Ultra-low emittance electron beams from two-color laser-ionization injection

In this paper we discuss how we used two laser pulses of different colors to generate femtosecond, ultralow emittance (∼10–2 mm mrad) electron beams in a laser-driven plasma-based accelerator. A long-wavelength pump pulse was used to excite a large wake without fully ionizing a high-Z gas. A short-wavelength injection pulse, with a small ponderomotive force and large peak electric field, ionizes a fraction of the remaining bound electrons into trapped wake orbits with a small quiver momentum. Two-dimensional (2D) Particle-In-Cell (PIC) simulations show that the transverse emittance of the injected beam can be an order of magnitude smaller than previously achieved in laser-plasma accelerators.