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Articles you may be interested inCompact high-speed reciprocating probe system for measurements in a Hall thruster discharge and plume Rev. Sci. Instrum. 83, 123503 (2012); 10.1063/1.4769052 Measurements of secondary electron emission effects in the Hall thruster discharge Phys. Plasmas 13, 014502 (2006); 10.1063/1.2162809 Investigation and modeling of plasma-wall interactions in inductively coupled fluorocarbon plasmasThe interaction of the plasma discharge with the ceramic walls of a Hall thruster leads to plasma recombination, energy losses, and extra electron collisionality. These three phenomena are included in a one-dimensional axial model of the discharge through source terms obtained from an auxiliary model of the radial dynamics. Spatial solutions are presented for different discharge voltages and wall materials, and agree satisfactorily with experimental data. The parameters related to wall effects are investigated extensively. The energy balance among Joule heating, wall-losses cooling, and heat conduction shapes the temperature profile; three different profile types are identified depending on the wall material and the discharge voltage. For long chambers, the main source of energy losses is the plasma interaction with the walls, even for zero secondary electron emission. By contrast, wall collisionality due to primary/secondary exchanges of electrons is negligible always. The current utilization is related directly to the total energy losses. The propellant utilization is set by the balance between gas ionization and wall recombination in the acceleration region. The rate of wall recombination suggested by the axial solution is much lower than the values given by radial models based on a Maxwellian electron distribution function.

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Particle-in-cell methods are used for ions and neutrals. Probabilistic methods are implemented for ionization, charge-exchange collisions, gas injection, and particle-wall interaction. A diffusive macroscopic model is proposed for the strongly magnetized electron population. Cross-field electron transport includes wall collisionality and Bohm-type diffusion, the last one dominating in most of the discharge. Plasma quasineutrality applies except for space-charge sheaths, which are modeled taking into consideration secondary-electron-emission and space-charge saturation. Specific weighting algorithms are developed in order to fulfil the Bohm condition on the ion flow at the boundaries of the quasineutral domain. The consequence is the full development of the radial plasma structure and correct values for ion losses at lateral walls. The model gains in insight and physical consistency over a previous version, but thrust efficiency is lower than in experiments, indicating that further model refinement of some phenomena is necessary.

An axisymmetric macroscopic model of the magnetized plasma flow inside the helicon thruster chamber is derived, assuming that the power absorbed from the helicon antenna emission is known. Ionization, confinement, subsonic flows, and production efficiency are discussed in terms of design and operation parameters. Analytical solutions and simple scaling laws for ideal plasma conditions are obtained. The chamber model is then matched with a model of the external magnetic nozzle in order to characterize the whole plasma flow and assess thruster performances. Thermal, electric, and magnetic contributions to thrust are evaluated. The energy balance provides the power conversion between ions and electrons in chamber and nozzle, and the power distribution among beam power, ionization losses, and wall losses. Thruster efficiency is assessed, and the main causes of inefficiency are identified. The thermodynamic behavior of the collisionless electron population in the nozzle is acknowledged to be poorly known and crucial for a complete plasma expansion and good thrust efficiency. V

A macroscopic model which accounts for the complex interactions between electrostatic, thermal, and kinetic effects in a Hall thruster is presented. The analysis establishes the one-dimensional steady structure of the flow as consisting of an anode sheath, a long electron free-diffusion region, with reverse ion flow, a thin ionization layer, and the acceleration region, which extends into the plume. The ion flow presents a forward sonic point around the exit of the ionization layer, which can be either internal, with a smooth sonic transition, or localized at the channel exit. The supersonic plume is included via a simple expansion model, allowing closure of the formulation and calculation of thruster performance. The results indicate good agreement with experimental data for the case of an internal sonic point, and they delineate the existence and nonexistence regions in the space of externally controllable parameters. They also unveil the importance of the electron pressure, the reverse flow of ions, and the ionization rate in shaping the plasma structure, whereas, contrary to common perception, the details of the magnetic field profile influence weakly that structure.

A paradigm for Hall discharge modeling is presented whereby only the time scale of the lowest-frequency mode is explicitly resolved. The ability of such a low-frequency model to reproduce with excellent accuracy the breathing mode is demonstrated through comparisons with a fully time-dependent numerical model. Based on this formalism, an approximate linearized model is derived which essentially constitutes a one-dimensional generalization of the classical zero-dimensional predator-prey model. The model highlights the interaction of standing plasma waves with the transport of neutral species, which involves standing and convective waves of similar magnitude. It predicts a frequency which is in cióse agreement with the frequency of the small perturbation modes observed in simulations. Finally, it is shown that unstable modes are in general strongly nonlinear and characterized by frequencies obeying a scaling law different from that of linear modes.

Plasma thrusters are challenging the monopoly of chemical thrusters in space propulsion. The specific energy that can be deposited into a plasma beam is orders of magnitude larger than the specific chemical energy of known fuels. Plasma thrusters constitute a vast family of devices ranging from already commercial thrusters to incipient laboratory prototypes. Figures of merit in plasma propulsion are discussed. Plasma processes and conditions differ widely from one thruster to another, with the pre-eminence of magnetized, weakly collisional plasmas. Energy is imparted to the plasma via either energetic electron injection, biased electrodes or electromagnetic irradiation. Plasma acceleration can be electrothermal, electrostatic or electromagnetic. Plasmawall interaction affects energy deposition and erosion of thruster elements, and thus is central for thruster efficiency and lifetime. Magnetic confinement and magnetic nozzles are present in several devices. Oscillations and turbulent transport are intrinsic to the performances of some thrusters. Several thrusters are selected in order to discuss these relevant plasma phenomena.