Pressure gradients in uniform magnetic fields induce unexpected instabilities in pair plasmas

Electron-positron pair plasmas are a class of plasmas with equal masses of matter and antimatter. Found mainly in energetic astrophysical settings and specially tailored experiments, these plasmas are generally thought to be stable under pressure gradients in homogeneous magnetic fields. However, Pueschel et al. demonstrated through simulations that electron-positron pair plasmas can develop instabilities and turbulence when physical conditions inject them into a magnetic field.

They found when an electron-positron pair plasma has a pressure gradient within a homogeneous magnetic field, the gradient will excite instability and turbulence within the plasma. As the electric and magnetic fields couple, turbulent transport of heat and particles arises, until a magnetic field gradient is established and suppresses the instability.

The emergence of turbulence has great impacts on plasma applications.

“For instance, if fusion plasmas were stable, we might have had all our energy from fusion reactors since the 1970s, and there would be no climate crisis,” said author M.J. Pueschel. “Some theories of gamma-ray bursts, which are among the most energetic events in the universe aside from the Big Bang, rely on expanding pair plasma shells, and if the shells individually become turbulent, that could conceivably change the gamma-ray spectrum.”

Using particle-in-cell and continuum simulations, the group was able to study this process in systems relevant to astrophysical and laboratory-based phenomena. With their results, they formulated a mathematically accessible fluid model that captures the instability dynamics in electron-positron pair plasmas.

The current model is sufficient for studying magnetic confinement, though a relativistic extension of the calculation is necessary in order to better apply findings to gamma-ray bursts and laser-based electron-positron experiments.

Source: “Pair plasma instability in homogeneous magnetic guide fields,” by M.J. Pueschel, R.D. Sydora, P.W. Terry, B. Tyburska-Pueschel, M. Francisquez, F. Jenko, and B. Zhu, Physics of Plasmas (2020). The article can be accessed at https://doi.org/10.1063/5.0020234 .