Suchergebnisse

Mención Internacional en el título de doctor ; This Thesis presents an experimental and theoretical investigation of the fundamental physical phenomena behind magnetic nozzle expansions. The work is motivated by the emerging electric propulsion thruster concepts which include magnetic nozzles to confine and accelerate the plasma beam. This research has been carried out under a bilateral agreement between the Office National d'Etudes et de Recherches Aérospatiales and the Electric Propulsion and Plasma Team from the University Carlos III de Madrid. The first part of this research constitutes an experimental investigation of an Electron Cyclotron Resonance thruster. An innovative procedure based on the integration of a diamagnetic loop signal at the thruster shutdown for estimating the mean perpendicular electron pressure inside the thruster source is presented. The signal is then used to estimate the mean perpendicular electron temperature by means of 1D and 2D theoretical models. Results fairly agree with the direct magnetic thrust measurements of a previous experiment. The magnetic nozzle of the thruster is characterized by means of electrostatic probes and non intrusive diagnostics (laser induced fluorescence). Profiles for the ambipolar plasma potential, ion velocity, plasma density and electron temperature are obtained as functions of the propellant mass flow rate. By combining Langmuir probes measurements with the optical measurements, complete profiles (from the thruster source exit to far downstream) of the ambipolar plasma potential are obtained, which allows to estimate the total potential drop along the magnetic nozzle. Different effective electron cooling rates are measured along the expansion for the thruster operating with permanent magnets and for the one with solenoids, which is linked to the different magnetic nozzle topologies. The experimental data are compared with a paraxial steady-state model, showing a good correlation with respect the plasma potential, ion velocity and plasma density. The comparison between the experimental data and the model allows to estimate the sonic transition of the plasma flow, which appears to be shifted from the magnetic throat. A recent publication from another research group verifies our results. The second part of this Thesis presents a theoretical investigation of the ion and electron thermodynamics along the magnetic nozzle expansion. A collisionless, paraxial, steady-state kinetic model is used to investigate in several directions: first, the macroscopic effect of the kinetic aspects along the expansion is discussed. Electrons are barely affected by magnetic mirroring, since anisotropy is only developed very far downstream. On the contrary, magnetic mirror is dominant for ions, especially in the "hot" ions limit. Parametric laws for the downstream properties are given, which can provide a quick estimate of the nozzle performance. The electron distribution function at the upstream source is formulated different from Maxwellian. The case of two electron populations with different temperatures is presented, where the kinetic features of a quasi-neutral steepened layer are addressed. The total potential drop of the nozzle is a function of the density fraction of hot electrons and their temperature, and it determines the level of anisotropy of this population. Finally, the electron distribution function at the upstream source is formulated as a bi-Maxwellian anisotropic function. The plasma response along the expansion is investigated in a convergent-divergent nozzle and in an only divergent one. The magnetic mirror causes an isotropization effect in the convergent side, while increases the initial anisotropy on the divergent side. Additional potential and density gradients are developed as a consequence of expanding a species with a non-isotropic pressure tensor. The gradients sign depends on the convergent/divergent character of the nozzle and on the type of anisotropy. ; Esta Tesis presenta una investigación experimental y teórica acerca de los principios físicos que rigen la expansión del plasma en toberas magnéticas. Este trabajo está motivado por el surgimiento de nuevos conceptos de cohetes de propulsión eléctrica que incluyen toberas magnéticas para confinar y acelerar el chorro de plasma. Esta investigación ha sido llevada a cabo mediante un acuerdo bilateral entre el Office National d'Etudes et de Recherches Aérospatiales de París y el Equipo de Propulsión Eléctrica y Plasmas de la Universidad Carlos III de Madrid. La primera parte de este estudio consta de una investigación experimental en un motor de resonancia ciclotrónica de electrones. Se presenta un procedimiento innovador basado en la integración de la señal de una bobina diamagnética durante el apagado del motor para estimar la temperatura perpendicular media de los electrones dentro de la fuente. La señal se utiliza para estimar la temperatura perpendicular media a través de modelos teóricos 1D y 2D. Los resultados están en línea con medidas directas de empuje magnético en el motor llevadas a cabo en una investigación anterior. La tobera magnética del motor se ha caracterizado con sondas electrostáticas y fluorescencia inducida por láser. Los perfiles del potencial ambipolar del plasma, velocidad de iones, densidad del plasma y temperatura de electrones se han obtenido para distintos valores del gasto másico de propelente. Combinando ambas técnicas, se han obtenido perfiles completos (desde la salida del motor a aguas abajo en la pluma) del potencial ambipolar, lo que permite estimar la caída de potencial total en la tobera. Se han obtenido distintas velocidades de enfriamiento de electrones para el motor de imanes permanentes y el de bobinas, lo que está ligado a la diferencia en la topología magnética de sus toberas. Los datos experimentales se han comparado con un modelo paraxial estacionario, mostrando una buena correlación en los perfiles de potencial, velocidad de iones y densidad del plasma. La comparación del modelo con los datos experimentales permite estimar la posición de la transición sónica en la tobera, que parece estar desplazada de la garganta magnética. Una publicación reciente de otro grupo de investigación confirma nuestros resultados. La segunda parte de esta Tesis presenta una investigación teórica acerca de la termodinámica de iones y electrones en la tobera magnética. Un modelo cinético no colisional, paraxial y estacionario se ha utilizado para investigar en varias direcciones: primero, se discute el efecto macroscópico de los aspectos cinéticos de la expansión. Los electrones apenas están afectados por espejo magnético, ya que solamente desarrollan anisotropía muy aguas abajo. En cambio, el espejo magnético es un efecto dominante para los iones, especialmente en el caso de iones "calientes". Se han obtenido leyes paramétricas para las propiedades asintóticas del plasma, lo que permite realizar una estimación rápida de las actuaciones de la tobera. La función de distribución de electrones en la fuente se ha formulado diferente a Maxwelliana. Se presenta el caso de dos poblaciones de electrones con distintas temperaturas, evaluando los aspectos cinéticos de una capa acusada cuasineutra. La caída de potencial total en la tobera depende de la fracción de electrones calientes y de su temperatura, y determina el grado de anisotropía de esta población. Finalmente, la función de electrones en la fuente se ha formulado como una bi-Maxwelliana anisótropa. Se discute la respuesta del plasma en una tobera convergente-divergente y en una totalmente divergente. El espejo magnético causa una isotropización de la función de distirbución de electrones en la región convergente, mientras que aumenta el grado de anisotropía en la región divergente. Se desarrollan gradientes fuertes de potencial eléctrico y densidad como consecuencia de expandir una especie con un tensor de presiones anisótropo. El signo de los gradientes depende del carácter convergente/divergente de la tobera y del tipo de anisotropía. ; This work was supported by the Spanish R & D National Plan (Grant No. PN ESP2016-75887) and by the European Union Horizon 2020 project MINOTOR, that has received funding from the research and innovation program under grant agreement No 730028. ; Programa de Doctorado en Mecánica de Fluidos por la Universidad Carlos III de Madrid; la Universidad de Jaén; la Universidad de Zaragoza; la Universidad Nacional de Educación a Distancia; la Universidad Politécnica de Madrid y la Universidad Rovira i Virgili ; Presidente: Carlos Hidalgo Vera.- Secretario: Pablo Fajardo Peña.- Vocal: Manuel Martínez Sánchez

