ET low frequencies and controls: a proposal for ILIAS-next. A. Rocchi for INFN Napoli, Pisa and Tor Vergata and NIKHEF

2nd ET General Meeting - Erice, Seismic Gravity gradient noise Control noise ET sensitivity

OUTLINE Sensitivity at low-frequency is affected by: – Seismic noise – Gravity gradient noise – Control noise Main goals of this proposal: – underground sites characterization – development of techniques to quantify and reduce the GGN contribution – development of linear accelerometers for ET suspension chain – development of position sensors and actuators for payload local controls 32nd ET General Meeting - Erice,

PARTICIPANTS INFN (Italy) F. AcerneseUniversity of Salerno – INFN Napoli F. BaroneUniversity of Salerno – INFN Napoli E. CalloniUniversity of Napoli Federico II – INFN Napoli G. CellaINFN Pisa E. Coccia University of Roma Tor Vergata – INFN Tor Vergata R. De RosaUniversity of Napoli Federico II – INFN Napoli L. Di FioreINFN Napoli V. FafoneUniversity of Roma Tor Vergata - INFN Tor Vergata F. GarufiUniversity of Napoli Federico II – INFN Napoli A. GennaiINFN Pisa L. MilanoUniversity of Napoli Federico II – INFN Napoli Y. MinenkovINFN Tor Vergata L. PalladinoUniversity of L’Aquila – LNGS-INFN Tor Vergata D. PassuelloINFN Pisa A. RocchiINFN Tor Vergata R. RomanoUniversity of Salerno – INFN Napoli NIKHEF (The Netherlands) J.F.J. van den BrandVrijeUniversity, Amsterdam - NIKHEF T. BauerNIKHEF E. HennesNIKHEF M. DoetsNIKHEF 42nd ET General Meeting - Erice,

SITE SELECTION Ambient ground motion and gravity gradient noise At 1 Hz: Hiidenvesi cave: <1 nm/√Hz Moxa station: 0.5 nm/ √Hz Asse 900 m: 0.5 nm/ √Hz Ongoing studies at Homestake with seismic network Down to 1500m Ground motion is strongly site dependent 5

62nd ET General Meeting - Erice, Large geological variations in Europe large sediment regions homogeneous materials: crystalline granite Test candidate sites using a seismic network

Summary Site selection – Requires dedicated tests at candidate sites in Europe Sites characterization: – Array of commercial seismic sensors – Monolithic horizontal accelerometer Effects of geology Influence of cultural noise Use results as input for FEA Gravity gradient noise – Limits sensitivity at low frequencies (1 – 10 Hz) – FEA and analytical studies for GGN subtraction are in progress – See all talks later this morning 72nd ET General Meeting - Erice,

8 Mechanical Model: Folded Pendulum The Folded Pendulum (FP) “horizontal” configuration allows arbitrary low resonant frequencies It basically consists of an arm (pendulum) connected through a rigid bar to an inverse arm (inverted pendulum). It doesn’t need an elastic restoring force (material stress) It can be easily obtained from a single piece of material with electric discharge machining (EDM) Tunable Mechanical Monolithic Horizontal Seismometer Accelerometer Single block of Al alloy 7075-T6 of 140x134x40 mm EDM with a 250  m thick wire to cut the internal surfaces. 8 elliptical notch hinges for torsional flexures (ellipticity ratio 16/5) Flexure joints 100  m thick One central hole to place the calibration mass Mechanical design Pendulum Central Mass Inverted Pendulum Central Window

The Folded Pendulum Prototype (mod. 08F_100_AL1) 92nd ET General Meeting - Erice, Pendulum Mirror (inside) Interferometric and/or optical lever readout Coil-magnet (inside) for closed-loop configuration Inverted Pendulum Central Mass Hinges Mechanical Quality Factor 70 mHz Folded Pendulum Tuning Test Measured sensitivities with different optical readouts: optical lever PSD and quadrant photodiode laser interferometer

600m Operation at DUSEL 102nd ET General Meeting - Erice, PSD of ground 600m underground level (4 weeks of data) compared with theoretical FP noise and Peterson’s New Low Noise Model

