Figure 4
Plot of extracted fast electron energy against incident laser a0. Black triangles indicate EPOCH simulations for which no positrons were measured while the black crosses indicate equivalent OSIRIS simulations, the red line is a fit to both sets of data corresponding to \(T\propto {a}_{0}^{2.18\pm 0.18}\). The blue asterisks indicate results from EPOCH simulations for which positrons were detected. The green line is a fit corresponding to \(T\propto {a}_{0}^{5.12\pm 0.34}\). Filled-in black circles indicate the measured number density of positrons in the EPOCH simulations for which positrons were detected. There is a clear correlation between the onset of pair production and the shift in energy scaling. The region of interest for calculating the positron number density began at the point of laser focus (the boundary between the bulk plasma and skin layer), and extended 0.2 μm into the bulk plasma. Inset is a plot copmaring the fast electron energy in simulations where the EPOCH QED package was turned on (blue asterisks and green line), and off (black triangles and red line).
