Professor Felix Parra Diaz

Abstract:

Particle-in-cell methods are used for ions and neutrals. Probabilistic methods are implemented for ionization, charge-exchange collisions, gas injection, and particle-wall interaction. A diffusive macroscopic model is proposed for the strongly magnetized electron population. Cross-field electron transport includes wall collisionality and Bohm-type diffusion, the last one dominating in most of the discharge. Plasma quasineutrality applies except for space-charge sheaths, which are modeled taking into consideration secondary-electron-emission and space-charge saturation. Specific weighting algorithms are developed in order to fulfil the Bohm condition on the ion flow at the boundaries of the quasineutral domain. The consequence is the full development of the radial plasma structure and correct values for ion losses at lateral walls. The model gains in insight and physical consistency over a previous version, but thrust efficiency is lower than in experiments, indicating that further model refinement of some phenomena is necessary. © 2006 American Institute of Physics.

Abstract:

The effect of a third, active electrode placed inside the ceramic chamber of a Hall thruster is analyzed. Both electron-collecting and electron-emitting modes are considered. Significant efficiency enhancement with respect to the single-stage operation can be obtained for a good electron-emitting electrode, placed in an intermediate location of the acceleration region, and for an anode-to-electrode (inner-stage) potential significantly larger than the ionization potential. Optimum values of the electrode location and voltage are determined. The performance improvement is due to a reduction of the energy losses to the chamber walls. This is the consequence of lower Joule heating and thus lower electron temperature in the outer stage. When the ionization process is efficient already in the single-stage operation, (i) two-stage operation does not affect practically the propellant and voltage utilizations and (ii) thrust efficiency decreases when the intermediate electrode works as an electron collector. © 2005 American Institute of Physics.

Abstract:

Secondary-electron emission at the ceramic walls of a Hall thruster modifies the potential jump of the wall Debye sheaths and thus the electron energy losses to the wall. Because of the low plasma collisionality the two counterstreaming beams of secondary electrons are not expected to be totally trapped within the bulk of the discharge. In order to analyze the effects of partial trapping of secondary electrons on the presheathsheath radial structure, a macroscopic model is formulated. The plasma response depends on the secondary electron emission yield and the trapped fraction of secondary electrons. The sheath potential and wall energy losses are determined mainly by the net current of secondary electrons in the sheaths. For any practical value of the secondary emission yield, the zero-trapping solution is very similar to the zero secondary emission case. Space charge saturation of the sheaths is unattainable for weak trapping. In all cases, secondary electrons have a weak effect on the presheath solution and the ion flux recombined at the walls. © 2005 American Institute of Physics.

Abstract:

Two issues are discussed. First, a new sheath model that takes into account charge-saturation is implemented in HPHall. Second, the transition between the quasineutral solution and the sheaths at the lateral walls is found to be treated deficiently in the original code. The use of finer meshes yields better solutions but do not solve the problem completely. Hall thrusters; particle-in-cell codes; sheaths.

Abstract:

An axial model for a two-stage discharge with an electron-emissive electrode is further examined. Scaling laws are derived and help to understand two-stage physics. Efficiency gains are obtained when the second-stage is placed in the upstream part of the acceleration region and the first and two stage voltages are comparable. A parametric study to determine the best position and voltage of the intermediate electrode is carried out. © 2003 by The Authors.