Update on aLIGO Charging Issues Need a catchy title for the talk, but drawing a blank… Update on Charging Issues, First Contact/Contamination Control, and Other Headaches? TITLE Talk Notes Update on aLIGO Charging Issues Kate Gushwa LIGO Laboratory California Institute of Technology On behalf of LIGO Engineering Team LVC Meeting September 2015 LIGO-G1501076-v1 LIGO-G1501076-v1

Electrostatic Charging TITLE Talk Notes WHAT TO SAY This is where ideas go… Rai gave talk in March… so this is just a quick overview/recap of the problem QUAD damping & control Top 3 stages: coil- magnet actuators Bottom stage: electrostatic actuator Quads use electrostatic drive (ESD) system instead of coil-magnet actuators on lowest stage to reduce noise coupling due to magnets Each pair of electrodes forms a capacitor, whose fringe field attracts the test mirror surface (dielectric). Distance between electrodes and ETM is 5mm. 5 channels to drive electrodes; 1 common BIAS, 4 driving channels (1 per quadrant). Electrostatic Charging Electrostatic drive (ESD) 4 electrode pairs coated onto reaction mass xcfb Main (Test) Chain Reaction Chain Finish / fix: Slide content Animations What to say Did part Denis want get deleted? NOTES QUAD damping & control: Stage 1-3: coil-magnet actuators Stage 4: electrostatic actuator Electrostatic drive (ESD) Pattern of 4 interleaving electrode pairs coated onto the inner surfaces of the reaction chain masses (CP for ITMs, ERM for ETMs). Each pair or electrodes forms a capacitor 1 electrode from each pair/quadrant is connected to a common bias potential, and control signals are applied to the remaining electrode. Applying a potential difference creates a fringing field that attracts the dielectric test mass. Capacitance of the pattern depends on the distance between the test mass and reaction mass. So changing the voltage on the electrodes changes the attractive force between the masses. Effects of charging: Interferers with optical position control Attracts dust Reducing reflectance Increasing scattering & absorption Accumulation & motion of charge is a potentially limiting noise source References & Related Documents Diagram of reaction & main chain: LIGO-G1200077 Quadruple Suspension Design for Advanced LIGO Notes format good LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

Electrostatic Charging TITLE Talk Notes Electrostatic Charging Refer to LIGO-G1500264 Update on Test Mass Charging Rai’s slide 4: Best strategy (for the moment) is to eliminate charge MIT LASTI tests Showed charge on Test Mass was small & stable <10-10 C/cm2 or < ~10V of equivalent bias voltage Not expected to be a major noise issue Observations at LLO & LHO Charge much higher than expected Typical:100V bias equivalent, sometimes 200-300 V Surface densities: ~10-12 C/cm2 per TM Different story from each optic High enough to significantly affect the electrostatic actuation Why? Many potential sources… NOTES References: G1500722 LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1 Notes; [ref]: G1500722 - Reitze’s NSF talk G1500264 – Rai’s March LVC talk alog 19547 - Graph from alog referenced in email (leo/kissel) #’s: Both polarities, ~10-9 C per TM Surface densities ~10-12 C/cm2 per TM

How Much Charge? Refer to Rai’s March 2015 LVC talk LIGO-G1500264 TITLE Talk Notes How Much Charge? Refer to Rai’s March 2015 LVC talk LIGO-G1500264 NEEDS MASSIVE WORK NEEDS MASSIVE WORK LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

What are we doing about it? TITLE Talk Notes LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

Mitigate Known Sources First contact photo Ion Pumps First Contact

Remove Existing Charge TITLE Talk Notes In vacuum In-air LIGO-G1501076-v1 LIGO-G1501076-v1

