The dynamics of electron injection from a shock front under a laser condition of a0 > 3 and tight focusing (FWHM < 10 µm) laser condition has been studied by numerical simulations. Compared to a regular longitudinal shockfront injection, the transverse injection starts near the edge of the bubble with a narrow energy spread of < 13 MeV. The trajectories of the transverse injected electrons are more coherent than the longitudinal injection. By applying the tilted shock front, the betatron oscillation amplitude is significantly larger than the un-tilted shock front. The enhancement of the betatron radiation brightness has been observed.
We propose a table-top linearly polarized hard X-ray source by using a tilted shock-front injection in a laser wakefield accelerator (LWFA) to achieve comprehensive control of both polarization and energy of X-ray. By using shock-front injection, the electron bunches are injected during a sharp transition of plasma density. The length of density transition is significantly shorter than the plasma wavelength and offers a highly localized injection. In regular injection methods, such as self and ionization injection, the majority of electrons are injected radially symmetrically. Particle-in-cell (PIC) simulations show the tilted shock front breaks radial symmetry of injection and creates coherent in-plane oscillation of electrons. The coherence of electron bunches is maximized around 30 degrees which leads to a linearly polarized betatron radiation. The polarization of the resulting X-ray is analyzed by Bragg diffraction after collimation by a polycapillary lens.
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