  Free electron lasers (FEL) require strict time relations for the electron
bunch and RF field interaction, which must be precise and deterministic in
time. This is guaranteed by the timing and synchronization systems that should
be robust under the situations like a power cycle in the master oscillator,
timing master, reference frequency distribution devices or low-level radio
frequency (LLRF) devices. After the power cycles, the time relations should be
kept or be capable to recover quickly to improve the availability of the FEL
machine. This article focuses on the robustness of the timing and
synchronization systems, such as the time uncertainty of the RF pulse related
with the trigger, the phase uncertainty of frequency dividers after power
cycles and the race condition between the trigger and clock. The possible
solutions to achieve a robust design of the timing and synchronization systems
are also discussed.
