Abstract
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.
Highlights
Significant progress has been made in the field of plasma-based acceleration [1]
Such low slice energy spread may have significant advantages in applications relevant to future coherent light sources driven by plasma accelerators
In this paper we have examined ionization injection into plasma wave wakes, where both the wake and the injection are driven by lasers
Summary
Significant progress has been made in the field of plasma-based acceleration [1]. Our PIC simulation results show that it is possible to generate electron beams containing ∼4 pC of charge with small normalized emittances (∼50 nm), more than 1 order of magnitude smaller than that obtained in ionization injection using a single 800 nm wavelength laser. This indicates that it is difficult to further reduce the emittance by just fine-tuning the laser parameters of the single laser pulse for ionization injection In this simulation the ðδψÞmin is close to −1, only electrons with large initial transverse positions which lead to large transverse momepntaffiffiffiwffiffiffiffihffiffieffiffinffiffiffiffiffitffihffiffieffiffiyffiffiffiffifficross the z axis are injected (δψ < −1 þ 1 þ p2⊥=m2c2=γφ) [14,34] and form a phase space ring structure in 2D simulations. The injected charge is about 20 pC by assuming a cylindrical distribution around the z axis. (c) The x − px phase spaces at z 1⁄4 1 mm for the laser polarized in (left) and out (right) of the simulation plane
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