Mariano Andrenucci

Critical regimes and magnetohydrodynamic instabilities in a magneto-plasma-dynamic thruster

An extensive experimental investigation of magnetic and electrostatic fluctuations in a magneto-plasma-dynamic (MPD) thruster, with and without the application of an external magnetic field, has shown that gross magnetohydrodynamic instabilities develop whenever the current rises beyond a threshold value. These instabilities are helical kink modes with azimuthal m and axial n periodicity m/n=1/1 and their occurrence can be described by the Kruskal–Shafranov stability criterion. The presence of these modes is found to be the cause of the loss of efficiency observed in MPD thrusters at high current.

A Reduced-Order Model for Thermionic Hollow Cathodes

A reduced-order numerical model describing the plasma in an orificed hollow cathode is presented as a quick tool for the design of thermionic cathodes. A time-independent, volume-averaged model is developed to determine plasma properties, wall temperatures, and cathode lifetime without requiring experimental data as input. A system of particle and energy balance equations is numerically solved without invoking a Saha-type equilibrium under the hypothesis of a direct-impact ionization process. Further, a lumped-parameter thermal model is coupled with the plasma model to estimate the temperature profile along the cathode axis and the emitter lifetime. The obtained results capture most of the characteristic features of this class of hollow cathodes as compared with the available experimental data. In addition, the model gives insight into the most important power deposition processes affecting the emitter and orifice regions. The effect of the geometry on both plasma parameters and cathode performance is discussed to suggest design guidelines for the development of state-of-the-art hollow cathodes.

Numerical model of thermoelectric phenomena leading to cathode-spot ignition

This paper deals with the results of a numerical model of the predischarge heating process encountered by a microprotrusion on the cathode surface of a vacuum gap, due to the field-effect current density flowing throughout the protrusion. The model is one-dimensional and nonstationary. The protrusion is sketched as a truncated cone and the material considered is tungsten, whose physical and thermal properties have been assumed temperature dependent. Electron current density emitted by the tip and the Nottingham effect have been calculated solving the integral of the energy distribution of the emitted electrons. In all other existing models, these two quantities are approximated by algebraic equations, valid in a limited temperature range. Results seem to confirm the experimental evidence that breakdown starts from the explosion of microprotrusions in a time of the order of 1-10 ns. In order to induce the explosion, current densities could be as high as 10/sup 13/ A/m/sup 2/, while the corresponding electric field at the tip can reach the value of 10/sup 10/ V/m, slightly higher than the value found by others. Numerical results confirm that the more slender the protrusion, the more likely its explosion. Investigation of the role of the surface work function shows that decreasing its value at the cathode surface favors the explosion of a larger number of protrusions, inducing the distribution of the arc current among more spots, with a cathode damage reduction, especially on electrodes operating at high current and low temperature.

Theoretical Model of a Lanthanum Hexaboride Hollow Cathode

A model to predict the plasma properties inside a thermionic hollow cathode as a function of operational conditions and geometry is presented. The hollow cathode features a lanthanum hexaboride (LaB6) insert, which is capable of emitting current densities as high as 105 Am-2 at temperatures of ~1900 K, along with a tantalum orifice plate located at the downstream end of the cathode tube. The model self-consistently computes the plasma parameters in both the emitter and orifice regions. A simple semiempirical relation is suggested to evaluate the plasma penetration depth in the cathode interior, which is of primary importance to establish the plasma conditions. The heat transfer mechanisms and the related temperature gradients along the cathode are evaluated with the aid of a dedicated thermal model, which is coupled to the plasma model and accounts for temperature-dependent material properties. A parametric study of the cathode performance was conducted to assess the dependence of the power consumption and operational lifetime on discharge current and mass flow rate, as well as on the geometry. The results are in good agreement with both theoretical and experimental trends found in the literature as well as with experimental data collected by Alta. Further developments will include a deeper investigation into the cathode erosion phenomena, along with a broader comparison with empirical data.

