Igal Kronhaus

Investigation of physical processes in CAMILA Hall thruster using electrical probes

The CAMILA (co-axial magneto-isolated longitudinal anode) concept was developed to improve the anode efficiency in low-power Hall thrusters. Previous measurements, performed in Asher Space Research Institute, have shown that the thruster has the highest efficiency for its class. This paper presents an analysis of the discharge structure in an effort to improve understanding of the physical processes in CAMILA type thrusters. Internal measurements of the discharge parameters were performed using an emissive probe, a biased probe and a Faraday cup. The probes were mounted on a positioning system capable of mapping the channel in two dimensions. Maps for the plasma potential, the ion current density and the electron temperature were obtained. In addition, a one-dimensional fluid model was developed in order to compute the distribution of the plasma density and the ion velocity. The experimental investigations confirmed the basic assumptions used in the physical model of the CAMILA concept and revealed phenomena related to the radial non-uniformity of the discharge. In particular, focusing equipotentials were discovered in the area of intense ionization, reducing ion loss to the walls of the channel. This mechanism is principal in obtaining the high efficiency of the thruster. When operated with strengthened longitudinal magnetic field, the plasma density inside the anode cavity was significantly higher in the middle than near the anodes. The fraction of ion current generated inside the anode cavity was greater than in the simplified case, 19% compared with 13% respectively. In addition, it was shown that electrons in the cusp region, the region between predominately radial to predominately axial magnetic fields, were not well confined, however, no potential hump is created and ions are able to cross this region to the acceleration channel.

Plasma window characterization

Parameters of an arc Ar plasma discharge used as a plasma window with a discharge current of ∼50A and a voltage of ∼58V are presented. It is shown that this arc discharge allows one to decrease the pressure at the low pressure end of the plasma window almost 380 times using relatively low pumping at the low pressure end of the plasma window. Calculations of the plasma parameters and their spatial distribution using a simple wall-stabilized arc model showed a satisfactory agreement with the experimentally obtained data. It is shown that a significant decrease in gas flow through the plasma window occurs due to the increase in plasma viscosity. An improvement of the plasma window ignition and some of its design aspects are described as well.

Investigation of two discharge configurations in the CAMILA Hall thruster by the particle-in-cell method

The CAMILA (co-axial magneto-isolated longitudinal anode) concept was introduced to improve the ionization efficiency in low-power Hall thrusters. With relatively large coaxial anode surfaces and longitudinal magnetic strength, the CAMILA represents a significant departure from conventional Hall thrusters. In order to investigate the physical processes inside the CAMILA thruster, a two-dimensional particle-in-cell simulation of the thruster channel is used. The discharge parameters are analysed in two magnetic configurations: simplified CAMILA with a conventional magnetic field and full CAMILA with strengthened longitudinal component of the magnetic field. The simulation is fully kinetic with electrons, ions and gas atoms (xenon) represented as particles. Electron–neutral interactions are included together with particle–boundary interactions such as recombination and secondary emission. In addition, dielectric boundaries float and the cathode is represented as a free-space boundary, emitting electrons to satisfy quasi-neutrality on its surface. The high anode efficiency, observed in experiments, can be explained by several mechanisms found in this work. In the simplified case (magnetic configuration similar to the experiments) a focusing potential is created near the anode–dielectric boundary that directs ions away from the walls. It is created due to a combination of anode placement, in parallel with the channel, penetration of the plasma inside the anode cavity and the shape of magnetic force lines. Simulated steady-state results show good agreement with experimental measurements. In the full CAMILA case we demonstrate that the ionization region is found in the anode cavity. The electric field inside the anode cavity is substantial and it is directed towards the anode cavity centreline. Electrons are heated sufficiently to reach a high degree of ionization inside the anode cavity while ion currents to the anode surfaces are reduced significantly.

Schlieren characterization of gas flows generated by cathodic arcs in atmospheric pressure environment

Schlieren diagnostics of cathodic arc flows in air are presented. Three important processes are observed: (1) a luminous hemispherical region near the cathode with a radius of ≈1 mm, identified as the cathodic metallic plasma; (2) a luminous plasma jet extending ≈10 mm from the cathode, termed as the cathodic air jet (CAJ); (3) a background gas pushed to expand at subsonic velocities ≥100 m/s. The gas is accelerated in the direction of the CAJ. The main collisional processes in the CAJ are identified using optical emission spectroscopy and mean free path analysis. The CAJ plasma is shown to be composed of N2+ and Cu+ ions. It is concluded that the CAJ length is determined by the dissociative recombination of N2+. With external magnetic field, the CAJ rotates according to the Lorentz force direction. Observing the CAJ motion and its affect on stationary and flowing background gas, it is concluded that the CAJ has significant directed thrust.

