Low-Emittance Electron Bunches from a Laser-Plasma Accelerator Measured using Single-Shot X-Ray Spectroscopy

X-ray spectroscopy is used to obtain single-shot information on electron beam emittance in a low-energy-spread 0.5 GeV-class laser-plasma accelerator. Measurements of betatron radiation from 2 to 20 keV used a CCD and single-photon counting techniques. By matching x-ray spectra to betatron radiation models, the electron bunch radius inside the plasma is estimated to be $\sim 0.1\mu \mathrm{m}$. Combining this with simultaneous electron spectra, normalized transverse emittance is estimated to be as low as 0.1 mm mrad, consistent with three-dimensional particle-in-cell simulations. Correlations of the bunch radius with electron beam parameters are presented.

Figure 1

Setup for measurement of betatron x-rays from a LPA using a capillary-discharge waveguide plasma target.

Figure 2

Simultaneous single-shot electron energy distribution (top) and detected x-ray betatron spectrum (bottom). The detected x-ray spectrum (black; shaded region shows 1-$\sigma$ uncertainty) is matched by the angle-integrated theoretical spectrum, convolved with filters/detector response, for ${\sigma }_x=0.1\mu \mathrm{m}$ (red). Models for ${\sigma }_x=0.03$ (blue) or $0.3\mu \mathrm{m}$ (green) diverge from the data. Inset shows x-ray spectrum of the source, with instrument response extracted.

Figure 3

Binned detected x-ray betatron spectra (a)–(c) versus electron beam divergence. Shown are the detected spectrum (black; shaded region shows 1-$\sigma$ uncertainty) and angle-integrated theoretical spectra, convolved with filters/detector response. The best fit is identified. Best fit ${\sigma }_x$ versus divergence (d), with error bars at 95% confidence.