at conceiving, modelling and characterizing a valid alternative to LVDTs for harsh magnetic environments, which would guarantee magnetic-field-immune position sensing while keeping all the advantageous properties of LVDTs. The Ironless Inductive Position Sensor (I2PS) is an air-cored structure made of 5 coaxial coils. The position sensing is achieved by spatially-variable magnetic fluxes, which give rise to position-dependent coil voltages, just as for LVDTs. The complete electromagnetic model of the sensor is proposed, showing the working principle and demonstrating the magnetic-field immunity from a theoretical viewpoint. In addition, a high-frequency electromagnetic analysis is performed, in order to model the skin and proximity effects in the conductors and foresee their impact on the sensor’s functioning. The models are validated with FEM simulations and experimental measurements. The thermal behaviour of the sensor is also investigated and an effective compensation algorithm is proposed to cancel the temperature-dependence of the position reading. In addition, a smart real-time reading algorithm is proposed in order to significantly reduce the estimation error of standard three-parameter Sine-Fit algorithms when an additional sinusoidal signal is present on the main waveform. Finally, a generic optimization procedure is proposed in order to maximize the performances of the sensor in terms of sensitivity. Taking this procedure as a guideline, an actual I2PS optimized prototype is designed and manufactured, having the specifications of the LHC collimators application as a reference. The optimized prototype shows immunity to external ramped and sinusoidal fields, as expected. In addition, it is used for the experimental validation of the models and the reading techniques, which demonstrate their effectiveness.

In the framework of the UA9 experiment, the Cherenkov detector is useful to measure the amount of particles deflected by a bent crystal, proving the crystal collimation principle. The tank used to host this device is taken as a case study for an in-depth analysis of different damping strategies for electromagnetic modes which otherwise would give rise to important beam-coupling impedance contributions. Such strategies involve the use of ferrite, damping resistors and a mode-coupler, a solution which intercepts the modes inside the cavity but damps the related power outside the vacuum tank (potentially avoiding heating). Such solutions are discussed through experimental measurements and the relative quality factor is taken as a figure of merit., Proceedings of the 6th Int. Particle Accelerator Conf., IPAC2015, Richmond, VA, USA

International audience; Storm surge and wave runup are key determinants of the potential for beach overwashing during storm events. However, the prediction and quantification of wave runup on embayed beaches is strongly influenced by particular characteristics (e.g., irregular morphology, low tides, absence of swell, etc.) which differ from those on open beaches, and have rarely been investigated in literature. In the present paper, a model framework aimed at predicting wave-induced runup on an embayed sandy beach is validated by means of measurements derived from a video-monitoring station, recently installed in South Italy, during two storm events in 2016. The numerical approach employs MeteOcean forecasted waves within SWAN and SWASH models (in both 2-d and 1-d mode). The combination of multibeam and d-RTK surveys with Unmanned Aerial Vehicle (UAV) imagery provides high resolution depth grid (m 0.015), particularly required in shallow waters, where wave hydrodynamics is highly influenced by the bottom. The results show and discuss the agreement between video measurements and 2-d predictions of runup. A sensitivity analysis of the Manningfls roughness factor is needed in 1-d simulations. The accuracy of the empirical formulas in predicting wave runup in an embayed beach is also investigated , showing mainly an overestimation of the observations.