Mining for gravitational waves

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Mining for gravitational waves

• Enrico Campagna
• Giancarlo Cella
• Riccardo DeSalvo
• Seiji Kawamura

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Pushing the Low Frequency Limit of ground based
GWIDs

• Three limiting noise sources impede GWID at Low
  Frequency
• Newtonian Noise (NN, alias Gravity Gradient)
• Suspension Thermal Noise (STN)
• Radiation Pressure Noise (RPN)
• All three can be reduced by means of an
  underground interferometer
• Large payback if successful
• Opening new horizons

K. Weaver Astro-ph0108481/Sci. Am. July 2003
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Interesting physics from Low Frequency ground
based GWIDs

• Sensitivity reach of cosmological interest (red
  shift gt1) is achievable
• Explore population of Intermediate Mass Black
  Holes on their merging way to galactic size BH
• Fill part of the frequency gap between LISA and
  LIGO/Virgo

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Low Frequency ground based GWIDs

• What reach is possible on and in the Earth crust?

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The physics, frequency reach
.

NN limit
Under Ground ltgt Above Ground
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The physics, Universe range
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1
10
1
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Above ground Ad-LIGO See LIGO document
M-0300023-00 LF-GWID See R.DeSalvo, Class.
Quantum Grav. 21 S1145-S1154,(2004) G. Conforto,
Nucl.Instr.Meth. Vol 518/1-2 pp 228-232
(2004) Limited by Newtonian Noise Under
ground CEGO-1 Newtonian Noise reduced by
building an Underground Interferometer CEGO-2
Newtonian Noise further reduced by sensing
Ground motion and subtracting residual NN
.

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The physics, .
reach in the Universe
CEGO 1 and 2
CEGO 1-2
LF-GWID
LF-GWID
Ad-LIGO
Ad-LIGO
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Which knobs to turn for low frequency
Example surface LF-GWID (R.DeSalvo, Class.
Quantum Grav. 21 (2004))

• In LG-GWID the first limitation is
• Newtonian noise, followed by
• Suspension thermal noise and
• Radiation pressure noise

8 Watts laser Fused Silica Mirror 70 Kg
mirror Longer suspensions
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Which knobs to turn for low frequency

• Large symmetric underground halls for NN
• Longer suspension wires for STN
• Large mass mirrors for RPN
• Large beam spots for normal TN

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For lowest frequencies, turn more the
sameknobs

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Reducing Newtonian Noise

• NN derives from the varying rock density induced
  by seismic waves around the test mass
• It generates fluctuating gravitational forces
  indistinguishable from Gravity Waves
• It is composed of two parts,
• The movement of the rock surfaces or interfaces
  buffeted by the seismic waves
• The variations of rock density caused by the
  pressure waves

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NN reduction underground

• How to shape the environments surface to
  minimize NN?
• The dominant term of NN is the rock-to-air
  interface movement
• On the surface this edge is the flat surface of
  ground

Ground surface
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NN reduction underground

• If the cavern housing the suspended test mass is
  shaped symmetrically along the beam line and
  around the test mass tilting and surface
  deformations, the dominant terms of NN,
  cancel out
• (with the exception of the longitudinal dipole
  moment, which can be measured and subtracted).

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NN reduction underground

• Pressure seismic waves induce fluctuating rock
  density around the test mass
• The result is also fluctuating gravitational
  forces on the test mass

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NN reduction underground

• Larger caves induce smaller test mass
  perturbations
• The noise reduction is proportional to 1/r3
• The longitudinal direction is more important
  gtelliptic cave

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NN reduction from size
Reduction factor
Calculation made for Centered Spherical Cave In
rock salt beds
5 Hz 10 Hz 20 Hz 40 Hz
Width Length

• Cave radius m

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NN reduction underground

• Additionally deep rocks, if uniform, elastic,
  transmitting and non dissipative, can be measured
  with a small number of seismometers (or better
  density meters) to predict its seismic induced
  density fluctuations and subtract them from the
  test mass movements
• This subtraction is largely impeded on the
  surface by the fractal-like character of the
  rubble composing surface soil

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Contributions to NN

• Fraction of NN due to
• Surface Effects
• (balance from
• density waves)

Horizontal accelerometer on cave surface will
gain a factor of 2 Three-directional 3D matrix of
accelerometers or density meters needed for
further subtraction
Cave radius m
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Newtonian Noise gains

• Minimal (multiplicative) Gains
• 102 from going underground
• 103 from symmetry and size of cave
• Factor 2 from easy seismic measurements and NN
  subtraction
• Further subtractions of NN require rock density
  measurements

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Newtonian Noise Conclusions

• NN can be reduced by 106 by going underground
  and becomes irrelevant ! ! !
• Gain 101.5 in frequency (gt 1 Hz)
• NN is not an obstacle for GW detection down to
  much closer to LISA frequencies!

