Julien Vaudolon

Development and experimental characterization of a wall-less Hall thruster

An alternative Hall thruster architecture that shifts the ionization and acceleration regions outside the plasma chamber is demonstrated. This unconventional design is here termed a “wall-less Hall thruster,” as the bulk of the magnetized discharge is no longer limited by solid boundaries. A 200 W prototype with permanent magnets has been developed and characterized. Experimental results concerning the thruster operation, discharge oscillations, electric field distribution, and ionization zone characteristics are presented and discussed. Our first experiments show that the cross-field discharge can be moved outside the cavity without drastically disturbing the ion production and acceleration mechanisms. This design offers the benefit of reduced plasma-wall interaction and lower wall losses, while also greatly facilitating diagnostic access to the entire discharge ionization and acceleration regions.

Optimization of a wall-less Hall thruster

An experimental optimization of a Hall thruster in wall-less operation mode is performed with the PPS-Flex, a 1.5 kW class thruster capable of modifying the magnetic field topology over a broad range of configurations. The anode geometry and the magnetic topology have been modified to avoid interaction between the magnetic field lines and the anode surface, compared to the first wall-less Hall thruster prototype. The measurements of the thrust and far-field ion properties reveal that a satisfactory performance level can be obtained once the magnetic barrier is restored, and pave the way towards the development of a high-efficiency wall-less Hall thruster.

Visual Evidence of Magnetic Shielding With the PPS-Flex Hall Thruster

Magnetic shielding (MS) of insulating walls in a Hall thruster (HT) drastically reduces the damaging ion bombardment, therefore significantly increasing the thruster operational lifetime, as demonstrated in recent work. In essence MS rests on preventing the field line from intersecting the wall in the discharge chamber downstream section. We show here that such a topology can be attained with the newly developed PPS-flex HT. The latter is a unique prototype that offers a wide range of magnetic configurations through a combination of 22 coils with a sophisticated magnetic circuit that propagates the flux.

Perturbations induced by electrostatic probe in the discharge of Hall thrusters

Emissive and Langmuir probes are two widely used plasma diagnostic techniques that, when used properly, give access to a wide range of information on the plasmas ions and electrons. We show here that their use in small and medium power Hall thrusters produces large perturbations in the discharge characteristics. Potential measurements performed by both probes and non-invasive Laser Induced Fluorescence (LIF) spectroscopy highlight significant discrepancies in the discharge profile. This phenomenon is observed both in the 200 W and the 1.5 kW-class thrusters. In order to have a better understanding of these perturbations, ion velocity distribution functions are acquired by LIF spectroscopy at different positions in the smaller thruster, with and without the probes. Emissive probes are shown to produce the biggest perturbation, shifting the acceleration region upstream. The probe insertion is also shown to have significant effect on both the average discharge current, increasing it by as much as 30%, and its harmonic content in both amplitude and spectrum. These perturbations appear as the probe tip passes a threshold located between 0 and 5 mm downstream of the thruster exit plane.

Development and characterization of a wall-less Hall thruster

In this contribution, we propose an alternative Hall thruster architecture that shifts the ionization and acceleration regions outside the cavity. This unconventional design is here termed a “Wall-Less Hall Thruster”. A 200 W-class prototype with permanent magnets has been developed and tested with two anode variants. Experimental results concerning the thruster operation, discharge oscillations, electric field distribution and ionization zone characteristics are presented and discussed. Preliminary experiments show the magnetized discharge can be moved outside the cavity without drastically disturbing the ion production and acceleration. A wall-less Hall thruster, with an external electric field, could offer significant benefits in terms of integration, operating envelope and lifetime. As the thrust is generated outside, the dielectric channel can be considerably shortened, leading to economies in mass and volume. Furthermore, plasma interaction with the walls will have been significantly reduced. The cavity material will therefore have less influence on the thruster performance, and more importantly, operation at high voltage is presumably possible and lifetime can be extended.

