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.
