Narrow band and energy selectable plasma cathode for multistage laser wakefield acceleration

Stable, with a narrow energy band $<3%$, and energy selectable, from 1 to 40 MeV, electron beams with charges $\ensuremath{\sim}1\text{ }\text{ }\mathrm{pC}$ are produced from broadband electron bunches accelerated in the wakefield of femtosecond laser pulses with pulse energy $\ensuremath{\sim}0.4\text{ }\text{ }\mathrm{J}$. The beams are extracted with use of energy filtering based on a compact and controllable pulse-driven solenoid. Such a plasma cathode based on laser wakefield acceleration can be used as an injector for the multistage laser wakefield electron acceleration as well as for ultrafast electron imaging systems.