Observation of high-frequency ion instabilities in a cross-field plasma

This paper reports on the examination of the high-frequency ion dynamics in a low-pressure cross-field plasma. Measurements of the time-varying ion velocity distribution function were carried out by photon-counting laser-induced fluorescence spectroscopy. A temporal correlation was found between the low-frequency ionization process and the high frequency ion instability. The high-frequency spectral properties of the ion mean velocity are compared with a perturbed fluid model of the plasma flow on the source axis. The wavelength of the high-frequency mode is similar to the electric field extent outside the cavity. Surprisingly the mode appears only in the area of negative magnetic field gradient.

Azimuthal micro-instability inside a wall-less hall thruster

Summary form only given. In a Hall Thruster (HT), a discharge is ignited inside an annular ceramic chamber between an anode situated at the rear of the device and an electron-emitting hollow cathode downstream of the channel. Xenon gas propellant is injected through the anode plane. A magnetic field is created using internal and external coils to achieve a quasi-radial magnetic field profile. A discharge voltage applied between the anode and the cathode is responsible for the acceleration of the ions and the engine thrust. Nevertheless, a fraction of the energetic ions bombards the ceramic walls, resulting in erosion that reduces the thruster lifetime1.An alternative architecture has recently been tested for a low power HT. In this new configuration, the anode is now a ring very close to the channel exhaust. Magnetic coils have been replaced by permanent magnets. This new HT configuration has been named the Wall-Less HT (WLHT) because ionization and acceleration occur outside the channel2. Because in such a configuration the plasma-wall interactions are strongly reduced, the WLHT may offer the opportunity of an increased lifetime with performance levels similar to those of a conventional HT. We have developed fully Particle-In-Cell (PIC) Monte Carlo Collisions (MCC) model of the WLHT. The two-dimensional PIC MCC model describes the azimuthal and axial directions3. A hybrid parallel programming technique that combines OpenMP and MPI on 16 processors is employed to shorten computation time to a few days. Simulations reveal the presence of a micro-instability that participates to the cross-B field electron transport.

Impact of the magnetic barrier extent on the performance of a krypton-fuelled Hall thruster

While xenon will probably remain the standard propellant in the near future, some missions may require the use of alternative fuels, be it for performance or financial considerations. For a variety of reasons, krypton appears as an interesting alternative. This work is concerned with the development, and the preliminary characterization of a thruster optimized for an operation with krypton as a propellant gas. Three magnetic configurations are proposed. Far-field plume measurements of the ion properties, as well as thermal imagery, examine the ion properties and the thruster performance, both in xenon and krypton. Among the prominent results is the evolution of the ionization efficiency with the magnetized area extent. Assuming the sole presence of singly-charged species, it is possible to obtain the same ionization degree in krypton, compared to xenon, when modifying the magnetic topology in a suitable manner.

Investigation of the ion transit time instability in a Hall thruster combining time-resolved LIF spectroscopy and analytical calculations

The performance of Hall thrusters is greatly influenced by the discharge current oscillations. In order to characterize the oscillations of the plasma parameters, we have measured the time-varying Xe ion velocity distribution in the discharge channel and in the nearfield plume of a low-power permanent magnets Hall thruster. The distribution functions are obtained by means of a time-resolved laser-induced fluorescence technique based on a photon counting method, while time coherence is ensured by applying a sinusoidal potential modulation on a floating electrode located in the plasma to achieve resonance with the breathing mode. The measurements are therefore local, non invasive and the temporal resolution is 100 ns. The discharge oscillations are consistent with those of a standard operating point. The axial ion dynamics in the [100 kHz;1 MHz] band, i.e. the so-called ion transit time instability, is examined. The high-frequency spectral properties of the ion mean velocity are compared with a perturbed fluid model of the plasma flow on the source axis. The wavelength of the high-frequency axial mode is similar to the electric field extent outside the cavity. Surprisingly the mode appears only in the area of negative magnetic field gradient.