Investigation of effects associated with variation of electric charge on a fused silica test mass

Variation of electric charge on dielectric mirrors can produce excess noise in interferometric gravitational wave detectors. Charge located on the end face of a fused silica cylinder suspended as a torsion pendulum was measured using a multistrip capacitive probe. Two types of behaviour of this system were found in long-term measurements of the free decay pendulum amplitude and voltage from the probe. The first behaviour was characterized by the slow decrease of the pendulum amplitude corresponding to Q = (6–8) × 107 and the slow change of the averaged voltage from the probe corresponding to the charging of the test mass with a rate of the order of 104e/cm2 day. The second behaviour was characterized by the presence of jumps in the amplitude and the probe voltage in which these values changed several orders faster than in the first case. We did not find coincidences between these jumps and signals from plastic scintillators installed around the vacuum chamber to detect cosmic ray showers.