Ultra-low-emittance electron bunches from a laser-plasma accelerator measured using single shot X-ray spectroscopy*

Single-shot spectroscopic measurements of betatron X-rays are reported, and used to infer the transverse bunch size of both broadband sub-100 MeV and low-energy-spread 0.5 GeV electron beams produced by a laser-plasma accelerator. The measurements use semiconductor detector arrays, and spectra are obtained via single pixel absorption and cluster techniques. By matching the X-ray betatron spectra to analytical and numerical models of betatron radiation, the electron bunch radius inside the plasma is estimated to be $\sim$0.1$\mu$m. Combined with simultaneous electron spectrum and divergence measurements, the normalized transverse emittance is estimated to be as low as 0.1 mm mrad, consistent with three-dimensional particle-in cell simulations. This emittance is lower than previously measured, important for applications including gamma sources and colliders.