The velocity distribution function (VDF) of metastable Xe(+) ions was measured along the channel centerline of the high-power PPS (R) X000 Hall effect thruster by means of laser induced fluorescence (LIF) spectroscopy at 834.72 nm for various discharge voltages (300-700 V) and propellant mass flow rates (6-15 mg s(-1)). The development of the on-axis profile of the velocity dispersion reveals the interrelation between ionization and acceleration layers. The ion velocity profiles are in accordance with outcomes of a hybrid numerical model in which the electron mobility is assessed from particle-in-cell simulations. The axial distribution of the effective electric field is inferred from the mean ion velocity profile, despite the parasitic effect due to ions created in the acceleration region. Most of the acceleration process takes place outside the thruster channel. The electric field augments and it moves upstream when the applied voltage is ramped up. The impact of the xenon mass flow rates is found to depend upon the voltage. A novel approach based on the moments of the experimental VDFs in combination with the Boltzmann's equation is introduced in order to determine the real electric field distribution. The method also provides the ionization frequency profile. The LIF diagnostics reveals the existence at the end of the acceleration region of fast ions of which the kinetic energy is above the supplied energy. The fraction of these supra-sped up ions grows when the voltage increases. The ion VDFs were also recorded in the plasma plume far field by way of a retarding potential analyzer (RPA). The shape of the RPA traces as well as their evolution with operating conditions are in agreement with trends observed by means of LIF spectroscopy. Finally, physical mechanisms at the origin of supra-sped up ions are discussed in light of numerical simulation outcomes and a set of new experimental results.

The Velocity Distribution Function (VDF) of metastable Xe+ ions was measured along the channel axis of the 5 kW-class PPS(R)X000 Hall effect thruster by means of Laser Induced Fluorescence spectroscopy at 834.72 nm for various voltages, magnetic fields and mass flow rates. Axial velocity and dispersion profiles are compared to on-axis profiles obtained with the 1.5 kW-class PPS 100 thruster. Outcomes of the comparison are threefold. (i) The broadening of the FDV across the region of strong magnetic field is a general feature for Hall thrusters. It originates in the overlap between ionization and acceleration layers. The velocity dispersion increases with the discharge voltage; it reaches up to 200 eV in unit of kinetic energy at 700 V (ii) Most of the acceleration potential (approximate to 70%) is localized outside the thruster channel whatever the thruster size and operating conditions. The electric field moves upstream when the applied voltage is ramped up; in other words the fraction of potential inside the channel increases with the voltage; (iii) A non negligible amount of very slow and very fast (kinetic energy higher than the applied potential) Xe+ ions are always observed. Such ions may find their origin in space and temporal oscillations of the electric field as suggested by numerical simulations carried out with a hybrid model.

This paper presents recent efforts to better understand and quantify charged particle transport in Hall effect thrusters (HETs). Particle-in-cell (PIC) models, hybrid models, laser induced fluorescence (LIF) measurements and collective scattering (CS) experiments are combined to get a better insight into anomalous electron transport in HETs and to increase the predictive capabilities of simulation codes. PIC models have demonstrated that plasma turbulence associated with the development of a high frequency, short wavelength azimuthal instability can be responsible for anomalous transport. Scaling laws for anomalous electron mobility have not yet been derived and hybrid models, which are more practical than PIC models for parametric studies, must use empirical, adjustable transport coefficients that can be inferred from PIC results or LIF measurements of the ion velocity distribution function. CS experiments are aimed at validating the PIC model predictions of the azimuthal instability. The CS results show the first direct experimental evidence of the azimuthal instability predicted by the PIC code. The paper illustrates the synergy between experiments and models toward a complete and quantitative understanding of the physics of HETs.

The velocity-distribution function (VDF) of metastable Xe+ ions was measured along the channel axis of the 5-kW-class PPSX000 Hall effect thruster by means of laser-induced fluorescence spectroscopy at 834.72 nm for various voltages, magnetic fields, and mass flow rates. Axial-velocity and dispersion profiles are compared to on-axis profiles obtained with the 1.5-kW-class PPS100 thruster. Outcomes of the comparison are threefold: 1) The broadening of the VDF across the region of strong magnetic field is a general feature for Hall thrusters. It originates in the overlap between ionization and acceleration layers. The kinetic-energy dispersion increases with the discharge voltage; it reaches up to 200 eV at 700 V. 2) Most of the acceleration potential is localized outside the thruster channel whatever the thruster size and operating conditions. The electric field moves upstream when the applied voltage is ramped up, i.e., the fraction of potential inside the channel increases with the voltage. On the contrary, the electric field is shifted downstream when the gas flow rate increases. The magnetic field has a little impact on the potential distribution. 3) A nonnegligible amount of very fast (kinetic energy higher than the applied potential) and very slow Xe+ ions are always observed. Such ions may find their origin in space and temporal oscillations of the electric field as suggested by numerical simulations carried out with both kinetic and hybrid models.

The velocity-distribution function (VDF) of metastable Xe+ ions was measured along the channel axis of the 5-kW-class PPSX000 Hall effect thruster by means of laser-induced fluorescence spectroscopy at 834.72 nm for various voltages, magnetic fields, and mass flow rates. Axial-velocity and dispersion profiles are compared to on-axis profiles obtained with the 1.5-kW-class PPS100 thruster. Outcomes of the comparison are threefold: 1) The broadening of the VDF across the region of strong magnetic field is a general feature for Hall thrusters. It originates in the overlap between ionization and acceleration layers. The kinetic-energy dispersion increases with the discharge voltage; it reaches up to 200 eV at 700 V. 2) Most of the acceleration potential is localized outside the thruster channel whatever the thruster size and operating conditions. The electric field moves upstream when the applied voltage is ramped up, i.e., the fraction of potential inside the channel increases with the voltage. On the contrary, the electric field is shifted downstream when the gas flow rate increases. The magnetic field has a little impact on the potential distribution. 3) A nonnegligible amount of very fast (kinetic energy higher than the applied potential) and very slow Xe+ ions are always observed. Such ions may find their origin in space and temporal oscillations of the electric field as suggested by numerical simulations carried out with both kinetic and hybrid models.