Suppression of pair beam instabilities in a laboratory analogue of blazar jets*

The laboratory realization of dense, quasi-neutral electron-positron pair beams has been a decades-long pursuit, as a means to test theories of plasma instabilities fundamental to understanding the emission from Gamma-ray bursts and the jets of Active Galactic Nuclei. Experiments become possible for the first time due to a recent breakthrough demonstrating that dense pair beams can be produced using 440 GeV/c ultra-relativistic protons extracted from the Super Proton Synchrotron accelerator at CERN [1]. In the first application of this experimental platform, the stability of the pair beam is studied as it propagates through a metre-length plasma [2]. Theory predicts that non-idealized beam conditions such as finite thermal spread can lead to dramatic stabilization of the beam, which is particularly relevant to astrophysical pair beams where conditions are often far from idealized. We demonstrate experimentally that the growth of pair beam instability can be significantly suppressed when the finite thermal spread of pairs is accounted for and we discuss the implications for observations of blazar gamma-ray spectra.