The interaction between an electron emissive wall, electrically biased in a plasma, is revisited through a simple fluid model. We search for realistic conditions of the existence of a non-monotonic plasma potential profile with a virtual cathode as it is observed in several experiments. We mainly focus our attention on thermionic emission related to the operation of emissive probes for plasma diagnostics, although most conclusions also apply to other electron emission processes. An extended Bohm criterion is derived involving the ratio between the two different electron densities at the potential minimum and at the background plasma. The model allows a phase-diagram analysis, which confirms the existence of the non-monotonic potential profiles with a virtual cathode. This analysis shows that the formation of the potential well critically depends on the emitted electron current and on the velocity at the sheath edge of cold ions flowing from the bulk plasma. As a consequence, a threshold value of the governing parameter is required, in accordance to the physical nature of the electron emission process. The latter is a threshold wall temperature in the case of thermionic electrons. Experimental evidence supports our numerical calculations of this threshold temperature. Besides this, the potential well becomes deeper with increasing electron emission, retaining a fraction of the released current which limits the extent of the bulk plasma perturbation. This noninvasive property would explain the reliable measurements of plasma potential by using the floating potential method of emissive probes operating in the so-called strong emission regime.

An analytical expression is derived for the electron thermionic current from heated metals by using a non equilibrium, modified Kappa energy distribution for electrons. This isotropic distribution characterizes the long high energy tails in the electron energy spectrum for low values of the index K and also accounts for the Fermi energy for the metal electrons. The limit for large K recovers the classical equilibrium Fermi-Dirac distribution. The predicted electron thermionic current for low K increases between four and five orders of magnitude with respect to the predictions of the equilibrium Richardson-Dushmann current. The observed departures from this classical expression, also recovered for large K, would correspond to moderate values of this index. The strong increments predicted by the thermionic emission currents suggest that, under appropriate conditions, materials with non equilibrium electron populations would become more efficient electron emitters at low temperatures.

The design and operation modes of a small, low power ion plasma thruster and the properties of the emitted plasma plumes are discussed. The ion beam is extracted from a primary plasma produced by a stationary low pressure electric discharge where the ion production rate is essentially determined by the discharge current. The experiments evidence that the electron neutralization current controls the space charge levels of the outgoing ion current and also influences the spatial properties of emitted plasmas. The electron plasma density increases with the discharge and the electron neutralization currents, while decreases as the plasma plume expands. However, the corresponding electron temperatures decrease when the electron neutralization currents increments. The collisional origin of this electron cooling effect is excluded because of the large collisional mean free paths involved. Then, these electron energy losses during the neutralization of the ion beam would be caused by more subtle physical mechanisms than collisions. The experimental results are compared with previous numerical simulations and similar phenomena found in other experiments.

A three species one-dimensional kinetic model is presented for a spatially homogeneous weakly ionized plasma (WIP) subjected to the action of a time varying electric field. Planar geometry is assumed, which means that the plasma dynamics evolves in the privileged direction of the field. The energy transmitted to the charges is be channelized to the neutrals thanks to collisions and impacting the plasma dynamics.Charge-charge interactions have been designed as a one-dimensional collision term equivalent to the Landau operator used for fully ionized plasmas. Charge-neutral collisions are modelled by a conservative drift-diffusion operator in the Dougherty's form. The resulting set of coupled drift-diffusion equations is solved with the stable and robust Propagator Integral Method (PIM). This method feasibility accounts for non-linear effects without appealing to linearisation or simplifications, providing conservative physically meaningful solutions. It is found that charge-neutral collisions exert a significant effect since a quite different plasma dynamics arises if compared to the collisionless limit. In addition, substantial differences in the system evolution are found for constant and temperature dependent collision frequencies cases.

The stationary thermionic electron emission currents from heated metals are compared against an analytical expression derived using a non equilibrium quantum Kappa energy distribution for the electrons. This later depends on the temperature decreasing parameter κ(T ) which can be estimated from the raw experimental data and characterizes the departure of the electron energy spectrum from the equilibrium Fermi-Dirac statistics. The calculations accurately predict the measured thermionic emission currents for both high and moderate temperature ranges. The Richardson-Dushman law governs the electron emission for large values of Kappa or equivalently, for moderate metal temperatures. The high energy tail in the electron energy distribution function which develops at higher temperatures or lower Kappa parameters, increases the emission currents well over the predictions of the classical expression. This analysis also permits the quantitative estimation of the departure of the metal electrons from the equilibrium Fermi-Dirac statistics.