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Astrophysics, Instrumentation and Methods for Astrophysics, astro-ph.IM,General Relativity and Quantum Cosmology, gr-qc
Abstract:
Electrostatic force actuation is a key component of the system of geodesic
reference test masses (TM) for the LISA orbiting gravitational wave observatory
and in particular for performance at low frequencies, below 1 mHz, where the
observatory sensitivity is limited by stray force noise. The system needs to
apply forces of order 10$^{-9}$ N while limiting fluctuations in the
measurement band to levels approaching 10$^{-15}$ N/Hz$^{1/2}$. We present here
the LISA actuation system design, based on audio-frequency voltage carrier
signals, and results of its in-flight performance test with the LISA Pathfinder
test mission. In LISA, TM force actuation is used to align the otherwise
free-falling TM to the spacecraft-mounted optical metrology system, without any
forcing along the critical gravitational wave-sensitive interferometry axes. In
LISA Pathfinder, on the other hand, the actuation was used also to stabilize
the TM along the critical $x$ axis joining the two TM, with the commanded
actuation force entering directly into the mission's main differential
acceleration science observable. The mission allowed demonstration of the full
compatibility of the electrostatic actuation system with the LISA observatory
requirements, including dedicated measurement campaigns to amplify, isolate,
and quantify the two main force noise contributions from the actuation system,
from actuator gain noise and from low frequency ``in band'' voltage
fluctuations. These campaigns have shown actuation force noise to be a
relevant, but not dominant, noise source in LISA Pathfinder and have allowed
performance projections for the conditions expected in the LISA mission.
