All-optical GeV electron bunch generation in a laser-plasma accelerator via truncated-channel injection

We describe a simple scheme, truncated-channel injection, to inject electronsdirectly into the wakefield driven by a drive pulse guided by an all-opticalplasma channel. We use this approach to generate dark-current-free 1.2 GeV, 4.5% relative energy spread electron bunches with 120 TW laser pulses guided in a110-mm-long hydrodynamic optical-field-ionized (HOFI) plasma channel. Ourexperiments and particle-in-cell simulations show that high-quality electronbunches were only obtained when the drive pulse was closely aligned with thechannel axis, and was focused close to the density down-ramp formed at thechannel entrance. Start-to-end simulations of the channel formation, andelectron injection and acceleration show that increasing the channel length to410 mm would yield 3.65 GeV bunches, with a slice energy spread $\sim 5 \times10^{-4}$.