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SLAC tunnel motion and analysis

Explore systematic studies of slow tunnel motions at SLAC, revealing unexpected tidal components and their correlation with atmospheric pressure. Learn about tunnel displacement, diffusive motion, and spectral analysis, shedding light on the influence of landscape variations and pressure fluctuations. Delve into the dynamics of tunnel motion and atmospheric effects in the context of accelerator operations.

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SLAC tunnel motion and analysis

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  1. SLAC tunnel motion and analysis Andrei Seryi SLAC Originally 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

  2. 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. x3 x2 x1 Scheme of measurements Signals from the quadrant photo detector were combined to determine X and Y relative motion of the tunnel center with respect to its ends.

  3. SLAC tunnel drift studies • Unexpected facts: • The tidal component of motion is surprisingly • big ~10 micron. • Motion has strong correlation with external atmospheric pressure. Horizontal and vertical displacement of the SLAC linac tunnel and external atmospheric pressure.

  4. 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 where tidal motion is seen most clearly. Fit of 3 major tidal harmonics

  5. Influence of atmospheric pressure Very slow variation of external atmospheric pressure result in tunnel deformation. Explanations: landscape and ground property variations along the linac: Observed Dh=50mm for DP=1000 Pa is consistent with these estimations if DE/E~0.5, h~ l~ 100m, a~0.5 and E~109 Pa. Assumption E~109 Pa is consistent with SLAC correlation measurements. Taking v=500m/s (at ~5Hz, I.e. l~100m) and r=2*103 kg/m3, we get E= 109 Pa l - length of landscape change, a - variation of the normal angle to the surface

  6. Tunnel motion. Diffusive in time • Spectra of tunnel displacementsbehave as 1/w2 in wide frequency range, as for the ATL law for which P(w,k)=A/(w2 k2) Tidal peaks electronic noise Electronic noise of the measuring system was evaluated with a light diode fixed directly to quadrant photo detector

  7. Diffusive in time... • fit of the spectra to ATL gives A~ 10-7 -- 2*10-6mm2/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 Parameter A found in 1999/2000 SLAC measurements. xi2 shows the quality of fit to 1/w2 spectra.

  8. 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/w2 • 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 6mm/mbar in Y and 2mm/mbar in X. “A” vs amplitude of atmospheric pressure spectrum aP. Spatial l does not depend on f, but given spectra of landscape/ground properties. =>

  9. “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/w2and together they give 1/(w k)2that 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)! one of the causes ? for further studies

  10. Topography of many natural surfaces exhibits P(k) ~ 1/k2 behavior of the power spectra (k is spatial frequency, k=2p/l) ... ... (Note that definitions in this paper are different from ours. In the paper k is a coefficient and w is the spatial frequency.)

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