  In simulations of a 10PW laser striking a solid we demonstrate the
possibility of producing a pure electron-positron plasma by the same processes
as those thought to operate in high-energy astrophysical environments. A
maximum positron density of 10^26/m^3 is achieved, seven orders of magnitude
greater than achieved in previous experiments. Additionally, 35% of the laser
energy is converted to a burst of gamma-rays of intensity 10^22W/cm^2,
potentially the most intense gamma-ray source available in the laboratory. This
absorption results in a strong feedback between both pair and gamma-ray
production and classical plasma physics in the new `QED-plasma' regime.
