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Presents information on a study which examined a numerical model of thermoelectric phenomena leading to cathode-spot ignition. Description of the model; Results and discussion; Conclusion.

Iodine has proven to be a promising alternative to xenon as a propellant for Hall and ion thrusters. High atomic mass and favourable ionization characteristics, comparable to xenon in terms of propulsive performance, together with lower cost compared to xenon makes it an attractive option. However, its reactivity and corrosive nature raise concerns. Tests to assess material compatibility and determine whether thruster components are susceptible to corrosion are unavoidable for its safe adoption as a propellant. This paper describes the main results of an experimental campaign aimed at a qualitative assessment of the compatibility with iodine of different materials. The study included more than fifteen materials that find application in electric propulsion devices, such as cathodes, thrusters and feeding systems, including ceramics such as LaB 6 , polymers, iron and aluminum alloys, and graphite. The specimens were exposed to iodine vapor environments at temperatures reaching approximately 200 °C for extended periods (from three to over 30 h) or were directly exposed to liquid iodine. The experimental setup and results of morphological and elementary analyses are presented and discussed. The research provides some insights into the viability of using iodine as a propellant and the potential challenges associated with its corrosive properties. [ABSTRACT FROM AUTHOR]

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 10^5~\mathrmAm^-2 at temperatures of $\sim 1900$ 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. [ABSTRACT FROM PUBLISHER]

Thermionic hollow cathodes are currently used as sources of electrons in a variety of space applications, in particular as cathodes/neutralizers of electric thrusters (Hall effect and ion thrusters). Numerical tools are needed to guide the design of new devices before their manufacturing and testing, since multiple geometrical parameters influence the cathode performance. A reduced-order, numerical model was developed to assess the performance of orificed hollow cathodes, with a focus on the operational lifetime. The importance of the lifetime prediction is tied to its impact on the operational lifetime of the thruster to which the cathode is coupled. The cathode architecture consists of a refractory metal tube with an internal electron emitter made of lanthanum hexaboride (LaB 6 ). The choice of LaB 6 accounts for the reduced evaporation rate, the low sensitivity to poisoning and the absence of an activation procedure with respect to oxide cathodes. A LaB 6 emitter is thus a valuable option for long-lasting cathodes, despite its relatively high work-function and reactivity with many refractory metals at high temperatures. The suggested reduced-order model self-consistently predicts the key parameters of the cathode operation, shedding light on the power deposition processes as well as on the main erosion mechanisms. Preliminary results showed good agreement with both the experimental data collected by Alta and data available from the literature for different operating conditions and power levels. Next developments will include further comparisons between theoretical and experimental data, considering cathodes of various size and operating conditions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

Highlights • Thermodynamic analysis of a hybrid system with storage and production. • A method for comparing system performances with different kind of plant is defined. • Optimized arrangements, reach very high equivalent round trip efficiencies. • Oxy-combustion only produces carbon dioxide and water. • Carbon dioxide can be easily liquefied, using cold liquefied natural gas. Abstract The increasing penetration of renewable energy sources in the electricity generation scenario forces to face new challenges to achieve an effective management of the power system both in technical and economic terms. Traditional energy storage solutions, like electrochemical cells and pumped hydro energy storage appear critical in terms of economic sustainability and site-dependency. The use of compressed air as energy storage has been investigated since the 20th century, but, in its first configuration, it was affected by site constraints as pumped hydro plants do. Liquid air energy storage has the chance to overcome those limits, but the experimental studies have far reached low efficiency. However, by rising the highest cycle temperature with the addiction of fossil fuel energy, these results can be largely improved. The paper deals with the thermodynamic analysis of a hybrid system including energy storage and production based on a liquid air energy storage plant where only oxygen is liquefied, while liquefied natural gas is used as fuel. In the production phase, liquefied oxygen and natural gas react in an oxy-combustion chamber where a large amount of water is added to keep the temperature at an acceptable level by evaporation. The system does not require an external water supply since all the water needed is produced by the cycle itself, allowing the plant to be placed also in remote areas with poor water resources. At the beginning of the cycle, both the reagents are liquid at very low temperature (below −150 °C) and they need heat to be gasified; a large amount of this heat can be recovered from the combustion products, which, being cooled at suitable pressure, release liquid carbon dioxide which can thus be easily separated. Optimized arrangements, compared to the performances of the best available hybrid peak plants, even with sufficiently conservative hypotheses, reach high equivalent round trip efficiencies, even higher than 90%. [ABSTRACT FROM AUTHOR]

Low-power Hall effect thrusters (HETs) belong to a class of electric thrusters with an operating power lower than 500 W. The application of this class of HETs is suited for small satellites for telecommunications and Earth observation missions. Sitael is active in this field, through the development of two HETs, HT100, and HT400, belonging to a power class of 100 and 400 W, respectively. HT100 is a permanentmagnet thruster operating in the 100- to 250-W range, providing thrust between 4 and 13 mN, and specific impulse between 900 and 1400 s. HT400 operates in the 350- to 750-W range, providing thrust between 20 and 45 mN, and specific impulse between 1300 and 1700 s. Two cathodes have been developed and tested, referred to as HC1 and HC3, conceived for HT100 and HT400, respectively. Both cathodes are based on Sitael heritage in theoretical modeling and experimental activities for the development of such devices, and rely on lanthanum hexaboride emitters. HC1 is a cathode designed to provide a discharge current in the 0.3-1 A range, operating in steady-state conditions at mass flow rates between 0.08 and 0.5 mg/s of xenon. HC3 was designed for the range 1-3 A of discharge current, with 0.08-0.5 mg/s of mass flow rate. Both HC1 and HC3 have an expected lifetime higher than 104 h, based on the rate of material evaporation from the emitter surface, computed with the aid of a theoretical model developed to guide the cathode design. Experiments were carried out, including preliminary characterization campaigns, of each of the two cathodes and HET-cathode coupling tests. The collected data are presented and discussed with reference to the model predictions, showing a good agreement between theoretical and experimental results. [ABSTRACT FROM AUTHOR]

The current increase in the deployment of new renewable electricity generation systems is creating new challenges in balancing electric grids. Solutions including energy storage at small and large scales are becoming of paramount importance to guarantee and secure a stable supply of electricity. This paper presents a study about a hybrid solution including a large scale energy storage system coupled with power generation and fast responding energy storage systems. The hybrid plant is able to deliver the energy previously stored by using an air liquefaction process either with or without the contribution of additional energy from combustion. The paper also highlights how such hybrid plants may offer the chance of providing the grid with fast control services. An ideal energy storage technology should have a high power rating, a large storage capacity, high efficiency, low costs and no geographic constraints. The use of air as energy carrier has been studied since the 20th century with the first compressed air energy storage (CAES) systems. This technology is still considered to have a potential but it is geographically constrained, where suitable geological reservoirs are available, unless compressed air is stored in pressurized tanks with significant costs. Liquid Air Energy Storage (LAES) represents an interesting solution due to its relatively large volumetric energy density and ease of storage. Different process schemes for hybrid plants were modeled in this study with Aspen HYSYS® simulation software and the results were compared in terms of equivalent round-trip and fuel efficiencies. Equivalent round-trip efficiencies, higher than 80%, have been calculated showing that the proposed configurations might play an important role for power systems balancing in the near future. [ABSTRACT FROM AUTHOR]

The magnetohydrodynamics interaction around a conical test body in a hypersonic argon flow is experimentally investigated. The hypersonic flow is realized in the high-enthalpy arc-heated wind tunnel of AIta Società per Azioni (S.p.A.) (Pisa, Italy) at Mach 6. The aim of experimental activity is to produce an experimental data base to be used for the validation of numerical codes for the analysis of the magnetohydrodynamics interaction. The magnetohydrodynamics interaction is obtained in the shock layer of the conical test body at the exit of the hypersonic nozzle. The electrical configuration realized allows one to enhance the effect of the magnetohydrodynamics interaction on the plasma parameters. This is done using the Hall field to generate the magnetohydrodynamics interaction and by short-circuiting the Faraday current inside the plasma of the shock layer. The magnetic flux density is produced by an array of three magnets located in the test body. Tests are performed at three different stagnation pressures. Fluid-dynamic, electrical, and optical observations have been done. The experiment showed a considerable effect of the magnetohydrodynamics interaction on the values of the quantities measured. [ABSTRACT FROM AUTHOR]