This work presents a kinetic study of the plasma response in a fully magnetized plasma expansion. A paraxial, collisionless, steady-state plasma model is used to analyze the effect of expanding in a convergent-divergent nozzle a plasma with different types of VDF at the plasma reservoir. The first part of the paper studies the kinetic features, such as magnetic mirroring and anisotropy for both ions and electrons. The collective effects are analyzed by evaluating the momentum and energy equations based on the kinetic solution. The study of the collisionless heat fluxes allows estimating a simple closure to the fluid equation hierarchy for the electrons. The second part of the paper analyzes the solution with two species of electrons with disparate temperatures at the upstream source. A quasineutral steepened profile is formed, which impacts the plasma properties along the expansion. The collisionless electron cooling, as well as the anisotropy on the divergent side of the nozzle are analyzed by separating the different subpopulations of electrons (free, reflected, or doubly-trapped), of both the thermal and the suprathermal species. ; The work has been supported by Project ESP2016-75887 (Spain's National Research and Development Plan - MINECO/FEDER), and by the European Union Horizon 2020 project MINOTOR, that has received funding from the research and innovation program under grant agreement No 730028.

Proceeding of: 35th International Electric Propulsion Conference (IEPC), October 8-12, Atlanta, Georgia, USA ; Laser-induced fluorescence (LIF) is a non-intrusive technique that can provide useful information on ion production and acceleration in electric propulsion system. In this paper, spatially resolved LIF measurements of Xe+ are performed in the plume of an electron cyclotron resonance plasma thruster. The mapping of ion velocity distribution function in the magnetic nozzle shows that the ions are accelerated over a distance greater than 12 cm. A mean axial velocity up to 16 km/s has been obtained at 1 sccm and 26 W. The acceleration of ions is compared for different xenon flowrates. ; This work was made in the framework of project MINOTOR that has received funding from the European Union's Horizon H2020 research and innovation programme under grant No 730028.

