Generation of Highly-Collimated Pair-Plasma Electron, Positron, and Gamma-Ray Beams by Longitudinal Laser Fields*

An 'all-optical' set up for the acceleration of electron-positron (e^- e⁺) beams in pair plasma is investigated using 3D particle-in-cell (PIC) simulations.  By irradiating a pair plasma target with a highly focused ultra-relativistic laser, electrons and positrons are accelerated into beams in a similar mechanism to Linear Direct Acceleration (LDA) of electron-ion plasma.  Here, a pre-fabricated hollow micro-channel target acts as a wave-guide and stabilizes the acceleration process of electrons pulled into the channel.  Though, the set up can be sensitive to issues such as laser-channel alignment and pre-expansion of plasma into the vacuum channel.  These are largely avoided in the pair plasma set up where the vacuum channel is formed pondermotively by the laser as it propagates through the target.  Regardless, the LDA mechanism is similar in both; the vacuum channel acts as a wave-guide for the powerful laser, pulling bunches of particles from the walls and propelling them into the longitudinal laser electric field.  Their near speed-of-light forward motion, free of transverse oscillations, enables prolonged interaction with the accelerating region of the longitudinal electric laser fields.  The result, in our pair plasma simulation, are highly relativistic and collimated bunches of electrons and positrons that generate energetic and similarly directed beams of gamma-rays through synchrotron emission.