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[en] Plasma acceleration to supersonic velocities occurs both in space and laboratory plasmas. For a laboratory plasma accelerator, it is desirable that the plasma flow be stable, continuous and controllable. Yet, both experiments and numerical simulations of the Hall thruster exhibit in many cases an oscillatory behavior of the accelerated plasma. Steady flows rarely exist, a fact that is a serious restriction on the flexible use of the accelerator and an obstacle in the quest for an accelerator of variable thrust. Employing a simple model, we show that the acceleration of the plasma to supersonic velocities as a steady flow occurs only when certain relations between the flow parameters arc satisfied. The desirable smooth steady acceleration in which the sonic transition occurs inside the accelerator is therefore an exception rather than the rule, as we demonstrate by a comparison of the plasma acceleration to the acceleration of a gas to supersonic velocities at a Laval nozzle. We then demonstrate that by forcing an abrupt sonic transition, via the introduction of a discontinuity at the plasma flow (here by placing a floating electrode inside the channel), we enlarge the regime of parameters where a smooth sonic transition occurs. Moreover, the formation of a large electric field at the plane of the abrupt sonic transition provides a more efficient acceleration. We also show that the competition between the magnetic field pressure and the ionization in the Hall thruster is analogous to the competition between the plasma pressure and the sun gravitation in the solar wind acceleration, between the diverging geometry and Joule heating as happens in the expansion of the cathode spot plasma in vacuum arc discharges, and between a diverging geometry and ablation and dissipation in ablative discharge capillaries
[en] Pulsed plasma thrusters (PPT) are micro-propulsion devices used in satellites for station keeping. Conventionally the plasma discharge in a PPT is initiated by a spark plug. The primary objective of the present work was to develop and characterize a PPT that does not need a spark plug to initiate the plasma discharge. If the spark plug is eliminated, the size of the thrusters can be reduced and arrays of such thrusters can be manufactured using micro electro mechanical systems (MEMS) techniques, which can provide tremendous control authority over the satellite positioning. A parallel rail thruster was built and its performances were characterized inside a vacuum chamber to elucidate the effect of vacuum level on the performance. The electrical performance of the thruster was quantified by measuring the voltage output from a Rogowski coil, and the thrust produced by the developed thruster was estimated by measuring the force exerted by the plume on a light weight pendulum, whose deflection was measured using a laser displacement sensor. It was observed that the thruster can operate without a spark plug. In general, the performance parameters such as thrust, mass ablation, impulse bit, and specific impulse per discharge, would increase with the increase in pressure up to an optimum level due to the increase in discharge energy as well as the decrease in the total impedance of the plasma discharge. The thrust efficiency is found to be affected by the discharge energy.
[en] Results from experimental studies of integral characteristics of laboratory models of second-generation ATON stationary plasma thrusters are presented. Special attention is paid to high-voltage modes with a sufficiently high specific anode propulsive burn. Integral parameters of the thrusters were measured using a test bench with diffusion evacuation at the Moscow State Institute of Radioengineering, Electronics, and Automation and that with cryogenic evacuation at the Fakel Experimental and Design Bureau. The values of the thrust, specific propulsive burn, and efficiency measured in these test benches in the main operating mode coincide to within measurement errors. At a discharge power of 2 kW and voltage of about 700 V, the specific anode propulsive burn and anode thrust efficiency reach 3000 s and 60%, respectively. The experimental data show that the efficiency of the ATON stationary plasma thruster operating in a high-voltage mode is higher than that of other similar thrusters.
[en] AEA Technology, the commercial arm of the UK Atomic Energy Authority, is conducting an internally funded, detailed definition study of a spacecraft 'bus' and propulsion system for small, economical, off-the-shelf satellites, launched quickly by low cost boosters. These 'lightsats', as they are called, could use an ion thruster to increase payload capability or launcher enhancement. This article discusses the concept and its novel propulsion system, which may fly a demonstration mission later this decade. (Author)
[en] Morozov’s stationary plasma thrusters (SPTs) operating on xenon have been successfully used in space technology for many years. At the same time, due to the high cost of xenon, now there is an increased interest in alternative working substances. One of them is krypton. Therefore, research has been conducted earlier on the features of operation and characteristics of SPTs using krypton, which have showed that they are traditional for SPTs. Here, we consider the results of studying the effect of krypton consumption on the thrust efficiency of SPTs. The importance of these results is determined by the fact that they determine the appropriate ranges of SPT regimes when working on krypton.
[en] A 1 kW-class arcjet thruster was fired in a vacuum chamber at a pressure of 18 Pa. A gas mixture of H2 : N2 = 2.8:1.5 in volume at a total flow rate of 4.3 slm was used as the propellant with an input power fixed at 860 W. The time-dependent thrust, nozzle temperature and inlet pressure of the propellant were measured simultaneously. Results showed that with the increase in nozzle temperature the thrust decreased and various losses increased. The physical mechanisms involved in these effects are discussed.
[en] Experimental investigations into the effects of the magnetic field configuration near the channel exit on the plume of Hall thrusters were conducted. The magnetic field configuration near the channel exit is characterized by the inclination angle between the magnetic field lines and the thruster radial direction. Different inclination angles were obtained by varying the current ratio in the coils. The plume divergence angles were measured by a dual-directed probe. The results showed that the plume divergence angle increased obviously with the increase in the magnitude of the inclination angle near the channel exit. Therefore, in order to optimize the magnetic field for reducing plume divergence, the magnitude of the inclination angle should be reduced as much as possible. It suggests that the magnetic field configuration near the channel exit is another important factor that affects plume divergence.
[en] The possible role of current filamentation in the operation of magnetoplasmadynamic thrusters is investigated here by means of a stability analysis of a current-carrying plasma in a simplified coaxial configuration. Magnetoplasmadynamic thrusters are known to enter a strongly unstable regime, named onset in the literature, when operated above a threshold current level, given the propellant mass flow rate. During onset, a transition from diffuse to spotty current pattern occurs, leading to intense fluctuations of thruster terminal voltage and to severe anode damage with commonly employed anode materials. Despite several experimental and theoretical efforts in the last few decades, no complete and definitive understanding of the physical nature of this phenomenon is yet available. In this work it is shown that conditions suitable for azimuthal symmetry breaking and the subsequent development of this instability can actually exist in magnetoplasmadynamic thrusters. A physically coherent explanation of the complex onset phenomenology is then proposed, showing that both the plasma dynamics and the voltage fluctuations can be ultimately explained in terms of the filamentation instability and its effects.
[en] Conventional annular Hall thrusters become inefficient when scaled to low power. Cylindrical Hall thrusters, which have lower surface-to-volume ratio, are more promising for scaling down. They presently exhibit performance comparable with conventional annular Hall thrusters. The present paper gives a review of the experimental and numerical investigations of electron crossfield transport in the 2.6 cm miniaturized cylindrical Hall thruster (100 W power level). We show that, in order to explain the discharge current observed for the typical operating conditions, the electron anomalous collision frequency νb has to be on the order of the Bohm value, νB ∼ ωc/16. The contribution of electron-wall collisions to cross-field transport is found to be insignificant. The optimal regimes of thruster operation at low background pressure (below 10-5 Torr) in the vacuum tank appear to be different from those at higher pressure (∼ 10-4 Torr)