LISA Pathfinder
at the University of Trento
An orbiting experiment to demonstrate sub-femto-g free-fall
How can an experiment demonstrate that sub-femto-g differential acceleration measurements at 1 mHz will be reliably achieved with the LISA observatory? And, equally important, how can we demonstrate that all realistic noise sources are under control even at the lower frequencies (0.1 mHz and below) and higher sensitivity needed for LISA?
As LPF approached its December 2015 launch from the ESA Kourou site towards its science orbit around the L1 Lagrange point the PI team in Trento is focused on the final experimental design and analysis techniques needed to answer these questions. In addition to the main measurement of the acceleration noise between the two TM, we are a wide array of dedicated tests to quantitatively estimate different effects that can limit a precision differential acceleration measurement, including:
elastic coupling of the test masses to the noisy spacecraft motion
forces from stray electrostatic fields and noisy electrostatic charge on the test mass
differential pressure from molecular and thermal photon impacts in the presence of a temperature difference across the test mass
force noise from the application of quasi-static electrostatic actuation forces needed to balance spacecraft gravitational imbalances
coupling of spacecraft and interplanetary magnetic fields to the motion of the nearly “non-magnetic” Au-Pt test masses
Together with our colleagues around Europe and in the US, we are preparing the measurement procedures and “real time” data analysis tools needed to run LPF as an orbiting laboratory and maximize the science return for LISA and precision gravitational experimentation in the future.
Designing and prototyping Gravitational Reference Sensors for LISA and LISA Pathfinder
In LISA and LISA Pathfinder, the test masses are surrounded by electrostatic sensing and actuation devices, known collectively as the Gravitational Reference Sensor or GRS. The GRS has three main responsibilities in the science phase of the mission:
Provide nm-level sensing in all degrees of freedom to allow the necessary spacecraft control
Provide nN-level electrostatic actuation forces on the test mass on different degrees of freedom
Provide the electromagnetic and thermal shield that defines an environment that limits all stray forces to the femto-Newton level
The GRS is a contribution from the Italian Space Agency. At the University of Trento we developed the original sensor design, and we built and tested several prototypes, before collaborating with the CGS (Milano) aerospace company in their successful realization of the complete flight GRS hardware.
The GRS design includes:
2 kg test masses (TM) made from a gold-platinum alloy
a surrounding gold-coated electrostatic shield / electrode housing with a high thermal conductivity Mo-sapphire construction
integrated gold-coated electrodes used for contact free, 100 kHz capacitive sensing the TM position in 6 degrees of freedom and for audio frequency-carriers for applying near-DC electrostatic forces
a vent-to-space vacuum system to allow a final pressure below 10-11 Bar to limit forces from molecular impacts
a 2000 N caging system to protect the TM and GRS during launch vibrations and then release the TM into free-fall once in orbit
Additionally, we have consulted with colleagues in England and Switzerland as they designed, respectively the UV photoelectric discharge and sensor electronics sub-systems.
The LPF GRS was designed for the demanding needs of the LISA observatory, and LPF thus represents a flight verification of a crucial part of the LISA apparatus.
Probing the limits of free-fall in the lab with femtoNewton torsion pendulums
A ground test of free-fall cannot reach the full LISA/LPF sensitivity at the relevant time scales of 1000 or more seconds. However, a lightweight (hollow) LISA-like TM suspended as a torsion pendulum element inside an LISA-like GRS electrode housing provides a test bench for surface forces, which are perhaps the most threatening to the LISA sensitivity and hardest to analyse from first principles.
In Trento we pioneered efforts to measure the fN surface forces relevant to free-fall inside realistic GRS hardware prototypes and have reached sensitivities that allow placing an upper limit of roughly 2 fm/s2 for the random rms amplitude of TM acceleration from surface forces at 1 mHz. This is within a factor 2 of the LPF goal, and was critical in demonstrating the readiness for a space mission. It also represents the current state of the art in small force measurement for the class of 100 g test-bodies.
Our lab measurements have also measured a number of key force noise sources for LISA test masses, including:
Detection of the Brownian motion (and proximity-effect damping) from residual gas impacts in vacuum conditions
100 pN/K-level thermal gradient coupling forces
Stray potential fluctuations between different points on a single gold surface, with limits below 10 microV at mHz frequencies
Electrostatic force gradients to well below the μN/m limit allowed for LISA
Electrostatic dissipation with loss-angles below 10-6
The bipolar discharging properties of the suspended TM inside the GRS with the UV photoelectric discharge system.
The torsion pendulum lab represent a critical ground companion to complete the physical model of small force disturbances that will be a legacy of the LISA Pathfinder mission.