Speaker
Description
In order to reach the targeted low-frequency sensitivity of the Einstein Telescope, large cryogenic mirrors will have to be mounted on ultralow-frequency seismic isolation stages. In this context, ET-CRISTAL is developing a full-scale prototype suspending a 100 kg silicon test mass cooled to cryogenic temperatures by radiative cooling and isolated at low frequency from seismic noise. The proposed isolation strategy is hybrid, combining a large inverted pendulum mounted on a soft active platform. This pairing of passive and active isolation enables a compact suspension design that achieves seismic isolation below 10 Hz and has been shown to fulfil the ET-LF DARM requirements. In addition, the two stages reinforce one another: the inverted pendulum provides an excellent reference to keep the active platform aligned, and the active platform in turn helps align the inverted pendulum and control its resonances. This talk will present the latest achievements in active seismic isolation on the ET-CRISTAL prototype. First, an overview of the objectives will be presented, focusing on the isolation performance targets and the control-oriented roadmap. It will then address the general strategies adopted for seismic isolation. This includes techniques for decoupling in all six directions, and virtual sensor fusion to boost low-frequency performances while mitigating the drift of inertial sensors. To conclude, the latest performances at the active platform level will be presented and correlated with theoretical predictions.