S. Mazouffre

Influence of magnetic field and discharge voltage on the acceleration layer features in a Hall effect thruster

The axial velocity of singly charged xenon ion is determined by means of laser induced fluorescence spectroscopy at the exhaust of a PPS100-LM Hall effect thruster by analyzing the Doppler shifted spectral profile of the 834.72 nm Xe+ ion line. Measurements are carried out both inside and outside the thruster channel. Ion velocity distribution functions (VDF) are used to compute the ion accelerating potential. It is shown that such a potential can in fact be defined in several ways due to the overlap between the ionization and acceleration layers, which translates into broad VDF. The PPS100-LM thruster is operated under an applied voltage ranging from 100 to 300 V and a magnetic field produced by coils whose current is varied from 2.5 to 5.5 A. The broad range of working conditions allows one to determine the influence of these two parameters on the ion dynamics. Finally, the experimental results are used to confirm the outcomes of a 2D kinetic model.

Ion diagnostics of a discharge in crossed electric and magnetic fields for electric propulsion

The velocity distribution function (VDF) of metastable Xe+ ions was measured along the channel centerline of the high-power PPS?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 Boltzmanns 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.

Spatio-temporal characteristics of ion velocity in a Hall thruster discharge

The time evolution of the axial velocity of a metastable Xe+ ion was examined in the crossed-field discharge of a PPS100-ML Hall thruster fired at 250 V by means of laser-induced fluorescence spectroscopy at 834.72 nm. A pulse-counting detection technique was employed to achieve a time resolution of 0.1 µs. A periodic break of 10 µs duration of the anode current is used to stabilize the discharge and allow the investigation of both forced and natural plasma oscillations. Measurements were carried out along the channel axis throughout the region of large magnetic field strength. The mean ion flow velocity was found to oscillate at the discharge breathing mode frequency of about 21 kHz. By contrast, the ion velocity dispersion appears not to depend on time, which suggests a strong correlation between ionization and acceleration processes. The spatio-temporal behavior of the electric field was computed from experimental data using a Lagrangian description of the ion fluid motion. As expected, the field amplitude varies significantly at 21 kHz. More surprisingly, an electric field front seems to propagate periodically from the exterior toward the interior of the discharge chamber with a speed close to the thermal speed of the Xe atom.

Ionization and acceleration processes in a small, variable channel width, permanent-magnet Hall thruster

A 200 W-class permanent-magnet Hall thruster has been operated with different channel widths. A series of experiments have been carried out for each geometry over a broad range of propellant mass flow rates and applied voltages to investigate the impact of a channel cross-section area variation on discharge and plume properties. Measurement outcomes allow calculation of several quantities, namely the propellant utilization, the beam voltage, the equilibrium wall temperature and the energy flux towards the walls. This study reveals that widening the channel of a low-power Hall thruster enhances ionization and acceleration processes and broadens the operating envelope. Two mechanisms are proposed to explain experimental observations. The surface-to-volume ratio decreases with h, which reduces wall-losses. The magnetic field strength near walls increases with h, which leads to a better plasma confinement.

Low-frequency electron dynamics in the near field of a Hall effect thruster

Time-resolved electrostatic probe measurements were performed in the near field of a SPT100-ML Hall effect thruster in order to investigate electron properties changes on a microsecond time scale. Such measurements allow one to monitor the electron temperature Te, the electron density ne, as well as the plasma potential Vp during a time period that corresponds to one cycle of a breathing-type plasma oscillation with f≈15–30kHz. Although Te(t) stays constant in time, ne(t) and Vp(t) oscillate with the discharge current waveform frequency. The observed time delay between ne and anode discharge current (Ida) waveforms, which is of approximately 7μs, is linked to the ion transit time from the ionization layer to the probed near-field region. The same time gap is measured between Vp(t) and Ida(t), however Vp(t) and ne(t) are in phase opposition. The electron density reaches its highest value at the very moment ions are ejected out of the thruster discharge chamber, which also corresponds to the instant the cat...

