Philippe JEAN-PIERRE

Following a rockslide triggered in 2018, a slow-moving landslide has been reactivated affecting the Viella municipality (Hautes-Pyrénées, Southwest France). The slope is now sliding inexorably down to the valley; several landslide kinematic sub-units characterized by several ground motion have damaged infrastructures and buildings. In the months following this reactivation, surface velocities of up to one centimeter per day were observed, endangering some buildings and necessitating the implementation of mitigation measures. To assess the water circulation and storage in the slope, a three-dimensional geological model was built as a prerequisite of 3D hydrogeological simulations carried out with GeoModeller. The geological model allows for improving the knowledge of the slope internal structures, the position of the bedrock and of the sliding surfaces. Three-dimensional finite element water flow simulations obtained with the FEFLOW (©DHI) model provide an understanding of the functioning of the landslide aquifer. The integrative approach allows proposing a robust hydrogeological model and the establishment of a piezometric map of the site at equilibrium. In the landslide, the piezometry is between 780m and 970m in elevation, and the direction of the groundwater flow is about 340° North. The hydraulic conductivities determined by the model are between 10 -4 and 10 -5 m.s -1 in the shallow sub-surface (colluvium), up to 10 -7 m.s -1 in depth (Devonian's shales). From the calibrated hydrogeological model, various simulations were carried out in order to estimate the impacts of mitigation works on the water storage and circulation. It further helped to simulate the piezometric response of the slope to an extreme flood event at the toe of the landslide. Model simulations showed that the "sealing" or "waterproofing" of a 650m section in the lower part of the Bayet-Badoueil stream would lower the piezometric height under the village up to a maximum of 30 m and reduce the hydraulic load uphill. A decrease of the water table height of 5 m seems achievable and would be sufficient to significantly reduce the sliding motion with the boundary conditions set (zero flux) in these simulations.

The GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN, situated on the antiproton decelerator ring (AD), is aimed at investigating the free fall of antihydrogen atoms prepared at rest, as suggested by J. Walz and Th. Hänsch [1]. This experiment employs two trapped ions: one is a Be+ ion cooled via laser, while the other, an Hbar+ ion, undergoes cooling through interactions with the Be+ ion (sympathetic cooling). We are developing a test-bed experiment designed to explore sympathetic cooling of a light ion using a cloud of laser-cooled heavy ions, mimicking the conditions anticipated in the GBAR project [2]. The experimental setup involves the pairing of 88Sr+ (laser-cooled ion) and 9Be+ (sympathetically cooled ion). The choice of these ions offers two advantages: the ability to optically address the 9Be+ ion for thermometry measurements and their mass ratio (88/9 ≈ 9.8) closely resembling that in the GBAR project (9/1). Preliminary experimental results [3] demonstrate sympathetic cooling of 9Be+ ions by laser-cooled 88Sr+ ions, forming a Coulomb crystal. Detection of sympathetically cooled ions relies on analyzing Coulomb crystal images, revealing dark areas where non-laser-cooled Be+ ions reside. Molecular dynamics simulations showed strong spatial segregation for both species, due to their high mass ratios [4]. However, the initial experiment did not include laser addressing of the 9Be+ ions for measuring cooling dynamics and control over the initial energy of Be+ ions. The next stage of the project therefore involves adding the 313nm laser for addressing Be+ ions and utilizing a 2-zone trap to control the initial energy of a single Be+ ion. This will enable us to measure for the first time the capture dynamics of a light ion by a Coulomb crystal and to follow its cooling over several decades (typically from 10000K to mK). We will compare these measurements with numerical simulations [4]. Initially, only Sr+ ions were employed to validate trapping and cooling conditions, characterize photon collection optics, and test ion transport protocols between trapping zones. A method utilizing Doppler recooling was developed to characterize the initial energy of Sr+ ions upon arrival in the target trap. Subsequently, using the Be+ ion cooling laser at 313 nm, it will be possible to cool a single Be+ ion and transport it with controlled kinetic energy to the second trapping zone, already loaded with a Sr+ Coulomb crystal. The kinetic energy loss of the Be+ ion is quantified by measuring laser-induced fluorescence rate at resonance, which produces no laser cooling or heating. Thermalization of the light ion via coulomb interactions will then be studied for different heavy-ion crystal temperatures, shapes and ion numbers.