Transcript Document

The status of VIRGO
Edwige Tournefier (LAPP-Annecy )
for the VIRGO Collaboration
HEP2005, 21st- 27th July 2005
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The VIRGO experiment and detection of gravitational waves
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The commissioning of VIRGO
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Conclusions
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VIRGO
French-italian collaboration (CNRS – INFN)
Annecy (LAPP), Firenze, Frascati, Lyon (LMA), Napoli, Nice (OCA), Paris
(ESPCI), Perugia, Pisa, Roma, Orsay (LAL)
Virgo site : Cascina close to Pisa
Virgo goal: detection of gravitational waves
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How to detect gravitational waves?
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Effect of a gravitational wave on free masses:
 A Michelson interferometer is suitable:
- suspend mirror with pendulum => ‘free falling masses’
- Gravitational wave => phase shift
Suspended
mirror
Measure: h = L/L
L  
Suspended
mirror
hL
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L  
L = length difference between the 2 arms
L = arm length
Beam splitter
Light
Detection
LASER ()
 
4

L
3
hL
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The shot noise and the VIRGO optical design
Limitation of a Michelson interferometer due to photon shot noise:
the minimum measurable relative displacement is ~  1 2
h
4 L
P
=> Can reach h ~ 3.10-23 with L=100km and P=1kW
How to achieve that?
1/ Fabry-Perot cavities to increase the effective length:
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( F = finesse )
L'   F  L

=> L’ = 100km for L=3km and F=50
2/ Recycling mirror to increase the effective power:
P’ = R P
(R = recycling gain)
=> P’ = 1kW with P=20W and R=50
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Gravitational wave signal
Noise sources in interferometers
Thermal
Noise
Acoustic
Noise
Seismic
Noise
Index fluctuation
Shot
Noise
Laser Noises
Detection
Noise
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Noise sources: seismic noise
Seismic noise spectrum for f few Hz:
a m
~
xs  2
f
Hz
a ~ 10-6 - 10-7
  shot noise !
 Need a very large attenuation!
1014
Solution:
suspend the mirrors to a chain of pendulums
Transfert function
With a chain of 6 pendulums:
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attenuation of the seismic noise by ~1014 at 10 Hz !
Suspensions and control of the interferometer
All mirrors are suspended to a cascade of pendulums:
 Large attenuation in the detection band ( > 10 Hz)
 Large residual motion at low frequencies: < ~1mm
 Need active controls to:
- maintain the interferometer’s alignment
- maintain the required interference conditions
The control is done in 2 steps:
1/ Local control of the suspensions:
 Residual motion ~2 m/sec
 Obtain interference fringes
2/ To keep the interferometer at interference conditions:
– Need to control the length of the cavities to 10-12 m
– Need to keep the interferometer aligned
 Use the interferometer signals: photodiodes
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VIRGO design sensitivity
Main sources of noise limiting the VIRGO design sensitivity
Shot noise
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Seismic noise
Thermal noise
Shot noise
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Gravitationnal wave sources and
VIRGO design sensitivity

