Ultralow-emittance measurement of high-quality electron beams from a laser wakefield accelerator.

By designing a cascaded laser wakefield accelerator, high-quality monoenergetic electron beams (<italic>e</italic> beams) with peak energies of 340–360 MeV and rms divergence of <0.3 mrad were produced. Based on this accelerator, the <italic>e-</italic>beam betatron radiation spectra were measured exactly via the single-photon counting technique to diagnose the <italic>e</italic>-beam transverse emittance in a single shot. The <italic>e</italic>-beam transverse size in the wakefield was estimated to be ∼0.35 <italic>μ</italic>m by comparing the measured X-ray spectra with the analytical model of synchrotron-like radiation. By combining the measured <italic>e</italic>-beam energy and divergence, the normalized transverse emittance was estimated to be as low as 56 <italic>μ</italic>m mrad and consistent with particle-in-cell simulations. These high-energy ultralow-emittance <italic>e</italic> beams hold great potential applications in developing free electron lasers and high-energy X-ray and gamma ray sources. [ABSTRACT FROM AUTHOR]