LCLS Conceptual Design Report - Stanford Synchrotron Radiation ...

8.1 Overview
8.1.1 Introduction
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
In a single pass FEL (free electron laser) operating in the SASE (self-amplified spontaneous
emission) regime, exponential gain of the coherent radiation intensity and saturation after about
twenty power gain lengths are predicted by theory (see Chapter 4). An FEL operating at
saturation will have a more stable radiation output. Therefore, a goal in the design of the
undulator line is to allow saturation to be reached while minimizing the required undulator length.
Minimizing the undulator length helped guide many of the parameter choices for the undulator
line and was also used in allocating error tolerances. Sufficient diagnostics must be included
between undulator segments to effectively and conveniently monitor the electron and x-ray beams
along the length of the undulator line. These diagnostics will be used as an aid in electron beam
tuning, to identify problems, to monitor the intensity gain in the x-ray beam, and to confirm that
saturation has been achieved.
The basis for choice of parameters for the undulator line is explained in Section 8.2. A
tolerance budget for trajectory straightness through the undulator segments, phase errors, and
positioning errors is also given in that section. Section 8.3 gives requirements for the magnetic
measurement of the undulator segments. Considerations to be used in the measurement and
sorting of the magnet blocks for the undulator segments are given in Section 8.4. Considerations
in the choice of the grade of NdFeB magnets are explained in Section 8.5.1, and a magnetic
design is given in Section 8.5.2, along with results from the magnetic modeling calculations.
Some considerations of the end tuning for the undulator segments are given in Section 8.5.3.
Section 8.6 shows the mechanical design for the undulator segments, including the scheme for
holding the magnets and poles (see Section 8.6.1), the provisions included for magnetic tuning,
both through the main part of the undulator segment and through the ends (see Section 8.6.2), the
supports for the undulator segments that also provide for overall position adjustment (see Section
8.6.3), and the impact of temperature changes on the undulator segments (see Section 8.6.4). The
design for the permanent magnet quadrupoles is given in Section 8.7. The effect of missteering of
the electron beam so that it strikes the vacuum chamber and subsequently the undulator segments,
and the possibility of using collimators are considered in Section 8.8.1, along with possibilities
for reducing the roughness of the inside of the vacuum chamber. A description of the vacuum
system is in Section 8.8.2, pumping and outgassing considerations are given in Section 8.8.3, and
thermal considerations are in Section 8.8.4. Section 8.9 discusses wakefield sources in the
undulator vacuum chamber. Section 8.9.5 considers the effect of the roughness of the inside
surface of the vacuum chamber on the wakefields and concludes that the smoothness
requirements can be met. Section 8.10 considers the means by which ions could be produced and
the effect that those ions could have on the beam, concluding that the necessary vacuum is readily
achievable. The electron beam diagnostics include pickup electrode beam position monitors,
Cherenkov detectors, optical transition radiation imagers, wire scanners, and current monitors, as
described in Section 8.11. Section 8.12 describes the beam-based alignment scheme, including
8-2 ♦ U N D U L A T O R