Seismic Isolation for LIGO Laser – Interferometer Gravitational

Wave Observatory

RANA LEVYINTRO. TO ACOUSTICS 2019

UNIVERSITY OF WASHINGTON

Content

1. What is LIGO?

2. How does it work?

3. What are requirements for the experiment?

4. Sources of noise

5. Solutions (cantilever blades and systems)

6. Internal Seismic Isolation Systems (ISI)

7. HEPI (Hydraulic External Pre-Isolators)

8. Where do experiments like LIGO go from here?

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What is LIGO?

It is a large-scale experiment developed by Caltech, MIT, and funded by The National Science Foundation. It has been in operation since 2002 but gravitational waves had not been detected until 2015.

The experiment itself focuses on measuring gravitational waves rippling through space time that were predicted by Albert Einstein in 1916.

September 14, 2015 LIGO discovers gravitational waves

The teams were awarded the Nobel Prize and also confirmed Einstein’s theory

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The experiment uses an interferometer, which

is composed of one laser focused to a beam

splitter at a 45-degree angle.

The beam splitter divides the laser’s light down

two arms with mirrors at the end of each arm

and a photodetector.

The merging of two sources of light should

create an interference pattern – whether it is

destructive or constructive depends on

whether a gravity wave is passing through

Constructive – gravity wave

Destructive – No gravity waves

3How Does it Work

LIGO Expectations4

Gravitational waves were expected to be in the range from 30-300Hz

The sources of these waves can be from spinning dense stars that provide continuous gravitational waves like supernovae

Another source can be pairs of spinning objects like black holes, neutron stars or white dwarf stars

Shorter signals mean more massive objects like black holes

Longer signals would imply lower mass objects

Discovery in 2015 - This signal was converted from electromagnetic data into audible sound like the “chirp” , and was from two merging black holes

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Obstacles 6

Things such as trucks, plows, trains and

human activity give off vibrations that disrupt the optics of the experiment

Tidal cycles also can be disruptive

Even components of the LIGO facility

such as nearby lights and computers

Seismic noise, Quantum noise, Mirror

coating thermal noise (from Brownian

motion), Suspension thermal noise

(silica fiber suspension) and gravity gradient noise (changes in

gravitational field) are all obstacles

Initial Measurements

Below 10Hz, displacement noise from seismic

motion is at both sites.

The ground moves by ∼ 10−9 m/ √ Hz at 10 Hz—

This was ten orders of magnitude larger than the

LIGO target sensitivity at this frequency

This had to be offset by having engineers comeup with ways to filter the noise using active and

passive stages.

The next question must be…

How does LIGO filter out the natural vibrations

around the facility?

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Solutions 8

The mirrors in each arm of the facility are hung like pendulums

We can think of them simplistically like a pendulum.

In order to prevent oscillations on our mass on a wire, there needs to be a way so that any movement to the apparatus is able to effectively destruct the vibrations.

Solutions

A mechanism designed by the engineers at LIGO provided cantilevers with mass on a wire in the middle to provide a robust way to eliminate as much noise as possible.

These special cantilever blades can rotate in many ways

Pitch, yaw and roll as well as vertical, horizontal motions

They can also provide the oscillations slowly if you combine and compress two blades together

These can then be finely tuned

You can also make these variable sizes

The vertical frequency can be as low as needed

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Prototype

Isolators

You build a system with these blades like the one to the right

It contains an inverted pendulum, with the cantilever blades

The optics is then suspended by the apparatus and vibrations are dampened.

The schematic is for one of LIGO’s Passive Isolation systems

Of course – things change

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The Hydraulic External Pre-Isolator (HEPI) A hydraulic external pre-isolator

system (HEPI) is used for low frequency alignment and control

These systems are designed to provide active isolation

The platforms contain quiet hydraulic actuators, geophones, ground seismometers and inductive position sensors.

They are based on a quiet hydraulic drive technique

Experiments with systems like these were done at the LIGO/MIT facilities with great success and then implemented to the LIGO Livingston observatory.

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HEPI 14

The HEPI system contains two configurations: one for auxiliary optics chambers and one for the core optics chambers.

