By designing a cascaded laser wakefield accelerator, high-quality monoenergetic electron beams (e beams) with peak energies of 340–360MeV 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 lm 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 um 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.
- cascaded laser wakefield accelerator
- emittance measurement
- electron beams
- single photon counting