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
NLC - The Next Linear Collider Project
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.
NLC - The Next Linear Collider Project
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.
NLC - The Next Linear Collider Project
A.Seryi
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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A.Seryi
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
NLC - The Next Linear Collider Project
A.Seryi
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
NLC - The Next Linear Collider Project
A.Seryi
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.
=>
NLC - The Next Linear Collider Project
A.Seryi
“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