Radiatively trapped <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:mi>e</mml:mi></mml:mrow><mml:mo>±</mml:mo></mml:msup></mml:math> pair plasma as a laser-to-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">

An energy equilibrium arises when an electron-positron pair plasma is generated by the nonlinear Breit-Wheeler interaction of a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>γ</a:mi></a:math>-ray bunch with laser photons and becomes radiatively confined. Ordinarily, ponderomotive scattering prevents leptons from accessing the high-field regions where quantum electrodynamic (QED) effects dominate, but if quantum (stochastic) radiation reaction is sufficiently strong, a balance of acceleration and recoil forces emerges. This unique scenario enables leptons to continuously mediate the transformation of laser photons into directed <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>γ</b:mi></b:math>-rays via inverse Compton scattering as they copropagate with the field. Importantly, it is observed that the lepton energy stabilizes while the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mi>γ</c:mi></c:math> energy increases in direct proportion to laser energy depletion. The proposed scheme enables fundamental tests of QED, and is investigated here using the particle-in-cell method in full dimensionality, in addition to test-particle calculations. The per-species energy conversion efficiency is analyzed as a function of the initial laser and <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mi>γ</d:mi></d:math>-beam properties.