LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

L C L S C O N C E P T U A L D E S I G N R E P O R T
has had positive experience with it and less satisfactory experiences with concrete for support
structures.
To eliminate the diurnal temperature cycles associated with the ground, it is only necessary
for the piers to extend 1–2 feet below the concrete floor [13]. Given that the piers should be set
into the sandstone ground structure, the piers will extend about 60 cm below the floor level or
more as required to reach the sandstone. The piers will be isolated from the concrete floor.
Figure 8.24 Eccentric cam alignment system to support and position one end of the undulator
segment.
8.6.4 Thermal Considerations
Due to the low thermal expansion of titanium, it is estimated that if the tunnel temperature
changes by ±0.5°C, the resulting length change of the undulator segment will be on the order of
only ±15µm. This change would not have a significant impact on the output wavelength of the
undulator. The temperature variation in the strength of the undulator magnetic field due to the
reduction in the remanent field of the permanent magnets at higher temperatures would have an
effect, however. The remanence of the NdFeB magnets decreases by about 0.1% per °C. This
results in a decrease in the on-axis undulator field strength of about 0.054% per °C. To
compensate for this effect, the magnetic gap can be reduced due to differential thermal expansion
as the temperature increases. Magnetic calculations show that to compensate for a 1°C
temperature rise, the decrease in pole gap needs to be on the order of 8.6×10 -4 of the gap. To
accomplish this, an aluminum plate was placed between the titanium core of the undulator
segment and the magnet/pole assemblies. The predicted decrease in pole gap for the model is
approximately 8.26×10 -4 , or almost the ideal value.
8.7 Permanent Magnet Quadrupoles
The FODO lattice of the undulator line incorporates permanent magnet quadrupoles. No
provision is planned for adjusting the strength of the quadrupole, so magnetic tuning and
U N D U L A T O R ♦ 8-37