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.
The conclusions drawn from this text are as follows:
- The electrostatic force actuation system is a key component of geodesic reference test masses (TM) for the LISA orbiting gravitational wave observatory.
- In the LISA mission, TM force actuation is used to align the TM to the spacecraft-mounted optical metrology system, without forcing along the critical gravitational wave-sensitive interferometry axes.
- In the LISA Pathfinder mission, actuation was used to stabilize the TM along the critical $x$ axis and directly affected the mission’s main differential acceleration science observable.
- LISA Pathfinder demonstrated the compatibility of the electrostatic actuation system with LISA observatory requirements.
- Dedicated measurement campaigns were conducted during LISA Pathfinder to amplify, isolate, and quantify force noise contributions from the actuation system.
- Actuation force noise was found to be a relevant factor, but not the dominant noise source in LISA Pathfinder.
- Performance projections for the conditions expected in the LISA mission were made based on the findings during LISA Pathfinder.
Based on these conclusions, a future roadmap for readers can be outlined:
Future Roadmap
Challenges
- Reducing stray force noise in the LISA orbiting gravitational wave observatory, especially at low frequencies below 1 mHz.
- Improving actuator gain noise and minimizing low-frequency voltage fluctuations.
Opportunities
- Further optimizing the electrostatic actuation system for geodesic reference test masses in LISA.
- Exploring alternative actuation methods to potentially reduce force noise.
- Continuing dedicated measurement campaigns to better understand and quantify force noise contributions.
- Further aligning the TM to the spacecraft-mounted optical metrology system and improving stabilization along critical axes.
By addressing these challenges and exploring these opportunities, the performance of the electrostatic force actuation system can be optimized for the LISA orbiting gravitational wave observatory, ultimately improving its sensitivity and ability to detect gravitational waves.