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LISA Pathfinder II (Ballroom A)
Thursday Chair: Paul McNamara
13:30 - 13:47 Giuliana Russano
University of Trento
A torsion pendulum test of the Lisa Pathfinder free-fall modeDownload
13:47 - 14:04 Eric Plagnol
APC/Paris
Characterization of the micro-thruster systemDownload
14:04 - 14:21 Ferran Gibert
Institut de Ciències de l'Espai (IEEC-CSIC)
In-flight thermal experiments for LISA PathfinderDownload
14:21 - 14:38 Carlo Zanoni
University of Trento
Summary of the results of the LISA-Pathfinder Test Mass releaseDownload
14:38 - 14:55 Catia Grimani
University of Urbino "Carlo Bo"
The LISA Pathfinder environment and mission performance in 2015Download
14:55 - 15:12 valerio Ferroni
University of Trento, TIFPA
Measuring and mitigating electrostatic forces aboard LISA PathfinderDownload
15:12 - 15:29 Andreas Wittchen
Max-Planck Institute for Gravitational Physics
Optimizing Test Mass Position for the LISA-Pathfinder Optical Metrology SystemDownload
A torsion pendulum test of the Lisa Pathfinder free-fall mode Download
Giuliana Russano - University of Trento
The LISA Pathfinder geodesic explorer mission for gravitational wave astronomy aims to demonstrate a residual acceleration noise at the femto-m/s^2/Hz^1/2 level. The relative acceleration between two test masses free falling in orbit is perturbed by a large (nm/s^2) and constant relative acceleration that must be actively compensated in order to keep the test particles centered inside an orbiting apparatus. The actuation force applied to compensate this effect introduces a dominant source of force noise. To suppress this noise source, a “free-fall” control scheme has been designed: actuation is limited to brief impulses, with test masses in free fall in between two “kicks”, with this actuation-free motion then analyzed for the remaining sources of acceleration noise. In this work, we present preliminary data for an on-ground torsion pendulum experiment to test the measurement technique, possible associated analysis algorithms, and the general experimental conditions in which the test can be performed, at a level nearing the performance required for LISA Pathfinder. Tests are performed with a pendulum facility, using the LISA Pathfinder gravitational reference sensor prototype, measuring acceleration noise while holding one suspended test mass centered on average with periodic impulses.
Characterization of the micro-thruster system Download
Eric Plagnol - APC/Paris
The LisaPathfinder (LPF) mission aims at demonstrating the possibility of performing drag-free flight... One of the key elements of this demonstration is the use for micro-newton thrusters coupled to precise interferometric measurement of test mass position. This presentation will focus on the micro-thruster system of LISAPathfinder, the algorithm used to control them, and the scheme that may be developed to characterize individual thruster performances in terms of amplitude and direction noise during the mission.
In-flight thermal experiments for LISA Pathfinder Download
Ferran Gibert - Institut de Ciències de l'Espai (IEEC-CSIC)
Thermal Diagnostics experiments to be carried out onboard LISA Pathfinder will yield a detailed characterisation of how temperature fluctuations affect the LTP (LISA Technology Package) instrument performance, a crucial information for future space based gravitational wave detectors as the proposed eLISA. Experiments to disentangle each temperature-induced noise contribution will be performed onboard by means of a dedicated Thermal Diagnostics Subsystem, based on series of heaters and high precision temperature sensors (10^-5 K/sqrt(Hz) in the millihertz regime). These series of investigations will address thermoelastic distortion, temperature-gradients effects on the LTP Inertial Sensors and thermal-induced optical pathlength's variation. In this presentation we will report how these experiments are going to be conducted and analysed during the in-flight operations.
Summary of the results of the LISA-Pathfinder Test Mass release Download
Carlo Zanoni - University of Trento
The challenging goal of LISA-Pathfinder in terms of maximum non-gravitational forces applied to the test mass poses tight constraints on the design of the Gravitational Reference Sensor. In particular, large gaps (on the order of millimeters) must exist between the test mass and its housing and any system there located must be either gold coated or made of a gold-based material. As a consequence, a significant adhesion may arise between the test mass and the mechanism designed to cage it during the spacecraft launch and release it to free-fall. The criticality of the latter phase is enhanced by the control force authority exerted to the test mass by the surrounding electrodes. This force is strongly limited (order of N) by the large gaps. Since the expected adhesion force between the test mass and its holding devices is much larger than the force authority, a dynamic release must be realized by performing their quick retraction. However, following this procedure adhesion converts into test mass velocity, which can be controlled by the capacitive force only if it is smaller than 5 m/s. At the University of Trento (Italy) the Transferred Momentum Measurement Facility (TMMF) has been designed and developed to measure the impulse produced by metallic adhesion upon quick rupture, in representative conditions of the LISA-Pathfinder test mass release to free-fall. Large sets of data have been collected and a mathematical model of the in-flight release dynamics has been developed, in order to estimate the test mass release velocity. A summary of the results is presented, together with an overview of the recent developments and a prediction of the in-flight performance. Authors: D. Bortoluzzi, J. W. Conklin, I. Koeker, S. Vitale, C. Zanoni
The LISA Pathfinder environment and mission performance in 2015 Download
Catia Grimani - University of Urbino "Carlo Bo"
The LISA Pathfinder performance will be affected by the radiation environment in L1. We present projections of galactic cosmic-ray energy spectra during the second half of 2015. Variations and fluctuations of these spectra in the LISA-PF frequency band are discussed. The occurrence and characteristics of expected SEP events during six months of data taking are also presented. We focus, in particular, on the role of SEP spatial distribution. To our knowledge, this work represents the first attempt to evaluate in detail how a space mission is affected by the spatial distribution of SEPs during the evolution of the most intense events at energies > 100 MeV.
Measuring and mitigating electrostatic forces aboard LISA Pathfinder Download
valerio Ferroni - University of Trento, TIFPA
Electrostatic forces from test mass charging and stray electrostatic fields are a potentially important source of force noise for the LISA Pathfinder mission. We foresee in-flight operations dedicated to monitor the charge and reduce it to near zero by UV illumination. We will also measure the relevant stray electrostatic fields on the surfaces of both the test mass and the electrode housing and compensate them with DC electrode bias voltages. We present here the basics of the coupling between spacecraft and test mass electrostatic potential; we explain the importance of periodic charging/discharging and of long-term charge measurements to limit the force noise at low frequency, which is particularly relevant for the eLISA mission and we describe the analysis techniques used during the mission.
Optimizing Test Mass Position for the LISA-Pathfinder Optical Metrology System Download
Andreas Wittchen - Max-Planck Institute for Gravitational Physics
Suppression of amplitude and phase noise in the LPF OMS is achieved by control loops as well as subtraction of a reference signal from the measurement signals. The efficiency of this subtraction depends on the phase difference between the signals. Extensive measurements of this effect were made over the past year. This talk will present the results of these measurements and compare them to theoretical predictions. To achieve more flight-like conditions on the LPF ground setup the position and attitude of the test masses have to be controlled, with the aim of suppressing temperature induced drifts. Having controlled test mass mirrors will also allow us to simulate planned experiments which involve modulation of the test mass set-points. The current setup features piezoelectric actuators to induce longitudinal and angular test mass motion. These are currently controlled by an analog circuit. A digital control loop to suppress test mass motion will reduce the influence of the lab environment and make it possible to perform more in-flight experiments on the ground and achieve a more detailed characterization of the OMS procedures. This talk will present the on-going work on this topic and the results achieved to date.