1,721,036 research outputs found
Neonatal leukemoid reaction and early development of bronchopulmonary dysplasia in a very low-birth-weight infant
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Cathode-less RF plasma thruster design and optimisation for an atmosphere-breathing electric propulsion (ABEP) system
Full text linkAtmosphere-breathing electric propulsion (ABEP) is a concept that ingests residual atmospheric gases as a source of propellant for an electric thruster, removing the need for onboard propellant storage. This would enable continuous low-thrust drag compensation, extending the lifetime of spacecraft in Very-Low Earth Orbit (VLEO); <250 km. VLEO is an appealing region for spacecraft operations, enabling new remote sensing missions with improved radiometric performance and spatial resolution, whilst reducing size, mass and power requirements, as well as mission cost. A preliminary design review and optimisation is therefore conducted for an ABEP system that uses the cathode-less radio frequency (RF) plasma thruster from Technology for Innovation & Propulsion (T4i) S.p.A. This removes the issue of thruster erosion by means of magnetic confinement and offers reduced susceptibility to varying atmospheric composition. A semi-empirical oxygen-nitrogen global source model (GSM) has been developed which considers the volume-averaged flux, momentum, and energy balance of the RF discharge. This includes a detailed chemistry model for the complex electron-molecular reactions and energy-loss channels of air plasma in the ionisation chamber. The GSM is coupled to an analytical model of flux balance for an air intake, verified by Direct Simulation Monte-Carlo (DSMC) simulation, to consider its design for maximum collection efficiency. This is then utilised in a robust multi-objective optimisation of the ABEP system, accounting also for spacecraft aerodynamics and power requirements
Atmosphere-Breathing Electric Propulsion (ABEP) System using a Cathode-Less RF Plasma Thruster: Design and Robust Optimisation for VLEO
No full textAtmosphere-breathing electric propulsion (ABEP) is a concept that ingests residual atmospheric gases as a source of propellant for an electric thruster, removing the need for onboard propellant storage. This would enable continuous low-thrust drag compensation, extending the lifetime of spacecraft in Very-Low Earth Orbit (VLEO); <250 km. VLEO is an appealing region for spacecraft operations, enabling new remote sensing missions with improved radiometric performance and spatial resolution, whilst reducing size, mass and power requirements, as well as mission cost. ABEP is equally applicable to any celestial body with atmosphere. However, the presence of reactive chemical species, including atomic oxygen in VLEO, is a lifetime-limiting cause of discharge channel, grid and hollow cathode erosion in conventional EP systems such as ion and Hall-effect thrusters. A preliminary design review and optimisation is therefore conducted for an ABEP system that uses the cathode-less radio frequency (RF) plasma thruster technology from T4i S.p.A. This removes the issue of thruster erosion by means of magnetic confinement and offers reduced susceptibility to varying atmospheric composition. A semi-empirical oxygen-nitrogen global source model (GSM) has been developed which considers the volume-averaged flux, momentum, and energy balance of the RF discharge. This includes a detailed chemistry model for the complex electron-molecular reactions and energy-loss channels of air plasma in the ionisation chamber. The GSM is coupled to an analytical model of flux balance for an air intake, verified by Direct Simulation Monte-Carlo (DSMC) simulation, to consider its design for maximum collection efficiency. This is then utilised in a robust multi-objective optimisation of the ABEP system, accounting also for spacecraft aerodynamics and power requirements
Development of a lumping methodology for the analysis of the excited states in plasma discharges operated with argon, neon, krypton, and xenon
Full text linkIn this paper, a methodology is presented to compute the plasma properties (e.g.,, density and temperature) accounting for the dynamics of the excited states. The proposed strategy applies to both zero-dimensional (0D) models and multidimensional fluid and hybrid codes handling low-pressure (<50 mTorr) plasma discharges filled with argon, neon, krypton, and xenon gases. The paper focuses on two main aspects: (i) a lumping methodology is proposed to reduce the number of reactions and species considered in order to keep at bay the computational cost without a major loss of accuracy; (ii) the influence that different datasets of cross sections have on the results has been assessed. First, the lumping methodology has been implemented in a 0D model accounting for singly charged ions, neutrals, along with 1s and 2p excited states (Paschen notation). Metastable and resonant are treated as two separate species within the 1s energy level respectively). The results have been benchmarked against those obtained treating each energy level of the excited states as an individual species. Differences lower than 1% have been obtained. Second, the results of the 0D model have been compared against measurements of electron density and temperature performed on an inductively coupled plasma. Numerical predictions and experiments present a disagreement up to 20%-30%, which is comparable to the uncertainty band of the measurements. Finally, the lumping strategy has been implemented in a 2D fluid code to assess its computational affordability, and the results have been compared against the experiments as well. A variance up to 30% in electron density and temperature is registered adopting different datasets of cross sections
Fully kinetic study of facility pressure effects on RF-source magnetic nozzles
No full textA fully kinetic 2D axisymmetric Particle-in-Cell (PIC) model is used to examine the effects of background facility pressure on the plasma transport and propulsive efficiency of magnetic nozzles. Simulations are performed for a low-power (150 W class) cathode-less radio-frequency (RF) plasma thruster, operating with xenon, between background pressures up to 10(-2) Pa and average electron discharge temperatures of 4-16 eV. When the electron temperature within the near-plume region reaches 8 eV, a decisive reduction in performance occurs: at 10(-2) Pa, in-plume power losses surpass 25% of the discharge energy flux. Given that the ionisation energy for Xe is 12 eV, the 8 eV threshold indicates that a consistent percentage of electrons has energy enough to trigger ionisation. On the other hand, when the temperature is below such threshold, the primary collisions are charge-exchange and inelastic ion scattering, and the power loss remains less than 10%. It is established that losses in the considered thruster are significant if the facility pressure is greater than 10(-3) Pa, at absorbed powers larger than 130 W. At the nominal 150 W, this results in a 15% thrust reduction. When facility pressure is taken into consideration over ideal vacuum simulations, numerical error is reduced to <30% when compared to experimental thrust measurements at 10(-3) Pa
