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
respect to each other by √2×50 µm ≈ 70 µm rms. Over longer distances the relative misalignment
increases.
The 1-µm BPM resolution listed is the net rms measurement resolution over many pulses.
Averaging can also be used to get better resolution (i.e. 100 pulses are saved at each energy, the
BPM single-pulse resolution can be closer to 10 µm). The BPM offsets must, however, be
constant to a level of ~1 µm over the few hour period during which the energy is being scanned.
This implies, for example, adequate temperature stability for the BPM electronics, etc. The
various ‘calibration’ errors in the table imply that the BPMs (or magnet movers) are misscaled so
that, for example, an actual displacement of 100 µm will read back as 110 µm. The ‘incoming
trajectory bias’ is a static (constant) beam launch error which is ten times that of the rms beam
size in both position and angle (~10×30 µm and ~10×1.5 µrad). The ‘incoming orbit jitter’ is a
randomly varying launch position and angle error, which occurs during the energy-scan data
acquisition. The simulation shown here includes no orbit jitter, but in fact the results are
insensitive up to a 10% rms launch jitter. The jitter can actually be reduced even further in
practice, to a level of a few percent, by acquiring ~100 orbits and using the 8-10 pre-undulator
BPMs to select only those orbits which produce a constant mean trajectory launch. In addition,
the variable trajectory can be included as additional fit parameters; an option which was found to
be unnecessary in these simulations.
A mover reproducibility error has also been studied [58], which provides a small random
mechanical error on the final position of the quadrupole magnet mover. This effect has been
ignored here since small dipole steering coils will be used to augment the magnet movers and
provide a very fine vernier steering control. The quadrupole magnet movers are then controlled to
a level of a few microns, and the steering coils are used for smaller corrections. Without the
steering coils the magnet movers would require a mechanical reproducibility precision of