Retention of Attosecond Pulse Structure in an Hhg Seeded Fel Amplifier

High-gain free-electron lasers (FELs) operating at short wavelengths have the potential to gather incredibly detailed information on how matter interacts and arranges itself at the atomic and molecular scales [1]. If high brightness pulses of sufficiently short duration ( 10 − 10s) and power can be generated, it may also be possible to capture ephemeral events, such as molecular bond formation, without temporal blurring. While HHG delivers trains of attosecond pulses, they currently lack the peak powers available from FELs. Several techniques to achieve such pulse durations have been proposed (see e.g. [2, 3, 4] and references therein) where the widths of the pulses generated are of the order of the coherence length lc = λr/4πρ, where λr is the resonant wavelength and ρ is the fundamental FEL parameter [5]. A different technique, that applies the concepts from mode-locked cavity lasers, has also been proposed [6] that may generate a train of pulses with lengths less than lc. In this technique, a series of spatiotemporal shifts are introduced between the radiation and the co-propagating electron bunch that define a set of axial radiation modes which couple via the FEL interaction. The spatiotemporal shifts are achieved by delaying the electron bunch using magnetic chicanes inserted between undulator modules. By introducing a modulation at the mode spacing, each mode develops sidebands that can overlap neighbouring modes, allowing mode-locking to occur. This is analogous with mode-locking in conventional cavity lasers. The spectrum generated by the XUV mode-coupled SASE system of [6] has a modal structure with modespacing ∆ω similar to the spacing of harmonics in a HHG source. Equivalently, the temporal structure of the two systems are a comb of attosecond pulses. By matching the spectral/temporal structures of a mode-coupled FEL amplifier to an HHG seed, it may therefore be possible to amplify the HHG seed to saturation while retaining its attosecElectron delay