A torsion pendulum ground test of the LISA Pathfinder Free-fall mode
  LISA Pathfinder is the technological demonstrator space mission for the future gravitational waves observatory in space eLISA, with the aim of measure the differential acceleration between free-falling test masses orbiting in the same apparatus at a level of 30 fm/s-2Hz-1/2 at 1 mHz. Because the satellite can't follow the two masses at the same time, the second mass must be forced to follow either the other one or the spacecraft. The actuation force applied to compensate this effect introduces a dominant source of force noise in the mission noise budget at frequency near and below the mHz. The free-fall mode actuation control scheme has been designed to suppress this noise source and avoid actuation instabilities: actuation is limited to brief periodic impulses, with test masses in free fall in between two kicks. This actuation-free motion is then analyzed for the remaining sources of acceleration ultra noise. A free-fall mode parallel testing has been successfully implemented on torsion pendulum facility at the University of Trento and the results of the measurement campaign are ready to be shown and discussed. The implementation of the data analysis algorithms to estimate the spectrum of the acceleration of the controlled test mass, and the estimation of system parameters such as the force gradients, stiffness terms, are also part of this work. The free-fall mode has been tested at the level of 2 fNm/Hz-1/2, at the frequencies of 1 mHz, corresponding to an acceleration of about 100 fm /s-2 Hz-1/2. The ground tests have nevertheless allowed to develop and test quantitatively this innovative method of noise reduction due to actuation noise, contributing to its development for the LISA Pathfinder mission. 
Giuliana Russano
physics.ins-det