On pair plasma instability due to pressure gradients in homogeneous magnetic fields*

With the advent of laboratory experiments on collective effects in electron-positron plasmas, theoretical prediction of their stability properties becomes increasingly relevant. Prior work without compressional magnetic fluctuations [Helander \& Connor, JPP 82, 905820301 (2016)] predicted complete stability of pair plasmas to density or temperature gradients in a homogeneous magnetic guide field. Here, it is shown that the inclusion of such fluctuations produces a Gradient-driven Drift Coupling (GDC) instability [Pueschel et al., PoP 22, 062105 (2015)] also seen in helium plasma experiments [Pueschel et al., PPCF 59, 024006 (2017)]. An analytical growth rate expression applicable to a wide range of plasma parameters is derived, and a subdominant, finite-$k_z$ GDC is discussed. Overall, the GDC is shown to have a potential impact on systems ranging from Gamma Ray Bursts to magnetic confinement experiments like APEX, to laser-based setups. In all of these configurations, GDC growth times are much shorter than plasma lifetimes.