Mitigation Investigate Charging Due to First Contact TITLE Talk Notes Mitigation Investigate Charging Due to First Contact Charging experiments at LHO March 2015 Setup: NOTES Part of the charging experiments performed at LHO in March 2015 (by T. Sadecki, B. Weaver, C. Torrie, K. Gushwa). See references below. Experimental setup: ETM06 (pilot optic) on V-block inside a grounded metal box. ~2 year old First Contact sheet on the AR side. Electrometer 1” from the center of the HR side. All other items, including the table, are also grounded. Procedure: Measured charge just before, during, and after removing the old First Contact. Stuck the dry FC sheet back on the AR side, and repeated. Variables: Top Gun Test #1 – 1 test with and 1 test without Top Gun (per procedure) Test #3 – TG during peel, after peel, after peeling and pausing, etc. References: LIGO-T1500170 Charge Experiments at LHO – March 2015 (Day 3, First Contact Tests #1 & 3) LIGO-G1500539 “You can look but you can’t touch” LIGO-E1300017 First Contact Spray Application & Removal Procedure LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

Mitigation Investigate Charging Due to First Contact TITLE Talk Notes Mitigation Investigate Charging Due to First Contact Procedure: Measured charge just before, during, and after peeling First Contact Stuck sheet back on AR side & repeated Variable: Top Gun NOTES Part of the charging experiments performed at LHO in March 2015 (by T. Sadecki, B. Weaver, C. Torrie, K. Gushwa). See references below. Experimental setup: ETM06 (pilot optic) on V-block inside a grounded metal box. ~2 year old First Contact sheet on the AR side. Electrometer 1” from the center of the HR side. All other items, including the table, are also grounded. Procedure: Measured charge just before, during, and after removing the old First Contact. Stuck the dry FC sheet back on the AR side, and repeated. Variables: Top Gun Test #1 – 1 test with and 1 test without Top Gun (per procedure) Test #3 – TG during peel, after peel, after peeling and pausing, etc. References: LIGO-T1500170 Charge Experiments at LHO – March 2015 (Day 3, First Contact Tests #1 & 3) LIGO-G1500539 “You can look but you can’t touch” LIGO-E1300017 First Contact Spray Application & Removal Procedure LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

Residual charge is not always negative TITLE Talk Notes Top Gun start Top Gun stop Peel start First Contact removed NOTES Residual charge is not always negative LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

Not as effective if not used per procedure LIGO-G1501076-v1 LIGO Laboratory

Mitigation Ion pumps First Contact Ion pumps TITLE Talk Notes Mitigation Ion pumps First Contact Investigated charging during removal procedure Residual charge is not always negative Removal procedure is effective when followed Performed other experiments associated with static residue First Contact is not the devil Many potential sources of charge The Top Gun is our friend Significantly improved FC removal procedure Charge levels of several volts or less achieved at chamber closing Low charge levels under vacuum subsequently verified Ion pumps Being moved 250 m down beam tube Investigating better UV baffling Adding non-evaporable getter (NEG) to chambers NOTES Reference: G1500722 Charging, Paramet instabilities, and Squeeze film damping (2015 NSF review) E1300017-v5 First Contact Spray Application & Removal Procedure LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

Removing Charge: In-Vacuum Test Mass Discharge System (TMDS) Ions injected at low pressure without opening the chamber MIT LASTI tests Can neutralize surface charge densities as large as 10-10 C/cm2 on mirrors in BSC chambers [ref] Status update from March LVC Meeting (LIGO-G1500264) Tested with fair success in-situ on L1 & H1 LHO: NEEDS MASSIVE WORK LIGO-G1501076-v1 LIGO Laboratory

TMDS TITLE Caveat: not able to neutralize charge in gap To do/future: Talk Notes TMDS Caveat: not able to neutralize charge in gap All performed on far side of chamber pointing centrally To do/future: Use TMDS via an injection point closer to the QUAD (and pointed at gap) via custom flange NOTES Drawings from T1500056 V3 = far side chamber port V5 = proposed chamber port LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

TMDS TITLE Caveat: not able to neutralize charge in gap To do/future: Talk Notes TMDS Caveat: not able to neutralize charge in gap All performed on far side of chamber pointing centrally To do/future: Use TMDS via an injection point closer to the QUAD (and pointed at gap) via custom flange NOTES Drawings from T1500056 V3 = far side chamber port V5 = proposed chamber port LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

TITLE Talk Notes LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

TITLE Talk Notes LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

TITLE Talk Notes LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1

TITLE Talk Notes LIGO-G1501076-v1 LIGO Laboratory LIGO-G1501076-v1