Hollow cathode modeling: II. Physical analysis and parametric study

Gaétan Sary (1) , Laurent Garrigues (2) , Jean-Pierre Boeuf (2)

1 LAPLACE - LAboratoire PLasma et Conversion d'Energie
2 LAPLACE-GREPHE - Groupe de Recherche Energétique, Plasmas et Hors Equilibre

A numerical emissive hollow cathode model which couples plasma and thermal aspects of the NASA NSTAR cathode has been presented in a companion paper and simulation results obtained using the plasma model were compared to experimental data. We now compare simulation results with measurements using the full coupled model. Inside the cathode, the simulated plasma density profile agrees with the experimental data up to the ±50% experimental uncertainty while the simulated emitter temperature differs from measurements by at most 5 K. We then proceed to an analysis of the cathode discharge both inside the cathode where electron emission is dominant and outside in the near plume where electron transport instabilities are important. As observed previously in the literature, the total emitted electron current is much larger 34 A () than the set discharge current collected at the anode 13 A () while ionization plays a negligible role. Extracted electrons are emitted from a region much shorter than the full emitter (0.9 cm versus 2.5 cm). The influence of an applied axial magnetic field in the plume is also assessed and we observe that it leads to a 10-fold increase of the plasma density 1 cm downstream of the orifice entrance while the simulated discharge potential at the anode is increased from 10 V up to 35.5 V. Lastly, we perform a parametric study on both the operating point (discharge current, mass flow rate) and design (inner radius) of the cathode. The simulated useful operating envelope is shown to be limited at low discharge current mostly because of the probable ion sputtering of the emitter and at high discharge current because of emitter evaporation, plasma oscillations and sputtering of the keeper electrode. The behavior of the cathode is also analyzed w.r.t. its internal radius and simulation results show that the useful emitter length scales linearly with the cathode radius.

Mots clés: thermionic emission; numerical modeling; Hall thruster; hollow cathode

Domaines: Physique des plasmas [physics.plasm-ph]

Gaétan Sary, Laurent Garrigues, Jean-Pierre Boeuf. Hollow cathode modeling: II. Physical analysis and parametric study. Plasma Sources Science and Technology, 2017, 26 (5), pp.055008. ⟨10.1088/1361-6595/aa6210⟩. ⟨hal-02326743⟩