Test mass design for the EP experiment on M3 STEP Original Research Article

The design strategy for pairs of test-masses for the Satellite Test of the Equivalence Principle is discussed. Limitations are placed on the possible density ratios for the materials used in each test-mass pair, leading to the conclusion that a fully cyclical test for systematic errors is not possible. The immunity of test-masses to local gravitational influences is considered, and a figure of merit, the ‘differential acceleration susceptibility’ χ(R,θ,ϕ)=Δaz/a, is introduced for any test-pair, such that different designs may usefully be intercompared. The electrostatic patch-effect is described in the context of its potential influence on the STEP test-masses, and the need to prevent azimuthal rotation of the masses is underlined. Two schemes for frictionlessly preventing such azimuthal rotation are discussed: longitudinal grooves, and chordal ‘flats’ on the masses. It is found that the minimum number of grooves (or ‘flats’) that may be used on the STEP test-masses is 6.