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Free Electron Laser are coherent sources of radiation based on the
interaction of a relativistic electron beam in an undulator field.
According to the energy of the accelerator, they presently cover a wide
spectral range, from the infra-red to the VUV. FELs combine the
diagnostics of typical laser systems (for the measurement of spectral and
temporal characteristics, the transverse mode pattern, the polarisation)
and the diagnostics of relativistic electron beams. The electron beam is
characterised in order to evaluate and control the FEL performances, but
also in order to measure the effect of the FEL on the electron beam. The
FEL characteristics are monitored with various types of detectors,
depending mainly on the spectral range. Diagnostics for Linac based Infra
Red FELs and storage ring FELs in the UV-VUV will be described.
Particular instrumentation, required for FEL operation, such as the
optical resonator, possible diagnostics inside the undulator will also be
analysed.
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