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
Figure 5.16 Power distribution along the bunch at saturation (solid line), and at a distance 100 m
after saturation (dotted line). The dashed line depicts the current profile in units of kA.
The head of the bunch is to the right.
5.5 Sources of Gain Reduction
5.5.1 Undulator Trajectory Errors
This section discusses the sensitivity of LCLS FEL performance on imperfections of the
electron orbit in the undulator.
5.5.1.1 Undulator Steering and Corrector Description
As shown in detail in Chapter 8, the undulator is designed as a planar NdFeB hybrid
structure with a period of 3 cm and a full gap height of 6 mm. 113 undulator periods form a
3.375-m long segment (Spacing between the centers of the first and last pole). Segments are
separated by breaks that accommodate electron beam position monitors as well as 5-cm long
permanent magnet quadrupoles with a gradient of about 107 T/m. The quadrupoles are used for
two purposes, electron beam focusing and steering. Steering is achieved by adjusting the
quadrupoles’ x and y positions with stepper-motor based systems that allow a total movement of
0.5 mm with a step size of 1 µm. The undulator is built from 33 segments resulting in a total
length of about 121 m, of which about 112.86 m is magnet length.
5.5.1.2 Magnetic Field Errors
The sources of magnetic field errors in the LCLS undulator are from misaligned quadrupoles,
undulator pole errors, the earth field, and other stray fields. The misalignment of the quadrupoles
has been strongly reduced by design. As described above, their transverse position can be
remotely adjusted and is used for beam steering. The finite resolution of the movers is
compensated by auxillary horizontal and vertical steering magnets. The effect of the earth field is
small and will be corrected by beam-based-alignment. Stray fields will be avoided or minimized
5-18♦ F E L P A R A M E T E R S A N D P E R F O R M A N C E