Field emission cathode with electron optics for use in Hall thrusters

This paper is devoted to the development and numerical modeling of a field emission cathode for low power Hall thrusters (100–300 W). Generally, Hall thrusters use hollow cathodes, which require a relatively large mass flow rate of xenon-gas to operate. For lower emission currents the cathode gas consumption is still substantial, which contributes to the drop in efficiency when operating a Hall thruster in a low power regime. Conventional field emission cathodes, which are considered as an alternative, do not provide the required low power consumption with an acceptable lifetime. In order to increase the efficiency of the field emission cathode while retaining an acceptable lifetime, an acceleration-deceleration electron optics is proposed. This system is used for the extraction of electrons from carbon nanotubes and the formation of the electron beam. Numerical modeling of the processes in the proposed cathode was carried out using a particle-in-cell approach. It has been shown that (1) it is possible to...

Discharge Characterization of the Coaxial Magnetoisolated Longitudinal Anode Hall Thruster

The coaxial magnetoisolated longitudinal anode concept was introduced to improve efficiency and lifetime of low-power Hall thrusters (≤350  W). The coaxial magnetoisolated longitudinal anode represents a significant departure from conventional Hall thrusters and has not been thoroughly studied yet. The high efficiency of the thruster, as validated by measurements, increases the need for a better understanding of the physical processes in this type of thruster. An analysis of the coaxial magnetoisolated longitudinal anode discharge based on experimental measurements was conducted for this aim. The experimental setup includes electrical probes mounted on a fast moving positioner, enabling one to obtain the spatial distribution of plasma parameters inside the thruster channel. The results confirmed the basic assumptions used in the physical model of the coaxial magnetoisolated longitudinal anode concept and revealed new phenomena related to the radial nonuniformity of the discharge. In particular, focusing e...

Analysis of Nanosatellite Formation Establishment and Maintenance Using Electric Propulsion

Nanosatellite clusters are one of the current and future trends in space technology. To maintain a satellite cluster over a long period of time, the nanosatellites need to mitigate the alongtrack d...

Investigation of Cathodic Arc Plume in an Atmospheric Pressure Environment Using Stark Broadening

The plume of a cathodic arc operated in atmospheric pressure gas is examined using an intensified camera and a spectrograph. A 0.2-J pulse arc is shown to generate a glowing jet 10 mm in extent. Images recorded in the first microseconds after ignition show that the jet has a smaller core ≈2 mm in length. A spatially resolved Stark broadening measurement is performed along the jet core. The measured electron density is 1-2 x 1020 m-3. A model for the cathodic arc plasma-gas interaction is presented. The obtained equilibrium between the forward and reverse charge exchange processes is suggested as enabling the extended jet core.

Investigation of Cathodic-Arc-Jet Influence on Cross Flow using Particle Image Velocimetry

A new type of plasma actuator based on cathodic arc discharge is presented. In atmospheric pressure air, the cathodic arc discharge generates fast > 100 m/s and powerful jets normal to the wall, therefore potentially applicable to high-speed flow control. The affect of a 4 J cathodic arc discharge on a subsonic cross flow of 56 m/s is demonstrated. The flow velocity field across the actuator is measured using particle image velocimetry. The added momentum to the flow by the so called cathodic air jet is significant with the impulse-bit to pulse-energy ratio estimated as ~ 5 μNs/J.

PICO-SATELLITE ORBIT AND ATTITUDE CONTROL BY ELECTRIC PROPULSION

Abstract The possibility of maintaining a group of pico-satellites in a bounded formation, using very low power electric propulsion, is presented. A model predictive controller is used to develop a simple control scheme that avoids the need for on-line orbit determination. The control law, verified using a high fidelity propagator, is capable of maintaining a group of pico-satellites within a 1500 km region at an average ΔV of 2.5 m/s per month. In order to reduce the power requirements of the attitude and propulsion systems, a combined control is utilized. Attitude accuracy is shown to be better than 0.5 deg. The combined attitude-orbit control and the simplicity of the control law provide important advantage for implementation in a pico-satellite with extremely limited resources.