MHD instabilities in magneto-plasma-dynamic thrusters

Magneto-plasma-dynamic thrusters (MPDTs) act as electromagnetic plasma accelerators and represent a high power, electric propulsion option for primary space missions. One of the major problems facing MPDT operation is the onset of a critical regime, which is found when the power is increased beyond a threshold value, depending mainly on the thruster geometry, the type and mass flow rate of the propellant and the intensity of the magnetic field applied. In this regime, large fluctuations in the electrode voltage signals, damage to the anode and decreased efficiency are observed. Since 2000, several test campaigns have been carried out to investigate the electrostatic and magnetic properties of plasma fluctuations using electromagnetic and optic probes and ultraviolet tomography. Results obtained have shown a strong relation between the onset phenomena and the growth of a large scale magnetohydrodynamic (MHD) instability, with helical kink mode features. On the basis of the experimental observations, a passive method to suppress instability is proposed and has been partially tested, with encouraging results.

Current filamentation and onset in magnetoplasmadynamic thrusters

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.

HET Scaling Methodology: Improvement and Assessment

The HET scaling method is an important tool to predict Hall thruster performance in a preliminary design phase. In this paper we present the recent development of the HET scaling model. The model has been further refined in order to take into account the presence of doubly ionized particles and to attempt a prediction of what happens when a thruster works off the design point. Then the model has been extensively tested, comparing its predictions with the experimental data for eight different HETs. A wide power range is covered by the sample thrusters, allowing us to understand if the applicability of our model can be extended both to low and high power levels. Further investigation has been dedicated to the off-design behaviour of scaled HETs and to assess the accuracy of the model when predicting the performance for different HET configurations, such as TAL.

A MFD Model for Aerospace Applications

This paper is a sequel to a previous paper , and presents the development of a 3D code for the solution of a complete MFD (magneto-fluid-dynamic) system of equations. First of all, the magneto-fluid-dynamic equations have been written down in their most general formulation, and then simplified under the single fluid multispecie approximation. The diffusion tensor has been properly derived in the case of electromagnetic interaction with fluid. Then, the magneto-fluid-dynamic system of equations was split in two separate subsystems: the former consisting of the Navier Stokes equations, properly modified with the addition of the electromagnetic body force; the latter consisting of the Maxwell equations whose solution describes the evolution of the electromagnetic fields. The basic idea behind this splitting scheme is to solve each subsystem with the most appropriate numerical scheme. To solve the Maxwell subsystem both implicit and explicit numerical schemes have been implemented, moreover non-reflecting boundary conditions have been imposed and tested. For the fluid-dynamic part, the CIRA existing code for hypersonic flow computations in a 5 species air (H3NS) was opportunely modified and utilized to solve the fluid dynamics equations in presence of ionization and electromagnetic field. In particular chemical ionization models for argon and air have been proposed, and gases thermodynamic properties have been extended at high temperature. The code thus modified in this fashion have bes tested in cases of gas expansion throughout nozzles with non equilibrium chemistry and vibration effects, in absence of electromagnetic field. Finally, the Maxwell code and the modified fluid dynamics code have been coupled in the new EMCNS code that has been tested on some trial cases, including a numerical reconstruction of a plasma flow control literature experiment.

Unsteady Ballistic Code for Performance Prediction of Solid Propellant Rocket Engines

Solid Propellant Rockets (SPRs) by their very nature embody in their chemical properties and geometry the whole history of grain combustion from ignition to burnout. Thus, the development of robust and reliable ballistic codes for accurate prediction of SPR performance is critical for their cost-effective utilization in space propulsion applications, especially in the case of launch vehicle boosters where it can greatly contribute to limit the number of extremely expensive experimental tests. In this perspective the present work illustrates a numerical code (compiled with Fortran 77) based on the unsteady quasi-onedimensional conservation laws for the flow in SPR thrust chambers. By means of an ENO scheme and a Roe solver the code is capable of simulating the whole history of solid propellant grain combustion from ignition to burnout, thereby eliminating the matching problems associated with the use of two different codes for the transient and steady-state combustion phases. After successful validation in a number of test cases, sample results are presented in order to illustrate the effectiveness and accuracy of the program for the prediction of the internal ballistic of real SPRs .

Kink instability suppression and improved efficiency in magneto-plasma-dynamic thrusters

Helical kink mode suppression in a magneto-plasma-dynamic thruster for space propulsion drives the plasma to a quasiquiescent state. As a result the power required to sustain the plasma current is largely reduced. Kink suppression has been obtained by interrupting the helical current components associated with the spontaneous distortion of the plasma column. This result, while confirming that power balance is largely influenced by plasma instabilities, makes, in principle, this kind of device a good choice for long-term space missions.