2025 , Volume 30, № 2, p.5-18

The research addresses numerical modelling for the process of the interaction of electromagnetic radiation with a spatially inhomogeneous plasma in the case of low frequency of electron collisions when there is a plasma resonance region. In order to compare the numerical results with analytical predictions, it was decided to simulate the linear problem of the incidence of an electromagnetic beam at a given angle onto a given smoothly inhomogeneous plasma layer. The simulation was carried out using the widely used FDTD method, which consists of directly solving Maxwell’s equations on a cross spatial grid. The influence of plasma was taken into account within the Drude model. A feature of this model is the use of a pulse source with a diagnostic pulse duration of about 5 periods of the central frequency. The computations has demonstrated the presence of numerical artifacts, which can serve as an unambiguous criterion for the unreliability of the simulation results. According to theoretical predictions, plasma oscillations cannot transform back into electromagnetic waves. So the time dependence of the reflected power should show one single pulse, corresponding to the partial reflection of the incident pulse from the plasma. However, calculations have shown that after some rather long time period after the first reflected pulse, additional pulses are observed in a number of cases. It happened due to the repeated emission of energy stored in the plasma in the form of plasma oscillations, called as “plasma echo”. Detailed studies have shown that echo pulses arise as a result of replacing the smooth plasma profile with a stepwise approximation. In our opinion, “plasma echo” pulses are not the effect of numerical procedure. To experimentally observe this effect, several rather exotic conditions must be met simultaneously. For the case of a smoothly inhomogeneous plasma, “plasma echo” is a parasitic effect. The paper proposes two cost-effective methods to reduce the influence of this parasitic effect without significantly slowing down the speed of calculations.