Alexander Kapulkin

Low-Frequency Instability in Near-Anode Region of Hall Thruster

This paper is devoted to building a theoretical model of relatively low-frequency (nui Lt omega Lt radic(omegaeomegai), where nui is the frequency of the ionization collisions of the electron and omegae and omegai are the electron and ion cyclotron frequencies, respectively) instability of plasma in the near-anode area of the Hall thruster acceleration channel. The behavior of 2-D perturbations, which are infinitely spread along the magnetic field lines, is considered. In the model, the two-fluid magnetohydrodynamic approximation is used. The finite temperature of electrons is taken into consideration. The instability is due to the finite temperature of the electrons and nonuniformity of both the plasma and magnetic field. By its nature, it belongs to the Rayleigh-Taylor type. The instability can be responsible for the enhanced transfer of the electrons between the ionization region and the anode.

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.

Ion Beam Instability in Hall Thrusters

Stability of an ion flux, bounded with an anode and cathode, in Hall thrusters is investigated by theoretical and numerical modeling. Two-fluid magnetohydrodynamic approximation with cold magnetized electrons and cold nonmagnetized ions is used. The inertia of the electrons is taken into consideration. The perturbations are assumed to be quasi-neutral, potential, and dependent on a single spatial coordinate only. For simplicity, the magnetic field is assumed to be uniform. It is shown that the presence of the boundaries, where the potential of the ion beam is fixed, can cause instability of the beam. The growth rate of instability and frequency excited oscillation is of the order of the reciprocal of the ion crossing time between the anode and cathode. In the limit of a uniform unperturbed state of plasma, the instability is analogous to the Pierce instability of an electron beam with the fundamental distinction that here, the perturbations are quasi-neutral. It is shown that the instability is alternately a periodical and oscillating depending on the range of the αlh parameter, which includes induction of the magnetic field (through lower hybrid frequency), thickness of the acceleration layer, and velocity of the ion flux.

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.

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...

Magnetic Sensing of Azimuthal Current in Hall Thrusters: In-Flight Diagnostic Potential

In this paper, the method of Hall current spatial structure estimation based on the measurements of steady-state magnetic fields induced by this current outside the acceleration channel is proposed. The approach to Hall current structure determination is based on the inverse magnetostatic problem solution using two-dimensional constrained regularization. The optimal number and positions of magnetic sensors are determined, and the solutions using simulated measurements with and without simulated noise are obtained.

Theoretical model of suppression of electron instability in Hall thrusters by boundary feedback system

Summary form only given. Plasma instabilities in Hall thruster (HT) deteriorate the performance of the thruster and its compatibility with the electronic equipment of the spacecraft. Therefore, a development of effective methods of suppressing the instabilities is an actual problem. Among the plasma instabilities in HT, large scale electron instability holds one of the main positions. Its arising brings about redistribution of electrical field in the acceleration layer that can increase losses of ions on the walls of the thruster. In Ref (1, 2), the theoretical model of the electron instability, based on Rayleigh mechanism of its arising, was developed. The application of a feedback system is a versatile method of suppressing the large scale plasma instabilities. Two kinds of the feedback system are possible: volume and boundary (surface) ones. At conditions of the HT, the boundary feedback system (BFS) is more preferable. At an application of the BFS, sensors and controlling electrodes are placed on the plasma boundary, which is parallel to the magnetic field, that is, on an anode3. For the correct choice of the BFS parameters, a theoretical model of the electron instability suppression by the BFS was first developed. It is a subject of the presentation.The theoretical model is built in hydrodynamic approximation with cold magnetized electrons. The perturbations are assumed to be two-dimensional and potential. It is assumed that BFS creates an azimuthal distribution of the potential on the surface of the anode which is function of the electrical field perturbation near the anode. The boundary eigenvalue problem is solved. From the solution, the requirements to the transformation coefficient of the BFS are defined. It is shown that in the frame of made assumptions, the suppression of the electron instability is possible for all lengths of the wave. . For the suppression of the electron instability, the transformation coefficient should lie in the region, limited as a lower value, so an upper value, which depend on the length of wave and the distance from the anode, where the unperturbed drift velocity sharply increases. The physical mechanism of the electron instability suppression is considered.

Characterization of CAMILA Hall Thruster Discharge using Electrical Probe Measurements

The CAMILA (co-axial magneto-isolated longitudinal anode) concept was introduced to improve the efficiency and the lifetime of low-power Hall thrusters (≤ 350 W). CAMILA represents a significant departure from conventional Hall thrusters and has not been yet thoroughly studied. The high efficiency of the thruster, as validated by measurements, increases the need for better understanding of the physical processes in this type of thruster. For this aim, an analysis of the CAMILA discharge based on experimental measurements was conducted. The experimental setup includes electrical probes mounted on a fast moving positioner, enabling 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 CAMILA concept and revealed new phenomena related to the radial non-uniformity of the discharge. In particular, focusing equipotentials were discovered not only in the anode cavity, but in the dielectric channel as well, where the area of intense ionization was located. The physical processes contributing to the formation of the focusing equipotentials are discussed.