Integration and testing activities dedicated to LISA space instrument

LISA is an ESA project dedicated to space gravitational waves observation in the very low frequency range from 10 ^{-1} to 10 ^{-4} Hz, opening a window on massive black hole binaries which are not observable from ground based interferometers. The LISA space observatory consists of three satellites planned to be launched in 2034 by Ariane 6.4 launcher. The three satellites will be positioned on their final orbits in a triangular shape constellation with an arm of 2.5 million of km, orbiting around the Sun at 1 AU and following the Earth at a distance of 50 to 65 million of km.The gravitational waves detection is based on laser interferometric measurement of very tiny distance variations (i.e. several tens of picometers) between pairs of free falling test masses (TM), core elements of the two instrument terminal units mounted in each satellite at the end of each triangle arm.One instrument terminal unit consists of a telescope, an optical bench, a gravitational reference sensor (including free falling TM) and a support structure, this entity being called MOSA (Movable Optical Sub-Assembly). An assembly of two MOSA's together with various electronics sub-systems (e.g. Phasemeter, Laser, Payload Commanding and Processing) forms the instrumental payload or the LISA Core Assembly (LCA) of every LISA satellite. Each MOSA emits a laser beam towards its associated MOSA receiver based on the opposite satellite and, reciprocally, it receives the beam emitted by the distant MOSA. These dual links form one of the three interferometric arms of the triangular LISA satellites constellation.The interferometric measurement accuracy, expressed in optical path length has to present a single pathlength stability of 10 pm/sqrt{?} Hz above 3 mHz, also implying a beam pointing stability of 10 nanoradians/sqrt{?} Hz in the same frequency range. These performances have to be verified (by measurements or modeling) at MOSA level and therefore require a very sophisticated testing and verification process. France will contribute to this project with the development of the data processing center and the achievement of integration and testing (AIV/T activities) for the six MOSA flight models. The presentation describes the integration and testing steps, the verification tests principle with associated specifications, and the ground support equipment's that are required for the MOSA AIV/T activities. The MOSA assembly activities will be performed in a clean controlled environment and they will consist of μm precision alignment of the telescope relatively to the optical bench by mechanical adjustments on the MOSA support ring structure, followed by the assembly of the gravitational sensor (GRS). At the end of the assembly process, the size of a MOSA will be of ∼ 1 meter length and 0.6 meter diameter, with a mass of ∼ 75 Kg. Once the assembly of various sub-assemblies and their mechanical alignment step was achieved, the MOSA optical testing will be carried out inside a vacuum chamber, in order to comply with high precision requirement for optical path length (OPL) measurement. A vacuum pressure below 10-5 mb is required to provide a very stable and homogeneous thermal environment and consequently a limited thermal expansion. Moreover, stable low pressure below 10-5 mb avoids also refraction disturbances on the laser beam path. A pressure of 10-5 mb induces an optical path elongation of several picometers. The aimed accuracy is about 1 picometer stability over 30 seconds' time span in order to verify the intrinsic optical length path stability inside MOSA. Slow and slight thermal drifts will be performed to quantify the thermal effects affecting optical path length and wavefront quality for each MOSA. A major test equipment, called far-field optical simulator, is required to simulate the distant satellite laser beam. Its mechanical stability must comply with OPL accuracy requirement and the wave front of the emitted beam has to verify a flatness of Lambda/30. The simulator must also respect pointing stability and strict polarization requirements. The verification of the beam pointing angle requires a measurement stability of one nano-rad over a 30 seconds time span.This far field simulator is fixed on a cradle also bearing the MOSA during testing under vacuum. The material of the cradle must have a very low coefficient of thermal expansion (CTE) and it can be the same material as the telescope one (e.g. Zerodur ceramics). Moreover, three interferometers will measure the relative far-field simulator displacements in order to compensate them for the OPL measurement intrinsic to MOSA. The full MOSA performance verification is based on interferometric measurement involving the main MOSA elements, but also two additional major LISA sub-systems: the Phasemeter and the Laser source. Moreover, the full test of gravitational reference sensor (GRS), of which principle is based on a free falling test mass, cannot be implemented on ground, under gravity. Therefore, an optical sensor simulating the flight movements of the TM inside of its enclosure will be used as GRS substitute.Cleanliness is of paramount importance since a single 10 μm size dust is able to generate interference pattern by scattering laser light that could impair the detection sensitivity or jam gravitational events detection. Drastic precautionary measures will be defined and taken to avoid particle contamination during assembly and testing.