Effects of the channel radius on the direct laser acceleration of positrons

Plasma acceleration has been lately considered to become an auspicious technology for building a future multi-TeV electron-positron collider, leading to higher compactness of the device. Self-generated fields from laser-plasma interaction are, however, in contrast to electrons, usually not well-suited for positron focusing and on-axis guiding. In addition, an external positron source is required. Here, we study the method of direct laser acceleration of positrons. The positron generation is assured by an orthogonal collision of a multi-PW laser pulse and a GeV electron beam by the nonlinear Breit-Wheeler process. The acceleration subsequently takes place in a preformed plasma channel with a finite (tens-of-microns-long) radius. In this work, we examine how the choice of channel radius influences the process of acceleration. We show that this scheme is robust regarding the radius size. A significant number of the positrons is kept near the propagation axis, even if the channel radius was increased by almost 100 µm. The mechanism was examined by quasi-3D particle-in-cell simulation carried out with the OSIRIS framework.