Kelvin probe measurements: investigations of the patch effect with applications to ST-7 and LISA

Abstract

One of the possible noise sources for the space-based gravitational wave detector LISA (the Laser Interferometer Space Antenna), associated with its test masses, is that due to spatial variations in surface potential (or patch effect) across the surfaces of the test mass and its housing. Such variations will lead to force gradients which may result in a significant acceleration noise term. Another noise source is that due to temporal variations in the surface potential, which in conjunction with any ambient dc voltage or net free charge on the test mass may also produce a significant acceleration noise term. The ST-7 demonstrator mission is designed to test technologies for LISA, including the gravitational reference sensor, which contains a gold-coated gold/platinum (Au/Pt) alloy test mass, surrounded by a housing that carries the electrodes for sensing and control. We have used a Kelvin probe at the Goddard Space Flight Center to make spatial and temporal measurements of contact potential differences for a selection of materials (Au/Pt, beryllia, alumina, titanium) and coatings (gold, diamond-like carbon, indium tin oxide, titanium carbide). Our investigations indicate that subject to certain assumptions all of these coatings appear to satisfy the ST-7 requirement that patch effect spatial variations should be less than 100 mV. The data also revealed evidence of behavioural trends with pressure and possible contamination effects. Regarding temporal variations, the current accuracy of the instrument is limiting the measurements at a level above the likely LISA requirements. We discuss our results and draw some conclusions of relevance to LISA.

Kelvin probe force microscopy of metallic surfaces used in Casimir force measurements

“…In this context, it is important to discuss carefully all possible sources of systematic effects, in particular the effect of electrostatic patches already discussed for various high precision measurements [10][11][12][13][14][15][16][17][18][19][20][21][22][23], and more recently in the context of Casimir force measurements [24][25][26][27][28][29][30]. The patch effect is due to the fact that the surface of a The force due to electrostatic patches can be computed by solving the Poisson equation, as soon as the correlations of the patch voltages are known.…”

Interaction between Stray Electrostatic Fields and a Charged Free-Falling Test Mass

“…These arise in different exposed crystalline facets and surface contamination. Typical observed average potential differences between roughly centimeter-size regions of a gold surface are of order 10-100 mV [16][17][18][19].…”

Temperature Dependence of Electric Field Noise above Gold Surfaces

“…These ''patch potential'' variations generate local electric fields, with a static component thought to originate from differences in the work function between crystal facets, further modified by adsorbates. Because of the impact these fields can have on precision measurements, static patch fields have been extensively characterized in studies of neutral atoms [1][2][3], gravitational forces [4,5], electron emission [6] and contact potentials [7][8][9] for a wide variety of materials and morphologies.…”