Unexpected transverse velocity component of Xe+ ions near the exit plane of a Hall thruster

The velocity component of singly charged xenon ions in a plane perpendicular to the thrust axis of the 1 kW-class PPS100-ML Hall effect thruster is deduced from laser induced fluorescence measurements on the 5du20092F7/2→6pu20092D5/20 electronic transition at 834.72 nm. Measurements are carried out at several locations in the near field of the channel exhaust. Thruster operating parameters, such as magnetic field strength, discharge voltage, and xenon mass flow rate, are varied over a wide range. The initial aim of this work was to measure the azimuthal velocity of the ions due to their weak magnetic deflection. Surprisingly, experimental results cannot be explained by the one and only Lorentz force acting on Xe+ ions. A realistic picture of the ion trajectory in the E×B drift plane is obtained when adding a velocity component directed toward the external cathode.

A time-resolved laser induced fluorescence study on the ion velocity distribution function in a Hall thruster after a fast current disruption

The temporal characteristics of the Xe+ ion axial velocity distribution function (VDF) were recorded in the course of low-frequency discharge current oscillations (∼14u2002kHz) of the 5 kW class PPS®X000 Hall thruster. The evolution in time of the ion axial velocity component is monitored by means of a laser induced fluorescence diagnostic tool with a time resolution of 100 ns. As the number of fluorescence photons is very low during such a short time period, a homemade pulse-counting lock-in system was used to perform real-time discrimination between background photons and fluorescence photons. The evolution in time of the ion VDF was observed at three locations along the thruster channel axis after a fast shutdown of the thruster power. The anode discharge current is switched off at 2 kHz during 5u2002μs without any synchronization with the current oscillation cycle. This approach allows to examine the temporal behavior of the ion VDF during decay and ignition of the discharge as well as during forced and natur...

A comprehensive study on the atom flow in the cross-field discharge of a Hall thruster

The flow properties of Xe atoms were investigated in the 1kW class PPS100-ML Hall effect thruster by means of Doppler-shifted laser-induced fluorescence spectroscopy in the near infrared. Fluorescence spectra of the 6s[1/2] o resonant level and the 6s[3/2] o metastable level have been acquired inside and outside the thruster channel under several operating conditions. Analytical treatment and modelling of the measured lineshapes indicate the atom axial velocity increases inside the channel to a value well above the sound speed before decreasing quickly in the near-field plume. Numerical simulations performed with a fluid/kinetic hybrid approach allow us to explain the shape of the velocity profile. Atomic flow acceleration originates in the combination of three processes, namely the selective ionization of slow atoms, the flow expansion and the creation of fast neutrals on BN-SiO2 walls owing to recombination of ions. Deceleration results from the invasion of the atomic jet by slow and relatively cold atoms from the residual background gas and from the cathode. In addition, it is shown that charge-exchange collisions have a non-negligible impact on the atom velocity in spite of the low background pressure in test chambers. (Some figures in this article are in colour only in the electronic version)

Kr II and Xe II axial velocity distribution functions in a cross-field ion source

Laser induced fluorescence measurements were carried out in a cross-field ion source to examine the behaviour of the axial ion velocity distribution functions (VDFs) in the expanding plasma. In the present paper, we focus on the axial VDFs of Kr II and Xe II ions. We examine the contourplots in a 1D-phase space (x,vx) representation in front of the exhaust channel and along the centerline of the ion source. The main ion beam, whose momentum corresponds to the ions that are accelerated through the whole potential drop, is observed. A secondary structure reveals the ions coming from the opposite side of the channel. We show that the formation of the neutralized ion flow is governed by the annular geometry. The assumption of a collisionless shock or a double layer due to supersonic beam interaction is not necessary. A non-negligible fraction of slow ions originates in local ionization or charge-exchange collision events between ions of the expanding plasma and atoms of the background residual gas. Slow ions ...

Impact of discharge voltage on wall-losses in a Hall thruster

Calibrated infrared thermal imaging is used to investigate the temperature of the BN-SiO2 discharge chamber walls of the high-power PPSX000-ML Hall thruster over a broad voltage range. The energy flux deposited by charged particles onto the channel walls is assessed by means of a semi-empirical time-dependent thermal model. Equilibrium temperature as well as power losses onto the channel walls are determined for low and high voltage operation states. For a given input power, the two quantities strongly depend upon the discharge voltage. Above ~ 500 V, losses augment in an exponential way whereas they vary linearly at low voltages. As suggested by many theoretical works, secondary electron emission yield and associated near-wall sheath potential lowering could explain experimental outcomes. The contribution of ion bombardment, however, cannot be fully ruled out, as shown here.