PhD lecture Ferran Gibert: “Thermal Diagnostics Experiments for LISA Pathfinder” [NOT TRANSLATED]
Thesis director is Dr. Miquel Nofrarias.
Summary:
The LISA Pathfinder project is an ESA/NASA technology demonstrator mission that must test the critical instrumentation required for a future space-borne gravitational wave observatory based on the LISA design. The satellite, to be launched by the end of 2015, carries two free-floating test masses and an interferometer that measures the relative distance between them.
The main objective of the satellite is to demonstrate that the residual acceleration noise between the masses is lower than 3e-14 m/s^2/sqrt(Hz) in the band between 1-30 mHz. To achieve such a high sensitivity, the instrument is provided with an accurate control system that allows to sense and actuate on any of the 18 degrees of freedom of the system composed of the two test masses and the spacecraft, avoiding interfering the scientific measurements. The whole instrument is called the LISA Technology Package (LTP).
At such low frequencies, the system is exposed to a broad list of external perturbations that eventually limit the sensitivity of the instrument. Amongst them, temperature fluctuations at different spots of the satellite can end up distorting the motion of the masses and the interferometer readouts through different mechanisms. In order to measure such fluctuations and to characterise their contribution to the system sensitivity, the satellite is equipped with a thermal diagnostic subsystem composed of a series of heaters and high precision temperature sensors.
Three different kind of thermal perturbation mechanisms are to be studied with such a subsystem:
- thermal effects inducing direct forces and torques to the test masses due to the presence of temperature gradients,
- thermo-elastic distortion due to temperature fluctuations in the structure hosting the test masses and the interferometer and
- thermo-optical distortion of two optical parts located outside the ultra-stable optical bench.
This thesis focuses on the design of the experiments aimed to study the first two mechanisms. These experiments essentially consist in the injection of a series of heat loads near each of the thermal-sensitive locations in order to stress their associated thermal mechanism.
Such an induced perturbation is visible with high SNR at both the optical measurements and the nearby temperature sensors, and allows to derive coupling coefficients for each of the effects or, at least, bound their contribution to the acceleration noise.
The analysis of the impact of forces and torques on the test masses has followed two approaches: first, a simulator environment has been designed and implemented to estimate the impact of any kind of heat signal applied close to the test masses and, secondly, a test campaign has been carried out by means of a LTP-test mass replica installed in a torsion pendulum facility.
Regarding the simulator, a state-space model has been developed including a thermal system of the whole spacecraft and a specific design for each of the mechanisms that generate forces and torques from temperature gradients: the radiometer effect, the radiation pressure effect and asymmetric outgassing.
This model has been integrated to a general simulator of the whole LTP performance, what has allowed to simulate the whole chain between the heater activation and the final impact to the closed-loop performance of the LTP. In parallel, the experimental campaign by means of a torsion pendulum facility of the University of Trento has allowed to characterise the impact of each of the effects in different scenarios of absolute temperature and pressure.
On the other hand, the analysis of thermo-elastic noise in the LTP is based on the results obtained during a spacecraft Thermal Vacuum test campaign. In this test, a series of heater activations in the suspension struts that attach the LTP core assembly to the satellite structure allowed to bound the impact of temperature fluctuations at these locations and to characterise the main mechanical distortion mode associated to them. [NOT TRANSLATED]