When considering a plasma expanding through a nozzle, on the ions acts not only the electric field but also the fluid dynamic forces, transferring energy from neutrals to ions by means of collisions. These forces are produced by grad(P). Moreover there is also the diffusion that induce ion fluxes. Usually in the an arcjet the fluid dynamic forces on ions are larger than that of the electric field.
Thank you very much!
But anyway, why not to allow the ions to move with the field and get additional acceleration? Is there any specific consideration for opposite arrangement?
The limiting factors are the pressure and ionization degree. For sure, there is an acceleration of the ions, and as a consequence, through collisions to the neutrals, gives a contribution to the momentum equation. However, due to the mass difference, the energy transferred to the gas through electrons is more efficient therefore the force due to pressure gradient and the heating due to the Joule effect dominate to ion acceleration. Neutral-ion collision frequency depends on the pressure and the efficiency of energy transfer is proportional to ion density and therefore to the ionization degree.