07. July 2004 Andreas Freise Status of VIRGO A. Freise For the Virgo Collaboration European Gravitational Observatory

07. July 2004 Andreas Freise

Status of VIRGO

A. FreiseFor the Virgo Collaboration

European Gravitational Observatory


The VIRGO Collaboration

VIRGO is an Italian-French collaboration for Gravitational Wave research with a ground-based interferometer.

ITALY - INFN

Firenze-UrbinoFrascatiNapoli PerugiaPisaRoma

FRANCE - CNRS

ESPCI – ParisIPN – LyonLAL – OrsayLAPP – AnnecyOCA - Nice


The European Gravitational Observatory

Last mirror installed June 2003Inauguration August 2003


Expected Sensitivity


The VIRGO Interferometer

Laser 20 W

Output Mode Cleaner4 cm long

Michelson Interferometer with3 km long Fabry-Perot cavitiesin the arms and Power Recycling

Input Mode Cleaner144 m long


Main Optics

High quality fused silica mirrors

35 cm diameter, 10 cm thicknessSubstrate losses 1 ppmCoating losses <5 ppmSurface deformation /100 (rms on 150mm) R: <10-4


Vacuum System

Two tubes: 3 km long, 1.2 m in diameter, installed and tested, in vacuum since June 2003, 10-6 mbarAll tower (6 long, 2 short) pumping systems: installed, tested and put in operation, 10-9 mbar

Tubes

Towers


The Suspension System

The Superattenuator (SA) is designed to isolate the optical components from seismic activities (local disturbances). It is based on the working principle of a multistage pendulum.

Expected attenuation @10 Hz: 1014

Residual mirror motion (rms) rotation <1 rad

longitudinal <1 m


The Top Stage

Top of an inverted pendulum: - Inertial damping (70 mHz to 5 Hz) - Possibility to move the suspension point with small forces


Passive Filters

Five seismic filters:

Suspended by steel wires Vertical isolation by a combination

of cantilever springs and magnetic anti-springs


The Local Control

Marionette control:CCD camera, optical levers and fourcoil-magnet actuators: <2 Hz


Fast Control (Global)

Reference mass and mirror,four coil-magnet actuators: >2 Hz


Injection System

20 W Nd:YAG master-slave laser

Input mode cleaner: 144 m long ring cavity

Reference cavity: 30 cm long ring cavity, mirrors contacted to a rigid ULE spacer

Both cavities: Finesse=1000

IMC

RFC


Laser

Located on an optical table outside the vacuum

Nd:YAG master commercial CW single mode (700 mW) @1064 nm

Phase locked to a Nd:YVO4 slave (monolithic ring cavity)

Pumped by two laser diodes at 806 nm (40 W power)

Output power: 20 W

Slave

Master


Input Mode Cleaner

Triangular cavity, 144 m long, Finesse=1000Input optics and two flat mirrors are located on a suspended optical benchEnd mirror suspended with a reference massfor actuationTransmission 50%

Injection Bench

Mode Cleaner Mirror


Detection System

Suspended bench in vacuum with optics for beam adjustments and the output mode cleaner (OMC)Detection, amplification and demodulation on external bench

Suspended bench External bench

Output Mode-Cleaner


Output Mode Cleaner

Output Mode-Cleaner

Detection Bench

4 cm long ring cavitySuppression of TEM01 by a factor of 10Length control via temperature (Peltier element)Lock acquisition takes 10 min, lock accuracy is /60000


Photo Diodes

16 InGaAs diodes for the main beam (dark port)6 additional photo diodes (and 8 split photo diodes) for control purposes

External bench

Output Mode-Cleaner


Data Acquisition and Storage

16-bit ADCs, up to 20 kHz sampling frequency

Data is transferred via optical linksto workstations and then written to disk in Frame format:- full signal- down-sampled to 50 Hz- down-sampled to 1 Hz (trend data)

Frames available for data monitoringwith 3 s delay

Current rate: 7 Mbytes/s (compressed)

On-site storage on spinning media: - 5 TB for online analysis- 70 TB for offline analysis


Control

Fully digital control, local and global

Feedback is send with 20-bit DACs @ 10kHz to thesuspensions

The suspension control is performed by decicated DSPs (one per suspension)

