rrcat lecture 5 vibration isolation

8/17/2019 RRCAT Lecture 5 Vibration Isolation

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5: Vibration isolation (passive)

Peter Saulson

Syracuse University

25 December 2013 ICTS Winter School on Experimental

Gravitational-wave Physics

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.

2. Interactions of waves and detectors . o no se an ra a on pressure no se

4. Theory of linear systems

. rat on so at on pass ve6. Thermal noise

. pt cs o a ry- erot cav t es

8. Feedback control systems9. Description of LIGO and other current detectors

10. Future detectors in space



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. sp acemen no se goa

2. Seismic noise3. Seismic isolators in theory and practice

4 Limi f rf rm n



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n e nex wo morn ngs, we w a a ou

two kinds of noise that move ann er erome er s m rrors:

seismic noise

thermal noiseThis is in distinction to readout noise like shot

noise, which only affects our ability to see

where the mirrors are.Radiation pressure noise, shot noise’s quantum-

mechanical twin, was our first example of



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sp acemen no se.


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ow arge

Seismic: xrms ~ 1 m.Thermal

– mirror’s CM: ~ 3 x 10-12 m.

– mirror’s surface: ~ 3 x 10-16 m.



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’’,

as h ~ 10-21. What did I mean?

averaged over the 10 msec duration of, say, a signalfrom a su ernova or black hole rin down to be 10-21.

Let’s convert this spec to power spectrum language221

Hz1502

This means we want a noise amplitude spectral density

near 100 Hz of

Hz50

.Hz/10)Hz100( 22 f h



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a spec rum o sp acemen no se s

consistent with this goal?We’ll have four key mirrors (two in each arm to

make the Fabry-Perot cavities, see later

lecture.)If their displacement noises are incoherent, the

add in quadrature. The net result is

).(2

)( f x f h

Thus, we need x(f) = 2 10-19 m/Hz1/2.



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typically10-9 m Hz1/2* 10 Hz 2 uite diverse below 10 Hz.Note basic pattern: Strong at low frequencies, weak at high

frequencies.

At our target frequency of 100 Hz,x(f) = 10-11 m/Hz1/2,

ar rom x = - m z .

Seismic noise is serious!

e nee or ers o magn u e o so a on a z.



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… • … ree o move n response o grav a ona

wave• … free from large disturbances from the

environment



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Strategy to deal with

seismic noise ,

strategies for dealing with it. , .This will partly solve the problem, but will leave

.

2nd, build an instrument that can be sensitive athi h fr n i wh r w n m i minoise small), while tolerating the remaining

stron noise at low frequencies.This means ensuring linearity, and making

thoughtful use of servo control.



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mass on a spr ng ma es a goo so a or.

Frequency response goes like 1/ f 2

above resonance.



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ne suc s u n o our p ans a rea y –

each test mass must be suspended as apen u um, o a ow o respon ree y o egravitational wave.

It as a resonant requency near 1 Hz.Thus, we should multiply the input spectrum

y z o n ou pu spec rum .e., mo onof mirror.)



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Core Optics

ns a a on an a gnmen



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Optic Suspension10 kg Fused Silica25 cm diameter

• Magnet / coils control mirrors

• Suspending gives 1/f 2 isolation above ~1 Hz.

• Low loss music wire minimizes thermal noise

25 December 2013

ICTS Winter School on Experimental

Gravitational-wave Physics 15magnet


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Pendulum helps, but only makes seismic

noise good enough at f > 1 kHz



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Multiple stages of isolation

for better filtering ne ma es a goo er, u no goo

enough.If we make a chain of N oscillators, we have a

coupled system with N resonances, above

which the frequency response is2 N

. f

f G

.



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Add 3 stages of 3 Hz isolation,

LIGO can work down to 30 Hz



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-

isolation as well.

– If not, some of the large transverse motion would beconverted into optic-axis motion by asymmetries in the

pendulum.– Interferometer has some sensitivity to vertical motion,

’ .

The earth is curved!

So, we need quasi-isotropic isolation. Such isolators arecalled mass-spring “stacks”. They were invented by Joseph Weber.


Gravitational-wave Physics19


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Gravitational-wave Physics20


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25 December 2013 ICTS Winter School on ExperimentalGravitational-wave Physics

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Internal modes as limits to

performance o e a s ac s requency response

levels off above ~ 30 Hz, at a level ofa ou - a ou – .

Internal degrees of freedom in springs and

masses have resonant frequencies there.E.o.m. isn’t well approximated by the point-mass model in that regime.


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Virgo’s Free falling test masses


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aLIGO has more isolation

(see Rana’s talk on active SEI)

++

182

Stage 2

Actuation PlaneXX+

Y

+

Y

+

V

+

VOptical Table

Point (X , Y )[mm]+L+L +T+T

, .

ITM

+200 -369.8 ETM


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causes gravitational force noise.

a spring of resonant frequency

1

This is a low frequency (below 10-4 Hz), but is only likeone isolation sta e.

.

2

earthgrav

Net effect is un-isolatable noise that is stronger than

seismic noise below about 3 Hz.(Perhaps can measure seismic field, model this noise,

and subtract …)


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- ,

admitting all seismic noise below a few Hz.

,always be the Newtonian noise.N.B. There are some ideas for measurin the seismic noise at

many places around each mass, computing the Newtoniannoise, and subtracting it.

Why not get away from it all, in space?

goo ea. e more a ou propose spacemissions on Friday afternoon.


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rrcat lecture 5 vibration isolation

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