Speaker
Description
The pressure tensor in a collisionless, magnetised plasma is anisotropic with respect to the guide magnetic field. We derive evolution equations of the perpendicular and parallel pressure components for the case of a relativistically hot plasma. When a plasma is subject to a mean perpendicular flow, these reduce to double-adiabatic laws, with adiabatic indices dependent on the temperature and pressure anisotropy. For example, in the case of an ultra-relativistic plasma close to isotropy, our theory predicts the perpendicular and parallel pressure components evolve with adiabatic indices equal to 8/5 and 4/5, respectively. This differs from the non-relativistic result, whereby these values should be 2 and 1 independently of anisotropy. Our analytic picture is numerically confirmed using two-dimensional particle-in-cell simulations of a plasma in a compressing box.