Interferometer signals are acquired with 16-bit ADCs @ 20 kHz. The data is transferred via optical linksto Global Control, a dedicated hard and software that computes correction signals and sends them to the mirror DSPs


Computer Control

Monitoring and control of the detector by scientists and operators using: - 10 workstations - a digital video system


Digital Video System

Digital processing of video images


Video Surveillance


DSP / Suspension Control


Data Display


Database


Supervisor


Current Status

Commissioning of the central interferometer and the injection system from 2001 to 2003

Since September 2003 commissioning of the two arms and the full interferometer

At the end of this year the detector is planned to be operated in its final configuration

Scientific data taking starts 2005

Central Interferometer (CITF)


Commissioning of VIRGO

The commissioning of the full detector is divided into three

phases:

Phase A: the 3 km long arm cavities separatly

Phase B: recombined Michelson interferometer

Phase C: Michelson interferometer with Power Recycling

Currently we have completed phase B.


Commissioning Runs

C1: 14.-17. November 2003North arm cavity longitudinally controlled

C2: 20.-23. February 2004North arm cavity with longitudinal and angular control

C3: 23.-27. April 2004Recombined interferometerNorth arm with second stage of frequency stabilisation

C4: 24.-29. June 2004Recombined interferometer with angular control and second stage of frequency stabilisation

Continuous data taking periods are scheduled every second month:


Sensing and Control

Modulation-demodulationscheme with only one modulation frequency (6 MHz) to control:

4 lengths 10 angles


North Arm Cavity

PR misaligned

West arm misaligned

Phase A: the two arm cavities are used separatly, starting with the north arm; control systems are to be installed and tested.


North Arm Cavity

In October 2003 the North arm cavity was locked on first trial using a control algorithm that was tested before with SIESTA, a time domain interferometer simulationThe West arm cavity was locked in December 2003


North Arm Sensitivity


North Arm Sensitivity

Sensitivity limited by frequency noise between 7 Hz and 4 kHz (C1)By reducing the bandwidth of the IMC control loops the frequency noise could be improvedThe noise below 4 kHz (C3) still originates in the injection system but is not fully understood


Recombined Interferometer

Phase B:Recombined Interferometer

- B2 (P) used to control common mode (L1+L2)- B2 (Q) used to control beam splitter- B1/B1’ used to control differential mode (L1-L2)

PR misaligned

Recombined locked in February 2004


Automatic Alignment

Anderson technique:

- Modulation frequency coincident with cavity TEM01 mode

- Two split photo diodes in transmission of the cavity (at two different Guoy phases)

- Four signals to control the 2x2 mirror angular positions (NI, NE)


Automatic Alignment

• Alignment control allows to switch off local controls

• Power inside the cavities becomes more stable

• Installed and tested for the recombined interferometer

• Bandwidth ~3 Hz• Residual fluctuations ~0.5

urad rms (1 nrad @ 10 Hz)


Automatic Alignment


Frequency stabilisation

The `second stage´ of frequency stabilisation

The laser frequency is stabilised to the common lengths of thearm cavities (bandwidth ~17 kHz)The arm cavities are stabilised to the reference cavity (bandwidth ~2 Hz)Gains of the frequency control loops are increased


Present Sensitivity


Conclusions

The construction of VIRGO is finished, the commissioning of the full instrument is underwayBoth arm cavities were put into operation, longitudinal and angular control were implementedThe injection system and the detection system withboth mode cleaners are used routinely during thecommissioning The detector is now used in the recombined mode with automatic mirror alignment and the second stage of frequency stabilisation Starting now the final configuration including Power Recycling will be implementedThe first scientific data runs are planned for next year


End


Frequency stabilisation

The pre-stabilisation of the laser frequency was succesfully tested in the central interferometer

The frequency noise of the pre-stabilised laserlimits the sensitivity over a wide range

The second stage required for the designed stability was completed in June 2004

The performance will be tested in the commissioning run (C4) starting today


Recombined Interferometer

Sensitivity below 300 Hz limited by electronic noise from a temporary sensor setup

Documents

07. July 2004 Andreas Freise Status of VIRGO A. Freise For the Virgo Collaboration European Gravitational Observatory