Thermal Suspension Noise in Initial LIGO
Gregory Harry
LIGO / MIT
March 23, 2005 Detector Characterization LSC Meeting LLO
LIGO-G050113-00-R
Outline
Impact of thermal noise on sensitivity and commissioning Measurements of suspension thermal noise
Frequency domain Time domain Discrepancies
Questions and ideas
Feedback contamination Modeling
Impact of Thermal Noise
Suspension thermal noise
Suspension T hermal Noise
Structural damping Lower loss Thermoelastic can be relevant
= 6 10
-3
LHO 4K Noise
-3 -4
= 2 10
Mirror thermal noise
Coating (SiO2/Ta2O5) thermal noise dominant Silica substrate thermal noise not really a factor About factor of 5 below SRD Optic al (Shot)
= 3 10
SRD Noise
Three presented scenarios for suspension thermal noise
Pessimistic (worst measured) Nominal (average measured) Optimistic (material limit)
Coating T hermal
Sensitivity to Sources
Single Interferometer Sensitivity Neutron Star Inspirals SRD = 6 10-3 = 2 10-3 = 3 10-4 Thermoelastic Limit 16 Mpc 16 Mpc 20 Mpc 26 Mpc 29 Mpc 10 MO Black Hole Inspirals 63 Mpc 60 Mpc 84 Mpc 120 Mpc 140 Mpc Stochastic Background Crab Pulsar ( limit) 1.6 10-5 2.3 10-5 1.4 10-5 7.5 10-6 5.7 10-6 Sco X-1 Pulsar ( limit) 3.1 10-7 3.0 10-7 3.0 10-7 3.0 10-7 3.0 10-7
2.3 10-6 4.7 10-6 1.9 10-6 5.9 10-7 2.7 10-7
Suspension Thermal Noise
Sx(f) = 4 kB T g/(m L (2 f)5)
Dissipation Dilution
Restoring force in pendulum is due to both elastic bending and gravity Effective loss angle for thermal noise `diluted' by the ratio
= ke/kg (ke/kg)violin = 2/L (E I/T) (1+1/(2 L) (E I/T) n2 2) 2/L (E I/T) = 3.5 10-3
Correction for first three violin mode harmonics is negligible
5
Q Measurements
Frequency Domain
Collect data for ~ 2 h Associate peaks with mirrors Fit Lorentzians to peaks Limitations Optical gain drift ?
Get similar results with S2 data as current data with improved wavefront sensors
Temperature drift can cause central frequency to migrate
Minimal over a few hours
Graphic from R. Adhikari's Thesis
6
Q Measurements
Time Domain
Excited modes with onresonance drive to coil Let freely ring down Put notch filters in LSC loop Fit data to decaying exponential times sine wave Limitations Must ring up to much higher amplitude than thermal excitation
No consistent difference between Michelson and Full IFO locks
LLO ITMy Violin Mode
1 0.5 Amplitude 0 -0.5 -1 -1.5 0 20 40 60 Time (s) 80 100 120
10
2
Amplitude
10
0
10
-2
10
-4
0
20
40
60 Time (s)
80
100
120
Feedback can effect measured Q
7
Violin Mode Results
Overview
Ringdown Q's and frequency domain fits do not agree Ringdown Q's repeatable within a lock stretch but frequency domain fits are not Results different in different lock stretches High harmonics show a little more pattern
Still unexplained discrepancies
Highest Q's consistent with material loss in wires
Gillespie laboratory results
Similar (lack of) patterns in all three IFOs
Data from all 3, but more data on H2 than others
Violin Mode Results
Livingston
Comparison of Time Domain and Frequency Domain
x 10
5
Time Domain 2 Frequency Domain
1.5
Q
1 0.5 0
ITMy l
ITMy h l ETMy ETMy h ETMx l ETMx h ITMx l
ITMx h
Violin Mode Results
Hanford 2K
Comparison of Frequency Domain Q's in Same Lock
UTC 10:30 Jan 31, 2005
2 x 10
6
Comparison of Time Domain Q's in Same Lock
2 x 10
6
1.5
ITMx low ITMx high ITMy low ITMy high
1.5
ITMx low ITMx high ITMy low ITMy high
0.5
Q
20 40 60 80
1
1
Q
0.5
0 0
Time (minutes)
0 1
Ringdown Number
2
3
Violin Mode Results
Hanford 2K/Livingston
Comparison of Time Domain Q's in Different Locks
LHO2K IMTx low 8.6 104 1.6 105 1.6 105 1.2 105 LLO ITMx high 1.7 105 1.4 105
Higher Harmonic Results
Hanford 2K
10^7 Time Domain Frequency Domain 10^6
Q
10^5 10^4
x 2l
x 2h
x 3l
x 3h
ITM
y 2l
y 2h
y 3l
y 3h
Violin Mode Results
Hanford
Highest Q's Measured
Frequency Domain H2K ITMx Third Harmonic H2K ITMy Third Harmonic H4K ITMy Third Harmonic Time Domain H2K ITMy Third Harmonic Gillespie Lab Results Q 3.2 106 1.6 106 9.8 105 8.6 10-5 1.7 10-4 2.8 10-4
2.3 105
1.2 10-3 3 10-4
Questions from Violin Q Measurements
Why the disagreement between t and f domain?
Is f domain unreliable? Why? Changes in instrument over hour time scales? Optical drift? Thermal drift? Interaction between degenerate polarizations of modes?
Why changes in ringdowns between lock stretches?
Changes in suspension during lock acquisition? Feedback influence on Q's? ASC? LSC and optical spring?
Why are the highest Q's in f domain third harmonic?
Higher frequency gets away from unity gain frequency of loop? Why not seen in t domain?
How reliable are these numbers?
Changing thermal noise from lock to lock? Feedback contamination so Q's do not predict thermal noise? What about internal mode Q's?
Modeling
Some Hope for Answers
Is feedback mechanism feasible?
Violin modes coming soon to e2e
What about loss from optical spring?
Thomas Corbitt at MIT has done preliminary modeling Need to have cavity offset from...