PowerPoint-Pr

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Title
GEO 600 Commissioning progress
Max-Planck-Institut für Gravitationsphysik(Albert
-Einstein-Institut)
Stefan Hild Ilias WG1 meeting,
Sep 2005
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GEO 600 layout
0.09
1.5-2 kW
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Tuning signal recycling to 300 Hz

• Better knowledge of IFO parameters
• more accurate prediction
• from simulations
• downtuning to 200 Hz
• realized
• Fixed instability of MI auto-alignment by setting
  up a new telescope for DWS.

We will stay at 330 Hz !
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GEO 600 design sensitivity
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Calibration and demod phase _at_ 300 Hz

• Using signal recycling
• GW signal is split in
• P and Q quadrature
• P-response independant of demod phase
• Q-response only for larger than 20 to 30 deg
  demod phase compatible with current calibration
• (zero complex pole)
• Decided to use 40 deg for time being.

Drawback P used for MI servo is not really
only P
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High RF on HPD / Q-compensation

• Observations
• Found huge RF level after MI diode (1 Volt)
• Large DC level in Q after mixing.

Q-compensation Injected RF directly into
resonance circuit of the HPD to make Q DC level
zero. Improved sensitivity above 700 Hz
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RF setup
OLD setup

• Old setup
• Dominated by phase noise in the LO path
  (phaseshifter and splitter(0/90)
• Due to this noise from Q was mixed into P
  quadrature.
• That is why Q-compensation worked.

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Shot noise
Green line is the shot noise level calculated
from the DC current of the photodiode.
We are within a factor sqrt(2) shot noise limited
above 1kHz !
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Future RF setup
Proposed by Rana and others
GOALS

• Using only high quality hardware
• Guaranteeing high RF levels in all components

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Noise projections 21st of july
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Noise in the SR-long loop
PROBLEM
Noise in the detection band is entirely limited
by front-end-noise (shot noise from the
detector). Limits sensitivity from 100 to 500 Hz
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Analog SR electronics
Red acq amp, green acq pha Dark blue run
amp, brown run phase Light blue run2 amp,
pink run2 pha
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Signal Recycling digital
Phase _at_18 Hz -147 deg
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SR feedback analog vs. digital
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SR loop filter future design
Four complex integrators 2.25 (2x),1.3, 0.56 Hz
Filter not used at the moment. Can be implemented
when more gain is needed around pendulum
resonances
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Sensitivity improvement July to August
High frequency improvements reduction of RF
phase noise Low frequency improvements reduction
of Signal recycling feedback noise
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PR-bench
Historically grown layout of PR-bench not optimal
!

• beam clipping
• too many transmissive optical components
• too many polarizing components
• acoustic coupling

Goal Shorten and simplify the HPD path
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Sensitivity after PR bench work
Removed resonance structures between 100 and 200
Hz.
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Noise projections 21st of july
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Michelson length control
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Michelson length control
lt 0.1Hz
lt 10 Hz

• Reaction Pendulum
• 3 coil-magnet actuators at intermediate
  mass, range 100µm

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Michelson length control
lt 0.1Hz
lt 10 Hz

• Reaction Pendulum
• 3 coil-magnet actuators at intermediate
  mass, range 100µm
• Electrostatic actuation on test mass bias
  630V, range 0-900V 3.5µm

gt 10 Hz
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Using ESD as actuator

• Force is proportional to square of applied
  voltage.
• high voltages are needed
• for bipolar acting a bias voltage needs to be
  applied

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Sqrt circuits in MI loop
ESD F ? U2 Sqrt circuits are necessary to give
full linear force range for acquisition. Drawback
sqrt circuits are noisy 1µV/sqrt(Hz) (100µV/sqr
t(Hz) _at_ ESD)
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Sqrt circuits in MI loop
ESD F ? U2 Sqrt circuits are necessary to give
full linear force range for acquisition. Drawback
sqrt circuits are noisy 1µV/sqrt(Hz) (100µV/sqr
t(Hz) _at_ ESD) Bypassing sqrt circuits after lock
is acquired.
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Noise in MI loop
HVA noise 100nV/sqrt(Hz) (10µV/sqrt(Hz) _at_
ESD) HV-amplifier noise can be reduced by
decreasing bias voltage or active noise
suppression. Suppressing noise introduced by
loop electronics needs whitening
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MI loop whitening / dewhitening
Whitening right after mixer zero 3.5 Hz pole 35
Hz Dewhitening for both split passes Passive
dewhit-ening done in HV path (0-1kV)
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Sensitivity after fixing noise from MI loop
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Michelson servo design
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MI loop gain problem

• Strange observations
• We are not able to increase the low frequency
  loop gain, even though it should work from loop
  model (oscillation around 8 Hz)
• OLG measurements show that there is nearly no
  gain around 8 to 10 Hz.
• We needed to turn down the MI crossover gain
  continiously for the last few months
• Findings
• Influence of MI AA gain on maximum cross over
  frequency (AA-tilt ?, ? cross over possible.
• ? phase margin of crossover, can ? crossover
  frequency, but adding integrator still causes 8Hz
  oscillation
• ? IM gain (? crossover frequency), ? less noise
  in servo 4 to 8 Hz.
• ? IM gain, still low gain around 8 to 10 Hz (low
  gain cant be caused by IM)

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Injecting LF noise in MI loop
Injected noise from 5 to 11 Hz Between 6 and 7 Hz
the noise is not suppressed !
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Measurement of open loop gain (PRMI)

• Resonance structure
• is clearly visible
• Very low gain
• around 9 Hz
• Design gain
• 7 _at_ 15 Hz
• 20 _at_ 10 Hz
• 80 _at_ 6 Hz

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Mi servo design detail
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TF Alignment to longitudinal
Tilt couples a factor of 10 stronger than rot _at_
10 Hz. (IM-FF tilt2long) Why are the two
rot-TF so different?
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How to go on with the MI loop gain problem
Measure MI loop gain for various conditions to
decouple crossover and AA-tilt. Implementing
digital AA control to be more flexible (nearly
done). Measure tilt2long for single suspensions,
with the goal of setting up a FF
system. Investigate possible advantages of using
the ESDs for angular alignment. ....
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Segmented ESD for alignment
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Sensitivity improvement of GEO
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Latest noise projections
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Current sensitivity vs. Design sensitivity
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Discussion

• Michelson loop gain problem
• What experience exists in VIRGO with angular to
  longitudinal and longitudinal to angular
  couplings ?
• Is somewhere gain lost ?
• ...
• Digital filtering
• What kind of filters are used within VIRGO for
  steep roll off?

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