Numerical study on hydrodynamic and quasi-neutral approximations for collisionless two-species plasmas

We present a numerical study aimed at assessing the validity of certain approximations, chiefly the fluid and quasi-neutral ones, generally used in the theoretical and computational study of fusion plasmas. The impact of numerical artifacts has been minimised by using the very precise weighted essentially non-oscillatory (WENO) method, which can be applied to both kinetic and fluid simulations. This is the first application of WENO schemes to Eulerian simulations of Vlasov plasmas. The fluid modelling of plasmas appears adequate even when collisions are negligible, provided certain relationships hold between the characteristic speed of the phenomenon under consideration and the thermal velocities of the species which make up the plasma. The breakdown of this behaviour is probably caused by non-linear Landau damping; it appears linked with filamentation, which suggests that the magnitude of the departure from Maxwellian equilibrium may not be the one and only measurement of non-fluid behaviour. As for quasi-neutrality, our study suggests that the generally accepted criteria should be used with great care, according to the degree of neutrality expected.