  A controllable injection scheme is key to producing high quality laser-driven
electron beams and X-rays. Self-injection is the most straightforward scheme
leading to high current and peak energies, but is susceptible to variations in
laser parameters and target characteristics. In this work improved control of
electron self-injection in the nonlinear cavity regime using two laser-pulses
propagating in tandem is investigated. In particular the advantages of the
tandem-pulse scheme in terms of injection threshold, electron energy and beam
properties in a regime relevant to betatron radiation are demonstrated.
Moreover it is shown that the laser power threshold for electron self-injection
can be reduced by up to a factor of two compared to the standard, single-pulse
wakefield scheme.
