  We propose a novel method for changing the length of laser wakefield electron
acceleration in a gas jet by a cylindrical blast wave created by a
perpendicularly focused nanosecond laser pulse. The shock front destroys the
wake thus stopping interaction between the laser pulse and accelerated electron
bunch allowing one to directly control the interaction length and avoid
dephasing. It also improves the electron beam quality through the plasma
lensing effect between the two shock fronts. We demonstrated both
experimentally and numerically how this approach can be used to form
quasi-monoenergetic electron bunch with controlled energy and improved
divergence as well as to track changes in the bunch parameters during
acceleration.