Distance to the Virgo cluster = 10Mpc
Coalescing binaries (1.4 Mo)
Pulsars: upper limit (1 year)
Supernovae at 15Mpc
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The commissioning of VIRGO
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End of construction: 2003
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The steps of the VIRGO commissioning:
• Technical runs (3 to 5 days) at each step
C1(Nov 2003),…, C5(Dec 2004)
Lock stability
Sensitivity/noise studies
Data taking on ‘long’ period
- recombined (Michelson) ITF: Feb 2004
- recycled (full VIRGO) ITF: Oct 2004
West arm
- control of the north FP cavity: Oct 2003
- control of the west FP cavity: Dec 2003
Fabry-Perot cavities
input mode cleaner
l=150m
beam
splitter
laser
recycling
mirror
l=6m
output mode cleaner
Gravitational wave signal
North arm
L=3km
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The lock of the full VIRGO
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Lock of the recycled interferometer (full VIRGO):
– Need to control 4 degrees of freedom (3 cavities + Michelson on dark fringe)
– The lock is acquired in several steps (‘variable Finesse’ strategy):
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• Start without recycling
• Slowly increase the recycling gain and move to the dark fringe
Power stored in the recycling cavity (Watts)
Lock acquisition
With recycling
Recycling gain ~ 30
Laser
Without recycling
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Sensitivity summary
Single arm, P=7 W
Recombined, P=7 W
Recycled, P=0.7 W
P = 10W
h ~3. 10-21/Hz
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Typical unforeseen difficulties
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Injection bench:
– A small fraction (bigger than expected) of the light reflected by the
interferometer is retro-diffused by the input mode cleaner mirror
 spurious interferences
Frequency noise
Recycling mirror:
- aligned
- not aligned
Temporary solutions:
- tried to rotate the mode cleaner mirror
- reduce the incident light (/10)
 We are now working with only Pin = 0.7 Watts
Final solution: install a Faraday isolator
 A new input bench will be installed in September 2005
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Present sensitivity and perspectives
P=0.7 W
P = 10W
• Improvements since C5:
- local angular controls
- longitudinal controls
- low noise actuators
Shot noise for P=0.7 W
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Futur: the VIRGO sensitivity will significantly improve with
– full power (new input bench)
– the automatic alignment of the interferometer (global angular control)
– the improvement of the longitudinal controls
– lower noise actuators
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– …
Data analysis: some examples
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Test of the data analysis on real data
from the technical runs:
– Test the full chain of data analysis
– Learn how to put vetoes
– Inject events in the real data:
software and hardware injections
-> measure efficiencies, false alarm rate,…
- Injected events
Event amplitude
Quiet period
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Start collaboration with LIGO:
Coincident analysis will help the detection of GW
=>decrease false alarm rate
(rare events in a non gaussian noise)
Event amplitude
Combined data analysis is necessary to extract the source parameters
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Conclusion
– The recycled (full VIRGO) interferometer is working
– Next engineering run (C6), 29/07-12/08:
2 weeks of data taking with the best sensitivity
– The sensitivity will make big progress with
• New input bench (-> full input power)
• Automatic alignment of the mirrors
– The data analysis is been prepared and tested on real data
Collaboration with LIGO is starting
– First scientific run in 2006/7?
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Noise studies
Sensitivity measured during C4 run and identified sources of noise
Noise hunting:
1/ Identify the sources of noise which limit the sensitivity
2/ Perform the necessary improvements / implement new controls
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Comparison with LIGO first science run (S1)
Virgo May 2005
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Example of lock acquisition
Example of the lock acquisition of a Fabry-Perot cavity
Photodiode used for
lock acquisition
Power stored inside the Fabry-Perot cavity
/2
Error signal of the cavity
Lock acquisition:
Apply force on the mirror to keep
the error signal at zero
Mirror
Correction sent to the actuators of the mirror
Coil
Magnet
4 seconds
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The commissioning of the CITF
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Commissioning of the central interferometer: 09/2001 -> 07/2002
– CITF = Recycled Michelson interferometer (no Fabry-Perot cavities)
- a lot of common points with VIRGO
unit = meters!
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The evolution: configuration and
sensitivity: 4 runs of 3 days each
- E0/E1: Michelson
- E2: Recycled Michelson
- E3: + automatic angular alignment
- E4: + final injection system
Results:
– Viability of the controls
– Sensitivity curve understood
– And gain experience for the
VIRGO commissioning
- Improvements triggered by the CITF experience
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The mirrors
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Fused silica mirrors
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Coated in a class 1 clean room at SMA-Lyon
(unique in the world).
– Low scattering and absorption:
< few ppm
– Good uniformity on large dimension:
< 10-3  400 mm
• Large mirrors (FP cavities):
–  35 cm, 10 cm thick
– 20 kg
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The injection and detection systems
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Laser: powerful and stable
- 20W
- Power stability: 10-8
- Frequency stability: Hz
The input and output mode cleaners:
- optical filter => improve signal to
noise ratio
Signal detection:
- InGaAs photodiodes, high efficiency
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Future: how to improve the sensitivity?
The first generation of detectors might not be able to see gravitational waves
 Need to push the sensitivity further down:
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Seismic noise:
– The VIRGO suspensions already meet the requirements for next
generation interferometers
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The main limit: thermal noise
– Monolitic suspensions (silica)
– Better mirrors (material,
geometry, coating)
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Shot noise
– More powerful lasers
– Signal recycling technique
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And the technical noises
– Better sensors
– Better electronics
– Better control systems
Shot noise
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Recombined interferometer
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Recombined interferometer:
keep the two Fabry-Perot cavities on resonance + the Michelson on the dark fringe
Example of lock acquisition
Power ‘stored’ inside the FP cavities
Power at the interferometer output
Lock on the dark fringe
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