STEP Team Produces Electrodes for Positioning Test Masses

The Satellite Test of the Equivalence Principle (STEP) team at Stanford University has successfully manufactured a prototype set of electrodes of the Electrostatic Positioning System (EPS) for their differential accelerometers. The EPS is crucial to the operation of the experiment. Several electrodes will be arranged around each test mass, and control voltages can be applied depending on the position and velocity of the test mass. The most important use of the EPS electrodes is to measure electrical charge on the test masses.

The prototype EPS has five electrodes which are formed from bilayer Ti-Cu thin films on the inside of a 54-mm long and 40-mm inner diameter quartz cylinder. The first titanium layer with 100 nm thickness is used as a binding layer to the quartz substrate. The thickness of the overlying copper layer is adjusted to keep the total film thickness at about 1 micrometer. The copper film has good electrical conductivity. Bilayer Ti-Cu electrode films retain good adhesion after temperature cycling between 4 K and 300 K. Scanning electron microscopy surface morphology shows that the Ti-Cu films have smooth surfaces and grain sizes are about 0.15 micrometer.

A photograph of the EPS electrodes is shown below. Five electrodes are coated along the inside of the quartz cylinder. The spaces between the five electrodes are seen to be equal. These EPS electrodes meet the requirements for the STEP experiment. The width and the space of each electrode are critical for best performance of the differential accelerometers. The prototype EPS electrodes will be shipped to ONERA in France for further testing.

Test Masses