Molecular Dynamics Simulations of Temperature Relaxation Rates in Strongly Magnetized Antimatter Plasmas

Strong magnetization, defined by a gyrofrequency exceeding the plasma frequency, significantly modifies the temperature evolution of plasmas, such as those confined in antimatter traps at ALPHA. It is well known from previous studies of pure electron plasmas that the perpendicular energy becomes an adiabatic invariant. This leads to suppression of the temperature anisotropy relaxation rate, resulting in a prolonged time for temperature isotropization. A recent theoretical study on temperature evolution in strongly magnetized ion-electron plasmas [1] has shown that, in addition to the typical ion-electron temperature difference, temperature anisotropy also plays a crucial role in the temperature evolution. Here, we test theoretical predictions of this model using first-principles molecular dynamics (MD) simulations. The MD simulations employ two complementary methods: non-equilibrium MD, where a sudden ion-electron temperature difference is created; and equilibrium MD, which utilizes a novel Green-Kubo relation. The MD results agree well with theory. As predicted, strong magnetization enhances parallel energy exchange between ions and electrons, leading to rapid equilibration in the parallel direction. Perpendicular electron energy exchange is suppressed, as in pure electron plasmas. In contrast to pure electron plasmas, ion-electron collisions, in addition to electron-electron collisions, relax the perpendicular electron energy.

[1] Jose et al. Phys. Rev. E 111, 035201 (2025).