Two-Dimensional Electron Model for a Hybrid Code of a Two-Stage Hall Thruster

An axisymmetric model for magnetized electrons in a Hall thruster, to be used in combination with a particle-in-cell model for heavy species, is presented. The main innovation is the admission of exchanges of electric current at the chamber walls, thus making the model applicable to a larger variety of Hall thrusters. The model is fully 2-D for regular magnetic topologies. It combines an equilibrium law for collisionless dynamics along the direction parallel to the magnetic field with drift-fluid equations for perpendicular transport. These are coupled to sheath models for the interaction with different types of walls. The derivation of a parabolic differential equation for the temperature and the full computation of the electric field work improves clarity and accuracy over previous models. Simulations of a Hall thruster with an intermediate current-driving electrode, operating in emission, floating, and collection modes are presented. Enhancement of thrust efficiency is found for the electrode working in the high-emission mode if the magnetic field strength is adjusted appropriately. The two-stage floating mode presents lower wall losses, lower plume divergence, and higher efficiency than the equivalent one-stage configuration

Operational Characteristics and Plasma Measurements in a Low-Energy FARAD Thruster

Pulsed inductive plasma accelerators are spacecraft propulsion devices in which energy is stored in a capacitor and then discharged through an inductive coil. The device is electrodeless, inducing a plasma current sheet in propellant located near the face of the coil. The propellant is accelerated and expelled at a high exhaust velocity (order of 10 km/s) through the interaction of the plasma current with an induced magnetic field. The Faraday Accelerator with RF-Assisted Discharge (FARAD) thruster is a type of pulsed inductive plasma accelerator in which the plasma is preionized by a mechanism separate from that used to form the current sheet and accelerate the gas. Employing a separate preionization mechanism in this manner allows for the formation of an inductive current sheet at much lower discharge energies and voltages than those found in previous pulsed inductive accelerators like the Pulsed Inductive Thruster (PIT). In this paper, we present measurements aimed at quantifying the thruster's overall operational characteristics and providing additional insight into the nature of operation. Measurements of the terminal current and voltage characteristics during the pulse help quantify the output of the pulsed power train driving the acceleration coil. A fast ionization gauge is used to measure the evolution of the neutral gas distribution in the accelerator prior to a pulse. The preionization process is diagnosed by monitoring light emission from the gas using a photodiode, and a time-resolved global view of the evolving, accelerating current sheet is obtained using a fast-framing camera. Local plasma and field measurements are obtained using an array of intrusive probes. The local induced magnetic field and azimuthal current density are measured using B-dot probes and mini-Rogowski coils, respectively. Direct probing of the number density and electron temperature is performed using a triple probe

Recovering ‘lost’ information in the presence of noise: application to rodent–predator dynamics.

A Hamiltonian approach is introduced for the reconstruction of trajectories and models of complex stochastic dynamics from noisy measurements. The method converges even when entire trajectory components are unobservable and the parameters are unknown. It is applied to reconstruct nonlinear models of rodent–predator oscillations in Finnish Lapland and high-Arctic tundra. The projected character of noisy incomplete measurements is revealed and shown to result in a degeneracy of the likelihood function within certain null-spaces. The performance of the method is compared with that of the conventional Markov chain Monte Carlo (MCMC) technique

A multifrequency instability of cavitating inducer systems

Recent testing of high speed cavitating turbopump inducers has revealed the existance of more complex instabilities than the previously-recognized cavitating surge and rotating cavitation. This paper explores one such instability which is uncovered by considering the effect of a downstream asymmetry such as a volute on a rotating disturbance similar to (but not identical to) that which occurs in rotating cavitation. The analysis uncovers a new instability which may be of particular concern because it occurs at cavitation numbers well above those at which conventional surge and rotating cavitation occur. This means that it will not necessarily be avoided by the conventional strategy of maintaining a cavitation number well above the performance degradation level. The analysis considers a general surge component at an arbitrary frequency, ω, present in a pump rotating at frequency, Ω, and shows that the existence of a discharge asymmetry gives rise not only to beat components at frequencies, Ω − ω and Ω + ω (as well as higher harmonics) but also to circumferentially-varying components at all these frequencies. In addition, these interactions between the frequencies and the basic and complementary modes lead to “coupling impedances” that effect the dynamics of each of the basic frequencies. We evaluate these coupling impedances and show not only that they can be negative (and thus promote instability) but also are most negative for surge frequencies just a little below Ω. This implies potential for an instability involving the coupling of a basic mode with a frequency around 0.9Ω and a low frequency complementary mode about 0.1Ω. We also examine how such an instability would be manifest in unsteady pressure measurements at the inlet to and discharge from a cavitating pump and establish a “footprint” for the recognition of such an instability

Experimental Characterization of a 5 N Hydrogen Peroxide Monopropellant Thruster Prototype

In the framework of the LET-SME program funded by the European Space Agency, ALTA S.p.A. (Italy) and DELTACAT Ltd. (United Kingdom) jointly investigated the use of advanced catalytic beds on ceramic supports as a cost-effective alternative to metal screen reactors for the decomposition of high-concentration hydrogen peroxide in small monopropellant rockets. To this purpose ALTA S.p.A. designed and realized a reconfigurable test bench for the characterization of the operation and propulsive performance of small rocket thrusters. The present paper illustrates the experimental campaign carried out on a 5 N thruster prototype operating with two platinum catalysts on g− alumina supporting spheres, especially developed by ALTA in collaboration with the Chemistry and Industrial Chemistry Department of Pisa University, Italy. The results indicated that Pt/Al2 O3 is an effective catalyst combination for the decomposition of 87.5% propellant grade hydrogen peroxide, with good stability and performance comparable to silver screen beds of equal geometric envelope and operational conditions. Incomplete hydrogen peroxide decomposition and the onset of flow oscillations in the reactor were observed at the tested levels of bed loading, residence time and flow pressure. Thermal stresses due to the large temperature gradients occurring during the decomposition of high grade hydrogen peroxide (87.5% by weight) caused the ceramic pellets to break and the progressive occlusion of the bed. Based on the analysis of the test results, several ways to overcome these problems in future investigations have been tentatively identified, together with the necessary modifications to the present experimental set-up