We have experimentally realized a scheme to enhance betatron radiation by manipulating transverse oscillation of electrons in a laser-driven plasma wakefield with a tilted shock front (TSF). Very brilliant betatron x-rays have been produced with significant enhancement both in photon yield and peak energy but almost maintain the e-beam energy spread and charge. Particle-in-cell simulations indicate that the accelerated electron beam (e beam) can acquire a very large transverse oscillation amplitude with an increase in more than 10-fold, after being steered into the deflected wakefield due to the refraction of the driving laser at the TSF. Spectral broadening of betatron radiation can be suppressed owing to the small variation in the peak energy of the low-energy-spread e beam in a plasma wiggler regime. It is demonstrated that the e-beam generation, refracting, and wiggling can act as a whole to realize the concurrence of monoenergetic e beams and bright x-rays in a compact laser-wakefield accelerator.
By designing a cascaded laser wakefield accelerator, high-quality monoenergetic electron beams (e beams) with peak energies of 340-360 MeV and rms divergence of <0.3 mrad were produced. Based on this accelerator, the e-beam betatron radiation spectra were measured exactly via the single-photon counting technique to diagnose the e-beam transverse emittance in a single shot. The e-beam transverse size in the wakefield was estimated to be ~0.35 m by comparing the measured x-ray spectra with the analytical model of synchrotron-like radiation. By combining the measured e-beam energy and divergence, the normalized transverse emittance was estimated to be as low as 56 m mrad and consistent with particle-in-cell simulations. These high-energy ultralow-emittance e beams hold great potential applications in developing free electron lasers and high-energy x-ray and gamma ray sources.
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