Collimated GeV attosecond electron-positron bunches from a plasma channel driven by 10 PW lasers

High-energy positrons and bright {\gamma}-ray sources are unique both for fundamental research and practical applications. However, GeV electron-positron pair jets and {\gamma}-ray flashes are still hardly produced in laboratories. Here we demonstrate that, by irradiating two 10 PW-scale laser pulses onto a near-critical density plasma channel, highly-directional GeV electron-positron pairs and bright {\gamma}-ray beams can be efficiently generated. Three-dimensional particle-in-cell simulations show that GeV positron jets show high density (8*10^21/cm^3), attosecond duration (400 as) and a divergence angle of 14{\deg}. Additionally, ultrabright (2*10^25 photons/s/mm2/mrad2/0.1%BW) collimated attosecond (370 as) {\gamma}-ray flashes with a laser energy conversion efficiency of 5.6% are emitted. Once realized in experiment, it may open up new possibilities for a wide variety of applications.