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Progress Report on Charging Measurements at Trinity University

This report discusses the development and implementation of a vacuum-compatible capacitive probe to measure charge magnitude and relaxation time constant on LIGO optics at Trinity University. The report also outlines the setup, measurement results, and potential next steps.

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Progress Report on Charging Measurements at Trinity University

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  1. Progress Report on Charging Measurements at Trinity University Dennis Ugolini, Robert McKinney, Mark Girard Trinity University March LSC Meeting March 21, 2007 Goal: To develop a vacuum-compatible capacitive probe to measure the charge magnitude and relaxation time constant on LIGO optics D. Ugolini, LSC March 2007

  2. The Capacitive Probe • Use chopper to alternately expose/occlude sample • Aluminum probe body with Kapton-shielded wire • Sensitivity calibrated with surface DC voltmeter to be (3.5 +/- 0.5) × 105e-/cm2(to be published in Rev. Sci. Instr.) D. Ugolini, LSC March 2007

  3. Optical Measurement Setup • Chopper shielded by aluminum box • Optic held by viton O-rings epoxied to aluminum clamps D. Ugolini, LSC March 2007

  4. Measurement Setup (cont.) Viton O-rings D. Ugolini, LSC March 2007

  5. Vacuum System Pressure gauge goes here (returned for servicing) Feedthrough (on either side) Glass viewport Pfeiffer-Balzers Turbomechanical drag pump (~10-5 torr) D. Ugolini, LSC March 2007

  6. Obstacles in Moving to Vacuum • Pickup by probe signal lead from chopper driver wires • Used two feedthroughs, one for chopper, one for probe • Chopper wires tied to optical table, kept far from probe lead • Intermittent probe noise peak • Caused by contact between probe lead and nearly anything (chopper shielding box, optical table, walls of the vacuum chamber) • Vibrational or electrical? • Kapton wire is rigid, hold position well • Applied charge decays too rapidly to pump down system • Strong function of humidity, dehumidifier purchased • Spark? • Options for applying charge inside chamber (brush?) D. Ugolini, LSC March 2007

  7. Readout Problem • Finally trapped large charge on optic on February 20th • Probe readout oscillated, didn’t damp out over many hours • Drift in chopper frequency shifted peak on spectrum analyzer • Switched to lock-in amp, used chopper driver as reference D. Ugolini, LSC March 2007

  8. 16-Day Results Corresponds to time constant of ~170 days Appears to level off around day 13 Large phase change on “dropouts” – problem with reference? D. Ugolini, LSC March 2007

  9. UV Discharging Exposed to Hg discharge tube (no UV) for several hours – no effect Exposed to Pen-Ray UV Lamp (253.7nm, 4.4mW/cm2 at 19mm), exhibits discharging shown Glass viewport has ~15% transmission at this wavelength D. Ugolini, LSC March 2007

  10. Summary • Probe has been successfully moved to vacuum • Decay of charge deposited on optical sample has been observed with a time constant of ~170 days • What are the differences between this setup and Moscow State? Optical substrate? Surface contact? Pressure? • Demonstrated UV discharging with glass viewport • Next steps Longer time Triplicate setup Charge vs. position Different substrates, coatings, etc. Besocke Kelvin probe Quartz viewport? Discharging vs. λ? D. Ugolini, LSC March 2007

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