  We report new experimental results obtained on three different laser
facilities that show directed laser-driven relativistic electron-positron jets
with up to 30 times larger yields than previously obtained and a quadratic (~
E^2) dependence of the positron yield on the laser energy. This favorable
scaling stems from a combination of higher energy electrons due to increased
laser intensity and the recirculation of MeV electrons in the mm-thick target.
Based on this scaling, first principles simulations predict the possibility of
using such electron-positron jets, produced at upcoming high-energy laser
facilities, to probe the physics of relativistic collisionless shocks in the
laboratory.
