Characterization of the source size and transverse emittance of a laser wakefield accelerated electron beam by means of inverse-Compton-scattering cross-correlation

A recent focus in the development of laser wakefield electron accelerators (LWFA) has been the control of electron injection by ionization of inner shell electrons directly inside the accelerating structure [1]. Since electrons that are injected near the threshold of ionization are injected on axis, the resulting electron beams are expected to have lower divergence, as has been observed experimentally [2-5]; and thus they should also have lower transverse emittance. We used a technique based on inverse-Compton-scattering cross-correlation to measure the source size and normalized transverse emittance of an ionization-injected, laser-wakefield-accelerated electron beam. This is to our knowledge the first time a LWFA electron beam has been characterized by this means. \\[4pt] [1] A. Pak, et al., Phys. Rev. Lett. \textbf{104}, 025003 (2010).\\[0pt] [2] C. McGuffey, et al., Phys. Rev. Lett.\textbf{ 104}, 025004 (2010).\\[0pt] [3] J. S.Liu, et al., Phys. Rev. Lett.\textbf{ 104}, 035001 (2011).\\[0pt] [4] Y.-C. Ho, et al., Phys. of Plasmas, \textbf{18},063102 (2011).\\[0pt] [5] M. Chen, et al., Phys. of Plasmas, \textbf{19}, 033101 (2012).