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Simulation of Laser-Produced Light Sources

X-ray Sources

There is a growing demand for bright, ultrashort X-ray sources with femtosecond (fs) duration. Those provide insight into ultrafast, time-resolved structural dynamics of materials, such as chemical reactions, phase transitions, fs triggered crystal lattice dynamics, etc. Betatron radiation and Thomson/Compton scattering based on relativistic electron beams generated from laser plasma acceleration promise to provide such tabletop X-ray sources. Betatron radiation produces X-rays through transverse oscillation of the electrons in a laser wakefield acceleration process. Experiments demonstrated the production of ~100 keV, fs X-rays with peak brightness up to 1022 photons/(s mm2 mrad2 0.1% bandwidth). For Thomson/Compton scattering, high density electron beams from laser wakefield accelerators reflect a counter-propagating optical-frequency laser pulse. The frequency of the latter can be upshifted by 4γ2 due to the double relativistic Doppler effect, where γ is the Lorentz factor of the beam. A recent experiment demonstrated hundreds of keV, fs X-rays with peak brightness of 1021 photons/(s mm2 mrad2 0.1% bandwidth).

To explain the experimental observation, Particle-in-Cell (PIC) simulations are crucial. PIC simulations can reproduce the whole process from high energy electron generation by laser plasma interaction to the X-ray emission via the movement of the generated electrons in plasma fields and laser fields. On the other hand, simulations can test novel schemes to optimize the X-ray generation before experiments are performed. The figure is a typical result of a PIC simulation, showing the distribution of high energy electrons produced from a cluster plasma irradiated by a relativistic intense laser pulse.

Result from a PIC simulationResult from a PIC simulation: 2D snapshots of the electron density distribution of the gas (a) and clusters (c) at time t = 140 T_0 (T_0 is the laser optical cycle). (e) shows the electrical field in the interaction regime at the same time for the cluster target. The spatial distribution of the accelerated electrons are shown for the case with a gas target with energy >10 MeV (b) and clusters with energy >10 MeV (d) as well as >20 MeV (f)

Chen, L.M. et al. Bright betatron X-ray radiation from a laser-driven-clustering gas target. Sci. Rep. 3, 1912; DOI:10.1038/srep01912 (2013)