Laboratory tests of free-fall

Laboratory investigation of the small spurious forces that limit the purity free-fall

Small force measurements with torsion pendulums are the experimental anchor upon which a noise budget for low-frequency gravitational wave astronomy rests. The Trento group is currently operating two torsion pendulums investigating the experimental stray-force limits with which a LISA-like test mass inside a LISA-like GRS can serve as a free-falling geodesic reference.

With measurements of forces at the femto-Newton level, torsion pendulum measurements in Trento first demonstrated that the free-fall purity needed for LISA Pathfinder was even remotely possible and small force measurement campaigns are our best weapon to quantify some of the remaining mysteries for LISA.

Key measurements with torsion pendulums suspending LISA-like have included overall stray force noise limits, stray electrostatic fields and their fluctuations, Brownian motion from residual gas noise, GRS force gradients or “stiffness”, thermal gradient forces and electrostatic dissipation. The torsion pendulum tests, with prototype GRS flight hardware, have also been used to test key GRS functionalities, like electrostatic force actuation, capacitive sensing, the LISA Pathfinder free-fall mode, and UV photoelectric discharge.

We are currently preparing to test an “engineering model” of the LISA GRS in the next years with a 4-test mass (4TM) configuration directly sensitive to forces. The focus will be on surface related electrostatic effects, like stray fields and photoelectric discharge, thermal and outgassing effects, and the commanded and parasitic forces from the GRS actuation electronics.

In parallel, we are preparing a second single-mass (1TM) pendulum for dedicated small force studies and investigating improved small force detection sensitivity, including fused silica suspensions, interferometric readouts, and inertial platforms.

We are also equipped for dedicated specific tests to support investigation of low frequency electronics noise, femtoAmpere measurements of photoelectric discharge and various electromagnetic, gravitational and outgassing effects.

Staff contacts for information: Rita Dolesi, Bill Weber