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Beamline description


Overview and working principle

The standard process leading to CHG using FELs is based on the frequency up-conversion of a high-power laser pulse (the so-called seed laser) using a relativistic electron bunch as gain medium. The process is based on the layout modulator - dispersive section - radiator and the layout employed is sketched here:

The laser pulse is focused into a first undulator, called modulator so as to overlap (both in space and time) an incoming electron bunch. The interaction between the seed laser and the electron beam modulates the electron beam energy. Then, the beam passes through a magnetic chicane (referred in the following as the dispersive section) where the energy modulation is converted into a spatial modulation of longitudinal electron density into micro-bunches, with a periodicity equal to the seed wavelength and its higher order harmonics.

At the exit of the dispersive section, the beam is injected into a second undulator, called radiator, which is tuned at one of the seed harmonics. The micro-bunched electrons radiate coherently and the extracted power is proportional to the square of number of modulated electrons. If the radiator is sufficiently long, the amplification continues until saturation is reached. The radiation inherits both the properties of coherence and the temporal duration of the seed. 

Starting from a UV seed pulse permits the generation of coherent radiation in the VUV and soft X-ray ranges, offering the possibility to overcome the ionization potential of many materials and chemical species.


Ultima modifica il Mercoledì, 12 Dicembre 2012 13:43