Tommaso Misuri

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.

Advanced Erosion Diagnostics for Hall Effect Thrusters and Gridded Ion Engines

The present paper describes an advanced diagnostics for erosion detection in both Hall thrusters and Ion engines. This diagnostics has been developed by Alta in the framework of a Technology Research Programme funded by ESA, with the aim of further investigating the erosion phenomenon in such thrusters. The system is installed inside the vacuum facility hosting the test, so allowing for successive measurements of the erosion during the thruster operation. By using two different optical methods, according to the type of target engine, it has been possible to reconstruct the shape of the channel profile in a Hall thruster (Alta’s HT-100) and to acquire high-res images of the grid holes of an Ion engine. An extensive test campaign has been carried out on the HT-100 thruster, measuring the erosion on channels made of three different ceramic materials.

3D MFD Model Developments for Aerospace Applications

This paper is a sequel to a previous paper 1 , and presents the development of 3D code for the solution of a complete MFD (magnetofluiddynamic) system of equa tions. First of all, the MFD equations were written down in their most general formulation 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 MFD system of equations was split in two separate subsystems: the former consisting of the a 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 take the opportunity to solve each subsystem with the most appropriate numerical scheme. To solve the Maxwell subsystem both implicit and explicit numerical schemes was implemented, moreover nonreflecting boundary conditions have be en implemented and tested. For the fluiddynamic pa rt, the CIRA existing code for hypersonic flow computation (H3&SP) 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 so modified, have been tested in cases of gas expansion throughout nozzles with non equilibrium chemistry, in absence of electromagnetic field. Finally, the Maxwell code and the modified fluid dynamics code were coupled and tested on some trial cases, including the acceleration of a plasma flow in a cylindrical hollow channel, under the effect of a constant electric field.

A Magneto-Fluid-Dynamic Model and Computational Solving Methodologies for Aerospace Applications

Computational plasma physics is concerned primarily with the study of the evolution of plasma by means of computer simulation. The main task of this computational branch is to develop methods able to obtain a better understanding of plasma physics. Therefore a close contact to theoretical plasma physics and numerical methods is necessary. Ideally computational plasma physics acts as a pathfinder to guide scientific and technical development and to connect experiment and theory. To build a valid computer simulation program means to devise a model which is sufficiently detailed to reproduce faithfully the most important physical effects, with a computational effort sustainable by modern computers in reasonable time (Dandy, 1993). Computational models have played an important role in the development of plasma physics since the beginning of the computer age. Advances in our understanding of many plasma phenomena like magnetohydrodynamic instabilities, micro-instabilities, transport, wave propagation, etc. have gone hand-in-hand with the increased computational power available to researchers. Several trends are evident in how computer modelling is carried out: the models are becoming increasingly complex, for example, by coupling separate computer codes together. This allows for more realistic modelling of the plasma. Presently several efforts are carried out in different countries to develop plasma numerical tools for several applications such as fusion, electric propulsion, active control over hypersonic vehicles: these efforts lead to a growing experience in CMFD field (see Park et al. 1999, Kenneth et al 1998, Taku and Atsushi 2004, Cristofolini et al 2007, Miura and Groth 2007, MacCormack 2007, Yalim 2001, Giordano and D’Ambrosio 2004, Battista 2009). The chapter presented was carried out in the context of a research activity motivated by renewed interest in investigating the influence that electromagnetic fields can exert on the thermal and pressure loads imposed on a body invested by a high energetic flow. In this regard, spacecraft thermal protection and the opportunity to use active control surfaces during planetary (re)entry represent the driving engineering applications. The contents of the study should be considered, to a certain extent, a systematic re-examination of past work complemented with somewhat innovative ideas. So, in this chapter, methodologies for plasma modelling have been developed and then implemented and tested into a numerical code EMC3NS, developed in the frame of this