Noisy surface charges on gravitational wave detector optics

Paul Campsie

Paul Campsie completed his Ph.D. in the Institute for Gravitational Research at the University of Glasgow. He now works as a Product & Test Engineer for Freescale Semiconductor.

A direct measurement of the fluctuating force noise created by surface charge on dielectrics

It has been known that future interferometric gravitational wave detectors could have their low frequency sensitivity limited by excess surface charges on the detector optics. Though it is suspected that the limiting effects of this noise source have been observed in initial detectors, this was never directly verified because there was no measurement of the charge on the optic.

In our recent CQG article we present a direct measurement of the fluctuating force noise created by excess surface charges (charging noise) on a dielectric. This measurement is extremely important for the development of gravitational wave detectors since accurate charging noise estimates can now be made and appropriate precautions taken during installation and observations.

A torsion balance was used to measure the weak fluctuating forces generated by surface charges on samples of silica, the material that will be used for the advanced LIGO optics. Torsion balances have historically been used in weak force physics experiments including measurements of the Newtonian constant of gravitation and equivalence principle tests; however, they are still used in modern physics experiments due to the fact they can reach such high levels of sensitivity. The torsion balance used for our work was sensitive to forces of a femtonewton at 1mHz, under optimal conditions.

A Kelvin probe was used to monitor the decaying charge level on the dielectric sample. The Kelvin probe is a capacitive device that is primarily used to measure either the work function of materials or surface charge. From the exponentially decaying Kelvin probe signal it was possible to calculate the correlation time of the charge which could then be used to estimate the theoretical charging noise level. The average Coulomb force is also required for this calculation and can be estimated from the torsion balance measurement.

Our results show that the charging noise can be modelled accurately using the Weiss model of charging noise. In the near future our torsion balance apparatus will be upgraded and the sensitivity of the instrument should increase. This will allow more accurate measurements to be made and future work with the new setup will include measurements on optics with advanced dielectric coatings planned for use in future generation gravitational wave detectors.

Read the full article in Classical and Quantum Gravity:
A measurement of noise created by fluctuating electrostatic charges on dielectric surfaces using a torsion balance
P Campsie, J Hough, S Rowan and G D Hammond
2014 Class. Quantum Grav. 31 175007

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