Experimental characterization of plasma properties along the magnetic nozzle of an electron cyclotron resonance thruster is presented here. A permanent magnet (PM) prototype and a solenoid prototype are tested, whose main difference relies on the magnetic field strength and topology. A cylindrical Langmuir probe is used to measure plasma potential, plasma density and electron temperature. In the PM thruster setup, a laser induced fluorescence diagnostics is performed simultaneously with the Langmuir probe to measure the mean ion kinetic energy, and a Faraday gridded probe to characterize the angular plasma beam. An effective electron cooling rate has been identified, as well as the dependence of the total plasma potential drop with the mass flow rate. Results are compared with a supersonic collisionless fluid-kinetic 1D model where electron dynamics account for magnetic mirror effects and potential barriers, while ions are treated as a fluid cold species. The comparison allows to estimate the sonic transition of the plasma flow. ; The authors want to thank Mick Wijnen for his insightful comments and discussions. A preliminary version of this manuscript obtained the best PhD Communication Award at the Space Propulsion Conference 2018, 14–18 May, Seville. This work was made in the framework of project MINOTOR that has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 730028. Minor support came from the Spanish R & D National Plan (Grant No. PN ESP2016-75887).

Proceeding of: Space Propulsion Conference, SP 2018,14-18 mayo, Sevilla ; This paper presents experimental results on the magnetic nozzle of the 50 W Electron Cyclotron Resonance (ECR) thruster of ONERA, consisting of a 27 mm diameter ECR cavity and a fully divergent magnetic nozzle, created by a Neodymium permanent magnet. The diagnostics installed are a cylindrical Langmuir probe to measure plasma potential, plasma density and electron temperature, and a Laser Induced Fluorescence set-up to measure the mean ion kinetic energy. Both the ion velocity and plasma potential profiles seem to be independent of the mass flow rate when normalized with the electron temperature estimated at the sonic transition of the plasma flow. This sonic transition appears to be slightly shifted downwards of the thruster exit. Results are compared with a supersonic collisionless kinetic 1D model where electron dynamics account for magnetic mirror effects and potential barriers, while ions are treated as a fluid cold species. ; This work was made in the framework of project MINOTOR that has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 730028. This work was additionally supported by the Spanish R D National Plan (Grant No. ESP2013-41052-P).

Proceeding of: 35th International Electric Propulsion Conference (IEPC), October 8-12 2017, Atlanta, Georgia, USA ; This work presents experimental measurements along the magnetized plume of the ECR thruster developed by ONERA. Langmuir probes are used to determine the electron energy probability function (eepf) at different axial positions, revealing the non-local character of the distributions. The second part of the paper details a combined diagnostics of non-intrusive diamagnetic loop measurements and laser induced fluorescence to estimate the perpendicular electron temperature inside the plasma source. Averaged perpendicular electron pressure, plasma density, ion velocity at the exit plane and perpendicular electron temperature are determined as functions of the mass flow rate. ; This work was made in the framework of project MINOTOR that has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 730028.

A kinetic paraxial model of a collisionless plasma stationary expansion in a convergent-divergent magnetic nozzle (MN) is analyzed. Monoenergetic and Maxwellian velocity distribution functions of upstream ions are compared, leading to differences in the expansion only on second and higher-order velocity moments. Individual and collective magnetic mirror effects are analyzed. Collective ones are small on the electron population since only a weak temperature anisotropy develops, but they are significant on the ions all over the nozzle. Momentum and energy equations for ions and electrons are assessed based on the kinetic solution. The ion response is different in the hot and cold limits, with the anisotropic pressure tensor being relevant in the first case. Heat fluxes of parallel and perpendicular energies have a dominant role in the electron energy equations. They do not fulfill a Fourier-type law; they are large even when electrons are near isothermal. A crude electron fluid closure based on a constant diffusion-to-convective thermal energy ratio is shown equivalent to the much invoked polytropic law. Analytical dimensionless parameter laws are derived for the nozzle total electric potential fall and the downstream residual electron temperature. Electron confinement and related current control by a thin Debye sheath and a semi-infinite divergent MN are compared. ; This work was supported by the Spanish R&D National Plan (Grant No. PN ESP2016-75887) and by the European Union Horizon 2020 project MINOTOR, that has received funding from the research and innovation program under grant agreement No 730028. The authors thanks Dr Jesús Ramos for his interesting comments on this work.

The plasma-induced magnetic field in an electron cyclotron resonance plasma thruster is measured non-intrusively by means of a diamagnetic loop that encloses the plasma flow. The calibration process is described, and parasitic currents in the thruster walls and plasma oscillations are identified as the dominant sources of uncertainty. The integrated magnetic flux is seen to depend on the applied power and less significantly on the mass flow rate. The effect of the diamagnetic loop radius is also studied by testing two loops of different diameters. To estimate the perpendicular electron pressure in the plasma from the loop measurements, two plasma beam models, 1D and 2D, are used. While both models give similar results for the small loop, they differ significantly for the large loop, showing the relevance of 2D effects when a large diamagnetic loop is used. ; This work was made in the framework of project MINOTOR that has received funding from the European Union's Horizon 2020 research and innovation program under Grant agreement No 730028. Additional funding came from the Spanish R&D National Plan (Grant No. PN ESP2016-75887).