(color online) Schematics of two circularly polarized laser pulses colliding in a cylinder channel. Electrons are firstly extracted out of the cylinder inner wall and accelerated to high energy by the laser pulse (a), and then radiate high energy gamma photons in the counter-propagating pulse via the Compton back-scattering process (b). The radiated photons further interact with the laser photons to generate copious positrons (c).