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Relativistic electron–positron (pair) plasmas play a central role in extreme astrophysical environments, including gamma-ray bursts and pulsar winds, yet their microphysics remains poorly constrained by observations. The Fireball programme generates dense, relativistic electron–positron beams in the laboratory, enabling controlled studies of pair-plasma instabilities and particle acceleration under astrophysically relevant conditions.

Fireball is hosted at CERN’s High Radiation Materials facility (HiRadMat) and is currently the only scheme capable of producing relativistic pair plasmas terrestrially. In this talk, we present results from the two most recent experimental campaigns, Fireball II and Fireball III. Fireball II delivered the first experimental measurements of the magnetic-field amplification associated with beam plasma instabilities involving a relativistic pair jet. Fireball III revealed significant modifications to the longitudinal beam profile and energy spectrum, consistent with the acceleration and deceleration of electrons and positrons during the beam–plasma interaction.

These results provide new experimental constraints on kinetic processes thought to operate in extreme astrophysical sources and demonstrate the potential of laboratory pair plasmas as a platform for testing relativistic plasma theory.