Comparative Study of Vibration Stability at Operating Light Source Facilities and Lessons Learned in Achieving NSLS II Stability Goals Page: 4 of 5
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facility operations, etc.). As important as the amplitude of
ground vibration at a site is the coherence or cross
correlation of the motion. For light sources (and linear
colliders) this is essential since the temporal and spatial
movement of the entire electron beam lattice needs to be
known and feed-back corrected. The more uncorrelated
the motion on the lattice supports the more difficult the
task is in minimizing the beam jitter. Figure 3 reflects
coherence measurements at selected sites and for varying
distances. More relevant to a light facility are the
coherence data depicted in Figure 4a and representing the
motion on the experimental floor of Diamond 3rd
generation light source. These experimental floor
coherence data have been recorded in support of this
comparative study. Also shown in Figure 4b is the
filtering effect on the site ground motion introduced by
the structure. Clearly the placement of the ring building
on the green-field site results in the filtering of site ground
motion characterized by wave lengths smaller than the
characteristic wavelength of the ring structure (Rayleigh
wave velocity at the site divided by the ring diameter).
Figure 1: Recorded PSD at selected accelerator sites [1]Figure 4: (a) Coherence of motion on the experimental
floor of the Diamond LS measured in this study and (b)
recorded ground and experimental floor motion PSD
depicting the structure-induced filtering effect [4]
Figure 5 depicts power spectra and integrated rms
displacements in the free-field and on the APS
experimental floor. What the recorded PSD data indicate
is that while the structure filters the natural motion,
facility-generated cultural noise dominates the range
above -10 Hz where equipment operate. The rms
displacement on the other hand is "filtered" because of the
disproportionate reduction of the displacement in the
regime below 10 Hz. Quite different picture is seen in the
relationship between the free-field and the experimental
floor of Spring-8, Figure 6. The measured data confirmed
the extreme "quietness" of the site even on the surface.
The uniformity of the substrate and the type of rock that
exists ensures the low amplitude vibration. However, no
further filtering of the free-field motion due to the ring
structure is observed because minimal impedance
difference exists between the supporting rock and the ring
material. As seen in Figure 6 the facility cultural vibration
dominates the motion on the experimental floor.Figure 5: Recorded PSD and rms displacement at APS
Figure 2: Recorded power spectra and RMS
displacements at selected sites depicting depth effects [2]
Figure 3: Coherence of motion at selected sites [2]
To further address the filtering effect of the structure and
enable the establishment of a relation that links the site
ground motion with the motion that matters most in the
context of light source facilities which is the ring and the
experimental floor motion, measurements were conducted
at the APS and Spring-8 facilities.
- : L..D~[K-I
Figure 6: PSD and rms displacements at SPring-8
This finding demonstrates that in establishing the actual
criteria for the operation of a sensitive facility one must
consider not only the green-field conditions and how
favorable they are but also the role of the substrate in
determining the vibration within the accelerator facility
that will result from the interaction with both the natural
environment and the self-induced cultural vibration.
NSLS II SITE AND STABILITY GOALS
Electron beam stability of the order of 0.3 microns in the
vertical direction is desired in the NSLS II. It is also
desired, if achievable, to have a stable electron beam in
the order of 0.1 micron in the vertical direction. To ensure
that the baseline criterion of 0.3 microns the ring floor
integrated rms displacement must remain below 25 nm for
the frequency band above 4 Hz where, as indicated earlier,
the motion is random and uncorrelated and therefore-'y
t.
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Simos,N.; Fallier, M. & Amick, H. Comparative Study of Vibration Stability at Operating Light Source Facilities and Lessons Learned in Achieving NSLS II Stability Goals, article, June 23, 2008; United States. (https://digital.library.unt.edu/ark:/67531/metadc895440/m1/4/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.