Supersonically expanding stationary thermal plasma, formed by a thermal cascaded arc is studied. Due to the low chamber pressure (20-100 Pa) the results of continuum mechanics model can be doubtful. This is why these results are validated against kinetic Monte Carlo simulation and experimental data obtained by means of laser induced fluorescence. The analysis proves that continuum mechanics is still applicable for the velocity and temperature field predictions downstream of the shock region. However, the shock formation and some non-equilibrium effects typical for supersonic flow can be correctly studied only with the help of kinetic simulations. We show that the errors in the results using continuum mechanics can be attributed to the presence of flow gradients. These errors diminish when the shock regions are thickened due to rarefaction, viscosity and heat conductivity. Besides, both methods show that the effect of the chamber geometry on the plasma flow field is important.