Cosmic-ray and solar particles above 100 MeV penetrate the LISA experiment test masses. Consequently, electric charges accumulating there generate spurious Coulomb forces between the masses and the surrounding electrodes. This process increments the noise level of the experiment. We have estimated the amount of charge deposited per second on the LISA test masses by primary cosmic-ray protons at solar minimum and solar maximum and by solar energetic particle (SEP) events. This simulation has been carried out with the Fluka Monte Carlo program. A simplified geometry for the experiment has been considered. We have found an effective charge rate of 110 e s−1 for primary protons at solar maximum and 150 e s−1 at solar minimum between 0.1 and 1000 GeV. The amount of charge released by a medium intensity gradual event (7 May 1978) varies from 206 e s−1 in the first few minutes to 4575 e s−1 at the peak of the event. At the occurrence of medium or strong solar events, the LISA sensitivity curve at frequencies lower than 3 × 10−4 Hz is dominated by the noise due to the test-mass charging process.

Solar energetic particles and galactic cosmic rays with energies larger than 100 MeV cause progressive charging of the LISA experiment test masses. Consequently, Coulomb forces occur between the test masses and the surrounding conducting surfaces generating spurious signals that might be mistaken for gravitational wave signals. We have parametrized the energy spectra of galactic cosmic-ray nuclei and electrons near the LISA orbit in order to evaluate their role in the test-mass charging relative to the most abundant proton component. This work has been carried out using the FLUKA Monte Carlo program.

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LISA (Laser Interferometer Space Antenna) will be the first space-born antenna for Gravitational Waves in the range 10 −4 − 10 −1 Hz. It is a joint ESA-NASA effort, and it consists of a constellation of three satellites in heliocentric orbit, forming two semi-independent Michelson interferometers. Each spacecraft contains two test masses, nominally in pure geodesic motion, without any mechanical contact with the spacecraft. Distances between test masses on board each couple of spacecrafts (5 million Km apart) are monitored using on board lasers.On-ground tests are required to understand the couplings between the test-mass and environment, and between the various degrees of freedom of the test-mass itself. A two soft DOF system, a double roto-translational pendulum, is being studied as a test bench. Here we present an analysis of the system behavior, and the study of an active control system for the test-mass.Index Terms-LISA, LISA Pathfinder, ground based tests, rototranslational pendulum, torsion pendulum.