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Reducing the suspension thermal noise

• Reduce suspension thermal noise with long
  suspensions
• Noise 1/vL
• Suspensions tens of meters long
• How to shape the facility to allow this?

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Vertical cross section

A) Upper experimental halls contain all
suspension points, readout and control
equipment B) Wells (50 to 100 m deep allow
for long isolation and suspension wires for LF
seismic and STN reduction C) Lower large
diameter caves, immune from peoples and seismic
Noise reduce the NN
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Suspension and Seismic Isolation schematics
10-20 meter pendula Between all stages
2-3 meter tall Pre-isolator In upper cave
LF Vertical filters
marionetta
Composite Mirror Recoil mass
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Filter chain schematics
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Filter chain performance
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Filter chain schematics
Start at 10-11 m/vHz
Seismic noise 1.310-8 f -2.55 m /vHz
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Seismic attenuation requirements

• Need 4 seismic filtering
• Stages, each 20 m tall
• 100 m well
• with final controls

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Reducing the radiation pressure noiseThe Mirror
concept

• The Standard Quantum Limit would limit the
  performance of a LIGO-like mirror
• A heavy mirror is necessary to widen the
  radiation pressure/shot noise canyon
• One ton inertial mass required
• High transparency mirrors (311SV) only for masses
  below 70-80 Kg
• Lower transparency mirrors (for end masses or
  even for input masses if power requirements allow
  it) possible with higher weights

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Mirror concept

• A solution is composite mirrors
• Kenji Numata has proven that you can support a
  mirror from the nodes of a mode without affecting
  its Q-factor performance.
• Kazuhiro Yamamoto (using Levins theorem) has
  shown that
• if you consider the action of a pressure with the
  same profile of the laser beam and
• if you support a mirror from points where this
  pressure has no action,
• Then the supporting is does not affect the
  thermal noise performances of the mirror.

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Mirror concept

• Using Ansys simulations and Levins recipe Calum
  Torrie applied a mesa beam profile pressure and
  looked for null action areas on the mirror outer
  surface

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Mirror design

• A clear no action band is present

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Mirror design

• Indium coated Fused Silica wedges and two shims
  at 120o can be used to mount the mirror from its
  neutral plane inside the recoil mass with
  negligible losses for the beam pressure action

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Mirror design

• The null action band has less than a percent
  surface strain energy density than the laser spot
  area
• The mirror is supported from a very small
  fraction of the ring perimeter
• A mirror supported from this null action band
  inside a moderately high mechanical quality donut
  will present no relevant dissipation for the beam
  profile action
• A composite mirror can be expected to be a viable
  option

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Mirror design

• Large mirror pose a problem of gravitational sag
  modifying the mirror profile
• The mirror must be supported the same way during
  processing and during operation
• This clamping scheme can be a solution for the
  mirror processing problem

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Mirror suspension schematics
Marionetta for hierarchical controls 12-20 m
long suspension wires Recoil mass for final
stage controls Mirrors inertial mass Mirror
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Summarizing

• An underground facility permits to overcome or
  reduce Newtonian, Suspension Thermal and
  Radiation Pressure Noise the three limitations
  for Low Frequency operation of GWIDs
• Going underground is a very attractive option to
  explore the Intermediate Mass BH Universe

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A site example, The WIPP site
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6.46.4 Km WIPP land withdrawal area (no
commercial mining allowed)
1.52 Km WIPP facility area
55 Km interferometer
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Chlorides Dens. 2.1 More conv.
Sulfides Dens. 2.3 Less conv.
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Underground interferometer plan
Red top level experimental halls Black lower
level chambers (interferometer)
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