1. 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-G v1
LIGO-G v1

2. 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-G Quadruple Suspension Design for Advanced LIGO
Notes format good
LIGO-G v1
LIGO Laboratory
LIGO-G v1

3. Electrostatic Charging
TITLE
Talk Notes
Electrostatic Charging
Refer to
LIGO-G 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 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: G
LIGO-G v1
LIGO Laboratory
LIGO-G v1
Notes;
[ref]:
G Reitze’s NSF talk
G – Rai’s March LVC talk
alog Graph from alog referenced in (leo/kissel)
#’s:
Both polarities, ~10-9 C per TM
Surface densities ~10-12 C/cm2 per TM

4. 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-G
NEEDS MASSIVE WORK
NEEDS MASSIVE WORK
LIGO-G v1
LIGO Laboratory
LIGO-G v1

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

6. Mitigate
Known Sources
First contact photo
Ion Pumps
First Contact

7. Remove Existing Charge TITLE Talk Notes In vacuum In-air
LIGO-G v1
LIGO-G v1

8. 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-T Charge Experiments at LHO – March 2015 (Day 3, First Contact Tests #1 & 3)
LIGO-G “You can look but you can’t touch”
LIGO-E First Contact Spray Application & Removal Procedure
LIGO-G v1
LIGO Laboratory
LIGO-G v1

9. 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-T Charge Experiments at LHO – March 2015 (Day 3, First Contact Tests #1 & 3)
LIGO-G “You can look but you can’t touch”
LIGO-E First Contact Spray Application & Removal Procedure
LIGO-G v1
LIGO Laboratory
LIGO-G v1

10. 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-G v1
LIGO Laboratory
LIGO-G v1

11. Not as effective if not used per procedure
LIGO-G v1
LIGO Laboratory

12. 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:
G Charging, Paramet instabilities, and Squeeze film damping (2015 NSF review)
E v5 First Contact Spray Application & Removal Procedure
LIGO-G v1
LIGO Laboratory
LIGO-G v1

13. 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 C/cm2 on mirrors in BSC chambers [ref]
Status update from March LVC Meeting (LIGO-G )
Tested with fair success in-situ on L1 & H1
LHO:
NEEDS MASSIVE WORK
LIGO-G v1
LIGO Laboratory

14. 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 T
V3 = far side chamber port
V5 = proposed chamber port
LIGO-G v1
LIGO Laboratory
LIGO-G v1

15. 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 T
V3 = far side chamber port
V5 = proposed chamber port
LIGO-G v1
LIGO Laboratory
LIGO-G v1

16. TITLE
Talk Notes
LIGO-G v1
LIGO Laboratory
LIGO-G v1

17. TITLE
Talk Notes
LIGO-G v1
LIGO Laboratory
LIGO-G v1

18. TITLE
Talk Notes
LIGO-G v1
LIGO Laboratory
LIGO-G v1

19. TITLE
Talk Notes
LIGO-G v1
LIGO Laboratory
LIGO-G v1