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Hollow cathode life time modelling
No full textHollow Cathodes (HCs) are of primary importance in the field of electric space propulsion, being used as electron sources in ion and Hall-effect thrusters. Hence, their lifetime is a key factor in all these applications.HCs have demonstrated the capability of providing up to 30,000 hours of operation, whereas no direct experimental data exist above this limit.The importance of HC lifetime is a growing issue for deep space missions usingpropulsive systems based on ion or Hall-effect thrusters that may require longerlifetimes than those demonstrated up to now. To address these concerns about HCs andto prove the suitability of an ion thrusters based solar electric propulsion subsystem forfuture high-impulse missions (such as Bepi Colombo), a model able to predict the HClifetime is needed.The model that has been developed in this thesis consists of three parts: a barium oxidedepletion model, a low work function surface coverage model and a plasma updateprocedure to calculate the effects that a change in the insert surface work function willproduce on the cathode plasma.The barium-oxide depletion model has been validated by comparing its results withexperimental measurements performed at QinetiQ and NASA, showing a goodquantitative agreement.The low-work function surface coverage model is the first of its kind to include theeffect of ion bombardment. The plasma update procedure, even if semi-empirical, isable to produce results that are in good agreement with the measurements.Using these three models the lifetime of the NSTAR hollow cathode has beensimulated, yielding predictions that are in good agreement with the theoreticalexpectations
A model for hollow cathode surface work function evolution
No full textIn this paper a model to predict the evolution with time of the surface work function of an hollow cathode insert will be presented. The processes involved in BaO deposition and desorption on the surface is described and quantified using chemical data present in the literature. The model is compared with test results from the ELT NSTAR cathode. Predictions obtained from the model have been found to be qualitatively in agreement with the expectations and conservative, model predictions always overestimating the increases in work function with time. An explanation of this has been given noting that all the assumption made in the model tend to overestimate the desorption rates hence producing conservative estimate of the cathode lifetime. Possible ways to improve the model accuracy will be the development of an ignition model and the coupling of the model presented in this paper with a plasma model to take into account the effect that a change in the emission characteristics of the insert surface will produce on the cathode plasma
Numerical simulation of the insert chemistry of the hollow cathode from the Deep-Space 1 Ion Engine 30,000 hour life test
No full textA model for barium oxide depletion from hollow cathode inserts
No full textIn this paper, a chemical model to predict barium oxide depletion from hollow cathode insert will be developed.This model is based on the knowledge of the ternary diagramthat describes the BaO?CaO?Al2O3 system and takes also intoaccount the diffusive motion of barium oxide inside the insert. A comparison between numerical and experimental data is made to determine the diffusion coefficient inside the insert. The diffusion coefficient found presents an Arrhenius trend with activation energy similar to the one of barium oxide evaporation. A 2-D model is used to demonstrate how the temperature profile along the insert is a key parameter to calculate barium depletion. This is the first model the authors are aware of that includes boththe complex chemistry of the BaO?CaO?Al2O3 system and thediffusion motion of BaO from the insert core to the surface.<br/
Hollow cathode life time model
No full textIn this paper the latest advances in the hollow cathode lifetime modeling carried out at the University of Southampton will be presented. A barium oxide insert depletion model and a surface low work function compounds deposition/depletion model have been already developed by the authors. Here a procedure to update the plasma parameters relatively to changes of the hollow cathode surface work function and an end-of-life criterion will be presented. The procedure has been validated with experimental results and then coupled with the deposition/desorption model already developed by the authors. The evolution of the surface of the cathode tested for 28,000 hours by Sarver-Verhey has been simulated and from the data obtained an end of life criterion has been developed. Then using this criterion the ELT NSTAR discharge cathode has been simulated under the throttling conditions used during the Deep Space 1 Spare Ion Engine. The lifetimes found are in agreement with the theoretical expectation
Barium oxide depletion from hollow cathode insert, modelling and comparison with experiments
No full textIn this paper, the results of a barium oxide depletion model are compared with some experimental results. Themodel is used to simulate the T5 and T6 cathodes and the NSTAR discharge cathode. A comparison with the experimental data is performed. For the T5 and T6 cathodes, good qualitative agreement is found, but for theNSTAR cathode, the agreement is not as good. In both cases, the agreement is improved when the boundary conditions are modified to better reflect the experimental conditions. The model presented is the first three-dimensional axisymmetric insert model that includes the dependency of BaO depletion from both the impregnant chemistry and the diffusive motion inside the insert
A model for low work function compound deposition on hollow cathode insert surface
No full textThe chemistry of the low work function compounds on tungsten emitter surface has been presented qualitatively giving a possible explanation of the 4:1:1 ratio commonly used. The processes involved in BaO deposition and desorption have been described and quantified as well as possible given the chemical data present in the literature producing a simple model for deposition and desorption. The model has then been tested with the ELT NSTAR cathode. The results obtained from the model have been found to be very conservative as expected and a possible explanation has been given stressing the need of a coupled deposition-desorption-plasma model
Performance and lifetime testing of a pulsed plasma thruster for Cubesat applications
No full textThis paper describes the design and testing of the engineering model of a pulsed plasma thruster for Cubesat applications (PPTCUP-EM). It has been developed by Mars Space Ltd, Clyde Space Ltd and the University of Southampton with the main aim of increasing the in-orbit lifetime of Cubesats by providing drag compensation. Nevertheless, this thruster can be also utilized to perform formation flying, small orbit changes and Cubesat end of life deorbiting. A test campaign has been carried out to prove that the thruster and the conditioning electronics lifetimes are long enough to utilize all the propellant stored on-board. From the results of the test, the PPTCUP-EM can deliver a total impulse of 42.9±3.9 N s in about 1,125,000 shots. Moreover during the test campaign, a total of more than 1,800,000 shots have been achieved providing a safety factor of about 60% with respect to the number of required shots. The results gathered, including a preliminary characterization of the electromagnetic noise generated by the unit, are presented and show that the overall thruster performance is not influenced by the thruster aging
A thrust balance for low power hollow cathode thrusters
No full textA hanging thrust balance has been designed, manufactured and tested at the University of Southampton. The current design allows for direct steady thrust measurements ranging from 0.1 mN to 3 mN but this can be easily extended to measure thrust in a different range. Moreover the chosen balance design and the thrust measurement procedure allow for the cancellation of thermal drifts. The thrust balance was tested with a T6 hollow cathode thruster providing measurements with an uncertainty of about 9.7%. The thrust data were compared to those obtained with another direct thrust balance, and they are in quantitative agreement being the maximum difference only 6%
The thrust generated by a T6 ion engine hollow cathode
No full textThe thrust produced by a T6 ion engine main discharge hollow cathode was characterized using different propellants with a target-based measurement system, for discharge current values of 5–25 A and a wide range of mass flow rates. The calculated values of specific impulse are far in excess of those that could be attributed to the heating of a gas tothermal equilibrium with the walls, as in a resistojet. This would suggest an operation mechanism more similar to that of an arcjet. The main scaling parameter for the specific impulse appears to be the discharge power per unit mass flow rate (specific power)
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