Direct force measurements for testing the LISA Pathfinder gravitational reference sensor

We present results of testing of the LISA Pathfinder gravitational reference sensor (GRS) using a 4-test-mass torsion pendulum facility aimed at measuring low-frequency force-noise sources in the LISA and LISA Pathfinder frequency band. This pendulum, for the first time, allows us to make measurements which are sensitive to all forces acting along the sensitive axis of the test mass, not only those that create a torque. We will report on a campaign of testing using the LISA Pathfinder 'engineering model' prototype GRS which has set upper limits on the overall force noise acting on the test mass contributed by surface effects within the sensor at a level of 100 fN Hz-1/2 at 2 mHz and measured specific sources of unwanted disturbances. These sources include forces arising from the electrostatic coupling between the sensor and test-mass motion, electrostatic fields due to surface-potential variations and thermal-gradient effects within the sensor. Finally, we describe the extension of this campaign to the LISA Pathfinder flight-model replica GRS which will be crucial in verifying the design and performance of the flight instrument.