Monolithic Horizontal Seismometer/Accelerometers Goals Development and integration of the horizontal mechanical monolithic seismometer/accelerometer for: 1.Low frequency horizontal seismic noise measurement Long term tests of horizontal seismic noise acquisition at DUSEL and at Gran Sasso INFN Laboratory for their characterization in terms of sensitivity to environmental noises. Low frequency characterization (and modeling in term of seismic and Newtonian noise) of candidate underground sites for the implementation of the Einstein Telescope. 2.Integration of monolithic horizontal sensors in the control of the mechanical suspensions of ET. 112nd ET General Meeting - Erice,

Mirror and Marionetta Local Position Sensors - Local position sensors are used for the damping and pre-alignment of the payload; -The main characteristic of a such sensing system are: - Large dynamic in order to manage huge (some mm or mrad) displacements and rotations of the payload components; - Good sensitivity in order to allow the locking engagement within the residual displacement or angular noise (fraction of µm or µrad); - Often these requirements can only be achieved by using a set of hierarchical systems (Virgo: CCD cameras and optical levers) 122nd ET General Meeting - Erice,

- The sensor set-up is critical in order to be compliant with the cryogenic payload; -Seismic ground noise is expected to be about 10 times lower respect to the surface: in principle this allows to increase the sensitivity of the local sensing system, but the current performances of the Virgo sensing system are already under the limit; - There are two contrasting requirements: -“No” optical window for auxiliary beam transmission to increase the thermal isolation; -No dissipating devices inside the vacuum chamber; -Different solutions, based on optical levers, to investigate on. 132nd ET General Meeting - Erice, Constraints for ET

2nd ET General Meeting - Erice, Options for ET 14 -The investigation will be mainly focused on the performances of the different solutions in terms of sensitivity (compared to the standard position sensor) and compliance with cryogenic environment. Refer to E. Majorana yesterday talk

Mirror and Marionetta Actuators - No particular effort is required about the Marionetta actuators - Coil magnets pairs should easily fit the requirements - Electrostatic actuators are useful for the test mass control: -No need to use magnets on the test mass; -Better immunity from EM noise; - The idea is to use the test mass level actuator only for the lock acquisition; - Both longitudinal and angular controls from the Marionetta stage in standard conditions; - In this way the actuator could be placed on ground: no recoil mass; - The use of low temperature opens the possibility to adopt superconducting devices for the actuation on the Marionetta and the test mass; - The investigation will be focused on the actuator characterization and compatibility/optimization with cryogenic environment; 152nd ET General Meeting - Erice, Electrostatic actuator Rigid connection Marionetta Electrostatic actuator (Motorized ?) Actuator holder Marionetta Ground

TASKS AND OUTPUT OF THE PROJECT 162nd ET General Meeting - Erice, )Acquisition of seismic data in different underground sites in Europe, to get information about attenuation of seismic noise with depth, dependence on geology, e.g. sediments, hardrock and salt, and coherence length of signals among different sensors. 2)Analysis and validation of seismic models and study of suitable underground architectures and sensors configurations for GGN measurement and reduction in underground sites. 3)Development of seismic sensors and their application in the control of the suspension chains through their characterization in the facilities of INFN – Napoli and INFN - Roma Tor Vergata. 4)Data Analysis of the acquired data for geophysical applications and modeling of the underground sites. 5)Development and characterization of contactless sensors for the test mass and marionetta position sensing, optimization and integration in the suspended chain and test in cryogenic environment. 6)Development of actuators for test mass position control, characterization in different working configuration using the suspended chain, test in cryogenic environment. 7)Study of Earth tide with a suspended interferometer

INVOLVEMENT OF THE PARTICIPANTS IN THE TASKS PLANNING 172nd ET General Meeting - Erice, Task Napoli xxxxx Nikhef xxx Pisa xx x Roma TV xxx Year 1: Half seismic array + data acquisition ready; Preliminary test of actuators and position sensors; Year 2: Test of position sensor with in single probe configuration; Full seismic array installed for 1 st measurement; Characterization of standalone monolithic seismic sensors in low frequency range; Characterization of actuators and position sensors in vacuum; Year 3: Integration of monolithic sensors in the suspension; Completion of the experimental set-up for tests at low temperature; Year 4: Characterization of the underground sites on the basis of GGN measurement; Suspension chain control with local controls based on the developed sensors and actuators; Test of the developed sensors and actuators in cryogenic environment; Completion of the study of Earth tide study with a suspended interferometer.