NLC - The Next Linear Collider Project

SLAC tunnel motion and SLAC tunnel motion and analysisanalysisAndrei Seryi

SLACOriginally given at The 22nd Advanced ICFA Beam Dynamics Workshop

on Ground Motion in Future Accelerators

November 6-9, 2000, SLAC

http://www-project.slac.stanford.edu/lc/wkshp/GM2000/

Reported briefly to the Advanced Seismic Sensor Workshop, Lake Tahoe, March 24-26, 2004

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A.Seryi

Recent SLAC tunnel drift studies

• Goal: to perform systematic studies of slow tunnel motions

• Measurements were done from December 8, 1999 to January 7, 2000.

• These measurements were possible due to PEP- II shutdown.

• The linac alignment system working in the single Fresnel lens mode allowed submicron resolution.

• First measurements of this kind were done in November 1995 by C.Adolphsen, G.Bowden and G.Mazaheri for a period of about 48hrs.

Scheme of measurementsx1

x2

x3

Signals from the quadrant photo detectorwere combined to determine X and Y relative motion of the tunnel center with respect to its ends.

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A.Seryi

SLAC tunnel drift studies

Horizontal and vertical displacement of the SLAC linac tunneland external atmospheric pressure.

Unexpected facts:

• The tidal component of motion is surprisinglybig ~10 micron.

• Motion has strong correlation with external atmospheric pressure.

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Tidal motion of the SLAC linac tunnel

• Observed tidal motion is ~100 times larger than expected.(N.B. the system is not sensitive to change of slope due to tides, but only to

change of the curvature)• Higher amplitudes are caused by enhancement of tides due to ocean

loading in vicinity (~500km) of the shoreline.

• Tidal motion is slow, it has long wavelength and is not a problem for linear collider.

Subset of data wheretidal motion is seenmost clearly.

Fit of 3 major tidal harmonics

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Influence of atmospheric pressure

Very slow variation of external atmospheric pressure

result in tunnel deformation. Explanations: landscape

and ground property variations along the linac:

αE

Ph

E

E

E

hPh

- length of landscape change, - variation of the normal angle to the surface

Observed h=50m for P=1000 Pa is consistent with these estimations if E/E~0.5, h~ ~ 100m, ~0.5 and E~109 Pa.

Assumption E~109 Pa is consistent with SLAC correlation measurements.)1(ρ2

Ev

Taking v=500m/s (at ~5Hz, I.e. ~100m)and =2*103 kg/m3,

we get E= 109 Pa

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Tunnel motion. Diffusive in time

• Spectra of tunnel displacements behave as 1/2 in wide frequency range, as for the ATL law for which P(,k)=A/(2 k2)

Electronic noise of the measuring system was evaluated with a lightdiode fixed directly to quadrant photo detector

electr

onic

noise

Tidal peaks

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A.Seryi

Diffusive in time...

Parameter A found in 1999/2000 SLAC measurements.xi2 shows the quality of fit to 1/2 spectra.

• fit of the spectra to ATL gives A~ 10-7 -- 2*10-6 m2/m/s

• “A“ is higher for vertical plane.

• The value “A” varies in time. Why?

• The “A” value is consistent with FFTB measurements with stretched wire over 30 m distance

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Atmospheric pressure again

• Correlation X or Y and atmospheric pressure is significant from 10-6 up to about 0.003 Hz.

• Spectra of pressure also behave as ~ aP/2

• The amplitude of “A“ correlates with amplitude of pressure spectrum aP.

• The ratio (X/P) almost does not depend on frequency in 10-6 -0.003 Hz and is about 6m/mbar in Y and 2m/mbar in X.

“A” vs amplitude of atmospheric pressure spectrum aP.

Spatial does not depend on f, but given spectra of landscape/ground properties.

=>

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“A” versus Young’s modulus

Spatial variation of ground and/or landscape + variation of atmospheric pressure is a major cause of diffusive-like motion of the SLAC linac tunnel

The spectra of ground properties/landscape vary as 1/k2 , the spectra of pressure behave as 1/2 and together they give 1/(k)2 that is (or mimic) diffusive motion

For the shallow tunnel, the “A” scales as 1/E2 or 1/v4 !!!

Look for strong media, (higher Young’s modulus E or shear velocity v)!

? for further studies

one of the causes

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Topography of many natural surfaces exhibits P(k) ~ 1/k2 behavior of the power spectra (k is spatial frequency, k=2/)

...

...

(Note that definitions in this paper are different from ours. In the paper k is a coefficient and is the spatial frequency.)