Low emittance electron beam generation from a laser wakefield   accelerator using two laser pulses with different wavelengths
  Ionization injection triggered by short wavelength laser pulses inside a nonlinear wakefield driven by a longer wavelength laser is examined via multi-dimensional particle-in-cell simulations. We find that very bright electron beams can be generated through this two-color scheme in either collinear propagating or transverse colliding geometry. For a fixed laser intensity $I$, lasers with longer/shorter wavelength $\lambda$ have larger/smaller ponderomotive potential ($\propto I \lambda^2$). The two color scheme utilizes this property to separate the injection process from the wakefield excitation process. Very strong wakes can be generated at relatively low laser intensities by using a longer wavelength laser driver (e.g. a $10 \micro\meter$ CO$_2$ laser) due to its very large ponderomotive potential. On the other hand, short wavelength laser can produce electrons with very small residual momenta ($p_\perp\sim a_0\sim \sqrt{I}\lambda$) inside the wake, leading to electron beams with very small normalized emittances (tens of $\nano\meter$). Using particle-in-cell simulations we show that a $\sim10 \femto\second$ electron beam with $\sim4 \pico\coulomb$ of charge and a normalized emittance of $\sim 50 \nano\meter$ can be generated by combining a 10 $\micro\meter $ driving laser with a 400 $\nano\meter$ injection laser, which is an improvement of more than one order of magnitude compared to the typical results obtained when a single wavelength laser used for both the wake formation and ionization injection. 
X. L. Xu (1), Y. P. Wu (1), C. J. Zhang (1), F. Li (1), Y. Wan (1), J.   F. Hua (1), C.-H. Pai (1), W. Lu (1,2), P. Yu (2), ...
physics.acc-ph physics.plasm-ph