DEVELOPMENT OF SENSORS, SYSTEMS AND TECHNIQUES FOR LOW-FREQUENCY SEISMIC AND NEWTONIAN NOISE MONITORING AND FOR REDUCTION OF CONTROL NOISE IN UNDERGROUND GW DETECTORS Proposal for ILIAS-next (JRA1)

PARTICIPANTS INFN (Italy) F. AcerneseUniversity of Salerno – INFN Napoli F. BaroneUniversity of Salerno – INFN Napoli E. CalloniUniversity of Napoli Federico II – INFN Napoli 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 L. MilanoUniversity of Napoli Federico II – INFN Napoli Y. MinenkovINFN Tor Vergata L. PalladinoUniversity of L’Aquila – LNGS-INFN Tor Vergata 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

OUTLINE Sensitivity at low-frequency is affected by: – Seismic noise – Newtonian noise – Control noise Main goals of this proposal: – development of suitable integrated sensors and techniques to quantify and reduce the LF noise contributions – development of suitable position sensors and actuators for the last two stages of the suspension chain

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) Pendulum Mirror (inside) Interferometric and/or optical lever readout Coil-magnet (inside) for closed-loop configuration Inverted Pendulum Central Mass Hinges

First tests on prototype Measured sensitivities with different optical readouts: - Optical lever (PSD and Quadrant Photodiode) - Laser Interferometer. Mechanical Quality Factor 70 mHz Folded Pendulum Tuning Test

600m Operation at DUSEL PSD of ground 600m underground level (4 week 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 GW interferometric detectors.

9 Need for tiltmeters Acceleration signal coming from tilt has the wrong sign: noise amplification Need independent information VIR-NOT-FIR (a) pure translation: the table moves leftward, the accelerometer gives a positive signal and the feedback push the table rightward towards the zero position; (b) tilt: the table tilts, the accelerometer gives a positive signal and the feedback push the table rigthwards: wrong direction

Angular Accelerometers Goals 1.Development and characterization of an angular accelerometer 2.Low frequency tilt noise measurement in underground sites 3.Integration in the control of the mechanical suspensions of GW interferometric detectors.

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

Large geological variations in Europe large sediment regions homogeneous materials: crystalline granite Test candidate sites using a seismic network

Finite element analysis Reaction to vertical point oscillation – Two layer geology Wave attenuation has two components – Geometrical (expansion of wave fronts) ~ r n Rayleigh, n=-1/2 Body waves at depth, n=-1 – Material (damping) Rayleigh Head Shear Pressure Surface waves Body waves Example: sandstone,  = 3.5 x f sec/cm, a plane wave disturbance at 1 Hz would be attenuated over 10 km by less than 4% Mark Beker, David Rabeling, Caspar van Leeuwen, Eric Hennes

Effects of seismic noise Seismic noise suppression – Development of superattenuators Gravity gradient noise – Cannot be shielded – Network of seismometers and development of data correction algorithms Figure: M.Lorenzini

Underground detectors - Cella Surface Z=-10 m Z=-100 m Z=-1000 m Equivalent strain noise amplitude (Hz -1/2 ) Reduction factor Frequency (Hz) Assumptions: C L = 1000 m/s (lower is better) C T /C L = 0.5 (lower is worse) Surface modes and transverse mode only Feasible Can we do better? especially in the low frequency region Volume waves! Analytical results by G. Cella The 58th Fujihara Seminar (May 2009)

axax azaz ayay cPcP cScS P-wave passing 600m depth S-wave passing 400m depth H=400 m H=500 m H=600 m Time [ s ] a [ m/s 2 ] -16  More realistic model and impulse response – All wave types included – GGN drops less than order of magnitude – Little geometric suppression Impulse response - halve space - damping

Decomposition of GGN signal × z x  GGN composition – Both surface and bulk contributions – GGN signal `instantaneous’, sensors delayed response – GGN subtractions schemes under study

Summary Site selection – Requires dedicated tests at candidate sites in Europe 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 studies (and GGN subtraction schemes) in progress

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)

Mirror and Marionetta Local Position Sensors - The ultimate sensitivity of the local sensor is limited by the seismic noise, since the sensing is performed respect to the ground; - For ground detectors it is not useful to use very sensitive set-up, since the seismic vibration will mask the sensor performances (about 0.1 µm at 1 Hz); - For second generation detectors the residual motion of the test masses, allowing the locking engagement, has to be very slow: < 0.3 µm/s, giving a displacement around: 0.1 µm. -Still above the seismic position noise of the local sensor by using some care; - For third generation detectors the residual allowed displacement could be easily lowered to less than 0.01 µm, depending on: cavity finesse, mirror mass, … -But this requirement could be achieved thanks to the lower seismic noise content in underground quiet locations (at least a factor 10 less respect to surface); -The target is the development of a position sensor with a sensitivity around 1 nm/Hz 1/2 in the band Hz;

Mirror and Marionetta Local Position Sensors - A little more critical is the sensor set-up in order to be compliant with the cryogenic payload; - 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: - Uncoupling optics and PSD inside the vacuum chamber outside the cold shields; Vacuum Fiber PSD and uncoupling optics Cold shield with holes

Mirror and Marionetta Local Position Sensors - Uncoupling optics inside the tank and probe beam driven outside by fiber bundles or fiber tapers; -No optical uncoupling, multiple probe beams for geometrical uncoupling driven outside by fiber tapers; -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.

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;

TASKS AND OUTPUT OF THE PROJECT 1.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 NN measurement and reduction in underground sites. 3.Development and validation of low-frequency, low-noise and large-band seismic sensors (seismometers and accelerometers) for Seismic and NN measurement; 4.Development and characterization of low-frequency, high sensitivity tiltmeters and performances test on inverted pendulum. 5.Optimization of the developed seismic sensors for their application in the control of the suspension chains through their characterization in the facilities of INFN – Napoli and INFN - Roma Tor Vergata. 6.Data Analysis of the acquired data for geophysical applications and modeling of the underground sites. 7.Development and characterization of contactless sensors for the test mass and marionetta position sensing, optimization and integration in the suspended chain and validation in cryogenic environment. 8.Development of actuators for test mass position control, characterization in different working configuration using the suspended chain, validation at cryogenic temperatures and optimization for cryogenic environment.

INVOLVEMENT OF THE PARTICIPANTS IN THE TASKS Year 1: Half seismic array + data acquisition ready; Preliminary characterization of actuators and position sensors in vacuum; Year 2: Position sensor with sensitivity better than 5·10 -9 m in the band Hz in single sensor configuration; Full seismic array installed for 1 st measurement; Monolithic sensors with sensitivity better than m in the band Hz; Preliminary measurements of actuators and position sensors at low temperature Year 3: Integration of monolithic sensors in the suspension; Comparison of the performances of ground based and suspended actuators for the test mass; Characterization of tiltmeter in low frequency range; Year 4: Characterization of the underground sites on the basis of NN measurement; Suspension chain control with local controls based on the developed sensors and actuators; Full characterization of the developed sensors and actuators in cryogenic environment; PLANNING