It is outside of the vacuum chambers

It was designed to address the low frequency isolation and alignment requirements

Actuation is required in all six degrees of freedom (DOF)

HEPI can generate a maximum force greater than 2000

Has a bandwidth from 0 to ~10 Hz

Its noise level does not exceeding 10-9 m/√ Hz at 1 Hz which was a requirement

HEPI In this figure (b), the hydraulic

actuators are used to drive the platforms.

The differential pressure in the two chambers of the actuator drives the tripod connected to the structure

Flexible bellows are used instead of pistons to suppress friction between moving parts.

The bellows are designed to maximize the ratio of stiffness

This makes a very reliable and robust system

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Internal Seismic

Isolation (ISI)

Made of two stages, and has suspension through stiff blade springs and short pendulum links

Use low noise inertial sensors and provides low frequency active isolation (0.1 Hz)

Attenuates seismic motion above 10Hz

These also position the optics in the vacuum chambers.

The vibration of each stage is reduced by sensing its motion in 6 degrees of freedom (up/down/left/right/yaw/pitch/roll) and applying forces in feedback loops which reduces motion

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HAM- ISI

HAM ISI is a six axis passive isolator

The platform can carry more than 500kg of payload and can position it with nanometer resolution

Three triangular blades are used to provide vertical isolation

Three flexing rods are used forhorizontal isolation

It contains customized commercial instruments for sensing

These components are in the high vacuum chambers

After strenuous testing, the HAM ISI platform exceeded all requirements

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BSC – ISI platform15

The BSC ISI platform is a two stage internal isolator

It solves problems needed for seismic motion requirements for the core optics

The optics require an isolation factor of 10 at 0.1 Hz and up to several thousands in the control bandwidth

To have this requirement met, an active isolation concept was needed

Three other design concepts were made and tested at MIT but the final design was tested and approved in 2011

BSC – ISI

platform

The BSC ISI platform is shown in figure A

It is the top most plate of all the systems

This system is very similar to the HAM ISI system

It contains three sets of blades and flexures as well as sensors for positioning in the vacuum

Just as the blades in the HAM ISI system are used, the primary difference is that the BSC blades are flat and undeformed.

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These seismic

isolation chambers

are located

wherever mirrors are

located along the 4km arms

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Where do experiments like LIGO go from here?

There is not much the team can do to eliminate human made vibrations

This would require an experiment that is in the vacuum of space, away from obstructions

Of course, this was indeed the next step for gravitational wave physics!

Introduced in 2017, LISA Pathfinder: The gravitational-wave observatory in space

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LISA Pathfinder

Launched on December 5 2015

The mission tested technologies needed for the theoretical LISA

LISA is planned to be launched in 2034

By sending the experiment in the vacuum of space, data gathered in 2018 showed that by having no residual gas in the chambers, the data collected was of higher quality

The largest sources of any interference with the mirrors was eliminated

In addition, there is no curvature of the earth to contend with, and so the laser arms can be extended to much longer lengths

While LIGO is best suited for rapidly merging or spiraling objects (like black holes or neutron stars), LISA can identify objects long before the final merger.

When distances are thousands of miles away from their center of mass, the spiraling objects would make periodic signals that LISA would pick up.

LISA would also be able to detect matter falling into super massive black holes

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Advanced LIGO (orange) is

only sensitive to gravitational

events on timescales under a

second.

The first week of the mission

exceeded the requirements.

It performed 30 times better than LIGO.

Tuesday July 18 2018, the LISA

pathfinder mission was shut

down.

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Thank You

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Sources

https://www.ligo.caltech.edu/page/vibration-isolation

https://iopscience.iop.org/article/10.1088/0264-9381/21/5/081/meta

https://www.osa-

opn.org/home/articles/volume_26/march_2015/features/ligo_finally

_poised_to_catch_elusive_gravitational/

https://arxiv.org/ftp/arxiv/papers/1502/1502.06300.pdf

https://phys.org/news/2018-02-results-lisa-pathfinder-satellite.html

https://arxiv.org/pdf/1604.00439.pdf

https://en.wikipedia.org/wiki/LISA_Pathfinder

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