Modelling and design of Earth and Mars atmosphere-breathing electric propulsion systems (ABEP) using a cathode-less RF thruster
No full textAtmosphere-breathing electric propulsion (ABEP) is a concept of electric propulsion system that has the potential to revolutionise space mission scenarios by using the air from the atmosphere as a propellant source instead of relying on a stored reservoir. This promising technology could enable very low Earth orbit (VLEO) mission scenarios, providing a clean, efficient, and sustainable propulsion system for spacecraft. Due to the significant change of atmospheric composition with altitude, which decisively affects the performance of the ABEP system, accurately simulating ABEP plasma chemistry plays a crucial role in the mission design. However, achieving a proper estimation of the propulsive performance surely represents a challenging task, as a result of the highly complex plasma dynamics as well as the large number of species involved. In this study, a numerical routine was developed with the aim of portraying the performance of a radiofrequency ambipolar thruster as a whole. First, a DSMC simulation of the engine intake is carried out at a particular pressure level and atmospheric composition; the resulting flow properties are then used as input to a 0D Global Source Model (GSM) that evaluates the generation of plasma inside the ionisation chamber. Lastly, the plasma expansion in the magnetic nozzle is simulated by means of a fully-kinetic 2D3V Particle-in-Cell model. The modelling of the background neutral density of the atmosphere and its interaction with the plasma plume has been included as well
Semi-Analytical Model of a Helicon Plasma Thruster
No full textThis article is devoted to the presentation of a semianalytical model of a helicon plasma thruster based on: 1) a 0-D global source model (GSM) for the simulation of the plasma source; 2) a monodimensional acceleration model for the plasma expansion and thrust production in the magnetic nozzle; and 3) a detachment criterion for the identification of the location in the nozzle where the plasma separates from the magnetic field lines. The developed GSM permits the simulation of magnetic topologies provided for radial cusps in the source region. Three different acceleration models presented in the literature have been implemented and employed for component 2) of the full thruster model. Two detachment criteria have also been considered for component 3). A comparison between the characteristics and the results of the different models employed has been carried out. Finally, the results obtained by all the combinations of acceleration models and detachment criteria presented have been compared against the solutions of a numerical fluid code and experimental thrust measurements. A maximum relative deviation between the results of the acceleration models of 3% was found, while a thrust variation of up to 20%-30% was recorded by changing the detachment criteria. A maximum relative error between the model's predictions and the experimental measurements of 25% was obtained
Analysis of the plasma transport in numerical simulations of helicon plasma thrusters
Full text linkThe accurate simulation of the plasma transport in helicon sources is a key aspect to improve the design of Helicon Plasma Thrusters (HPTs). Specifically, the 3D-VIRTUS code was proven to provide satisfactory estimations of the propulsive performance of realistic HPTs (difference between measures and numerical estimations of the thrust <30%). Nonetheless, further investigations are needed to deepen the influence that the plasma chemistry model, the formulation of the energy equation, and the definition of the diffusion coefficients have on the results of the simulation. First, a quantitative analysis has been conducted on a simplified configuration of HPT to study each phenomenon separately. Second, the generalized fluid model has been benchmarked against measures of plasma density performed on a helicon source. The radiative decay reactions affect the estimation of the performance (e.g., thrust) up to 40%. The quasi-isotherm formulation of the energy equation affects results (e.g., electron density) up to 30%. Accounting for anomalous transport or defining diffusion coefficients classically does not have a major effect on the simulation (e.g., thrust varies less than 20%). The generalized formulation of the fluid model provides estimations of the plasma density, which are within the uncertainty band of the measures (i.e., differences <20%)
Predicting the antenna properties of helicon plasma thrusters using machine learning techniques
Full text linkWhen designing helicon plasma thrusters, one important characteristic is the impedance of the radio-frequency antenna that is used to deposit power into the plasma. This impedance can be characterized both experimentally and numerically. Recently, a numerical tool capable of predicting the antenna impedance, called Adamant, has been developed. However, Adamant takes a long time to run and has high computer resource demands. Therefore, this work has been done to evaluate whether machine learning models, trained on Adamant-generated data, can be used instead of Adamant for small design change evaluations and similar works. Six different machine learning models were implemented in MATLAB: decision trees, ensembles, support vector machines, Gaussian process regressions, generalized additive models and artificial neural networks. These were trained and evaluated using nested k-fold cross-validation with the hyperparameters selected using Bayesian optimization. The performance target was to have less than 5% error on a point-to-point basis. The artificial neural network performed the best when taking into account both maximum error magnitudes and generalization ability, with a maximum error of 3.98% on the test set and with considerably better performance than the other models when tested on some practical examples. Future work should look into different solver algorithms for the artificial neural network to see if the results could be improved even further. To expand the model’s usefulness it might also be worth looking into implementing different antenna types that are of interest for helicon plasma thrusters
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