Non-invasive characterization of transverse beam emittance of electrons   from a laser-plasma wakefield accelerator in the bubble regime using betatron   x-ray radiation
  We propose and use a technique to measure the transverse emittance of a laser-wakefield accelerated beam of relativistic electrons. The technique is based on the simultaneous measurements of the electron beam divergence given by $v_{\perp}/v_{\parallel}$, the measured longitudinal spectrum $\gamma_\parallel$ and the transverse electron bunch size in the bubble $r_{\perp}$. The latter is obtained via the measurement of the source size of the x-rays emitted by the accelerating electron bunch in the bubble. We measure a \textit{normalised} RMS beam transverse emittance $<0.5$ $\pi$ mm$\:$mrad as an upper limit for a spatially gaussian, spectrally quasi-monoenergetic electron beam with 230 MeV energy in agreement with numerical modeling and analytic theory in the bubble regime. 
S. Kneip, C. McGuffey, J. L. Martins, M. S. Bloom, V. Chvykov, F.   Dollar, R. Fonseca, S. Jolly, G. Kalintchenko, K. Krushe...
physics.plasm-ph