D. Ugolini, LSC March 2007 1LIGO-G070079-00-Z

Progress Report on Charging Measurements at Trinity University

Dennis Ugolini, Robert McKinney, Mark GirardTrinity University

March LSC MeetingMarch 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 2LIGO-G070079-00-Z

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) × 105 e-/cm2 (to be published in Rev. Sci. Instr.)

D. Ugolini, LSC March 2007 3LIGO-G070079-00-Z

Optical Measurement Setup

Chopper shielded by aluminum box

Optic held by viton O-rings epoxied to aluminum clamps

D. Ugolini, LSC March 2007 4LIGO-G070079-00-Z

Measurement Setup (cont.)

Viton O-rings

D. Ugolini, LSC March 2007 5LIGO-G070079-00-Z

Vacuum System

Pfeiffer-Balzers Turbomechanical

drag pump (~10-5 torr)

Pressure gauge goes here (returned for servicing)

Feedthrough (on either side)

Glass viewport

D. Ugolini, LSC March 2007 6LIGO-G070079-00-Z

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 7LIGO-G070079-00-Z

Readout Problem

Charge on LIGO Optical Sample 02/20/07

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0 500 1000 1500 2000

Time (minutes)

Pro

be

sig

nal

(m

icro

volt

s)

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 8LIGO-G070079-00-Z

16-Day Results

Charge on Optical Sample 2/21/07

y = 2.156E+02e-5.916E-03x

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Time (days)

Pro

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sig

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volt

s)

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 9LIGO-G070079-00-Z

UV Discharging

Optic exposure to UV light

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0 5 10 15 20 25 30 35 40

Time (minutes)

Pro

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sig

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volt

s)

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 10LIGO-G070079-00-Z

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 stepsLonger time Triplicate setupCharge vs. position Different substrates, coatings, etc.Besocke Kelvin probe Quartz viewport?Discharging vs. λ?