ALL-OPTICAL FREE-ELECTRON LASERS USING TRAVELING-WAVE THOMSON-SCATTERING

Traveling-Wave Thomson-Scattering (TWTS) [1–3] is a scheme for the realization of compact optical undulators. TWTS utilizes high-power, short-pulse lasers providing submm undulator periods. These allow for operation of an X-ray light source at A wavelength with only a few hundred MeV electron energy. Here, we show that by carefully controlling dispersion of a TWTS laser pulse it is possible to achieve free-electron laser (FEL) operation, creating a TWTS optical FEL (OFEL). The electron energies needed for X-ray radiation are an order of magnitude smaller compared to existing hard-X-ray FELs requiring several GeV [4, 5]. Moreover, with sub-mm undulator wavelengths the FEL process saturates in sub-m interaction distances removing the need for electron beam focusing during the interaction. Using a laser undulator further has the advantage that no material is required for producing or containing the undulator field which become a source of unwanted wakefields when aiming for compact setups. In Traveling-Wave Thomson-Scattering the interaction distances needed to drive the FEL process until saturation are achieved in two steps. First, by choosing a side-scattering geometry, where laser and electron direction of propagation enclose the interaction angle φ. Second, by tilting the laser pulse-front by half of the interaction angle αtilt = φ/2 for full spatial overlap of the electron bunch with the laser pulse during the entire interaction (Fig. 1). Thereby, the maximum number of undulator periods experienced by the electrons becomes independent of the laser pulse duration which is the limit for head-on Thomson scattering [6,7]. With TWTS the interaction distance is limited by the laser pulse width and thus by the available laser power. The variability of TWTS with respect to the interaction angle φ gives control over the scattered wavelength λFEL independent of the available electron energy Ee = γ0mc and the laser wavelength λ0: