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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.

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]

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]

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]