1,721,093 research outputs found
Preliminary thrust characterization of a T5 hollow cathode
No full textPrevious testing of conventional T6 hollow cathodes has shown performance sufficient to consider displacing dual chemical/ion thruster propulsion systems with a common hollow cathode thruster (HCT)/ion thruster system. Future design optimization requires a detailed understanding of the influence of terminal parameters on all thrust generation mechanisms. As the next step in this process, this paper presents a preliminary thrust characterization of the T5 hollow cathode with argon using a pendulum thrust balance as a laser optical lever with a position sensitive photo-detector. Influence of terminal parameters such as geometry, flow rate and discharge current are discussed with respect to thrust parameters. This work identifies the possibility of using a T5 HCT as part of an all-electric propulsion system for the proposed European Student Moon Orbiter (ESMO). The all-electric spacecraft presents significant mass savings over conventional configurations while the preferable electrical characteristics of the hollow cathode discharge also make standalone thruster a candidate for micro satellite applications
Time distributions of solar energetic particle events: Are SEPEs really random?
No full textSolar energetic particle events (SEPEs) can exhibit flux increases of several orders of magnitude over background levels and have always been considered to be random innature in statistical models with no dependence of any one event on the occurrence of previous events. We examine whether this assumption of randomness in time is correct.Engineering modeling of SEPEs is important to enable reliable and efficient design of both Earth-orbiting and interplanetary spacecraft and future manned missions to Mars and the Moon. All existing engineering models assume that the frequency of SEPEs follows a Poisson process. We present analysis of the event waiting times using alternative distributions described by Le´vy and time-dependent Poisson processes and compared these with the usual Poisson distribution. The results show significant deviation from a Poisson process and indicate that the underlying physical processes might be more closelyrelated to a Le´vy-type process, suggesting that there is some inherent ‘‘memory’’ in the system. Inherent Poisson assumptions of stationarity and event independence areinvestigated, and it appears that they do not hold and can be dependent upon the event definition used. SEPEs appear to have some memory indicating that events are notcompletely random with activity levels varying even during solar active periods and are characterized by clusters of events. This could have significant ramifications forengineering models of the SEP environment, and it is recommended that current statistical engineering models of the SEP environment should be modified to incorporate long-term event dependency and short-term system memory
Development of an indirect measurement Micro-to-Milli-Newton thrust balance
No full textThe performance of an indirect pendulum thrust balance suitable for micro-Newton level thrusters is presented. A design based on a frictionless pendulum with a laser optical lever and detection system makes it possible for thrusters to be mounted independently of the thrust measurement system. This simplifies the process of calibration without having to compensate for electrical connections, flow lines and other equipment. Measurements are not limited by thruster mass and offer thrust measurements down to 80?N. The balance was demonstrated by measuring the performance of two micro propulsion devices; the T5 and T6 hollow cathode thrusters. The T5 generating thrusts from 0.1mN to 1.4mN and the T6 showed thrusts from 0.2mN to 1.9mN
Understanding hollow cathode thrust production mechanisms
No full textThis paper reviews some of the design considerations for improving hollow cathode thrust efficiency and investigates propellant acceleration mechanisms based on experimental testing of a T5, T6 and ¼ inch cathode. A pendulum micro-thrust-balance was built for the measurement of thrust levels in the T5, T6 cathodes while a hemispherical energy analyzer was used for ion energy measurements in the ¼ inch hollow cathode. The data suggest that the thrust mechanism of hollow cathodes is composed primarily of gas dynamic mechanisms, particularly as a result of an intense electron pressure at the cathode exit. At high currents and low flowrates magneto-hydro-dynamic (MHD) forces arising from the self-induced azimuthal magnetic field within the orifice become significant in the acceleration proces
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
Hollow cathode chemical modelling
No full textIn this paper the state of hollow cathode life time modelling at the University of Southampton will be reported. Two models have been developed: one for BaO depletion from the hollow cathode insert and another for low work function compounds deposition and desorption. The model developed to predict BaO depletion from hollow cathode insert will be presented together with some comparison between experimental and numerical data to prove its validity.A model for low work function compounds deposition and desorption will also be presented. This model will be used to simulate the NSTAR cathode showing a very conservative estimate of the cathode life due to conservative character of the hypotheses made in the model development and due to the chosen criteria for the end of life
Hollow cathode thrust mechanisms
No full textThis paper investigates propellant acceleration mechanisms within hollow cathode thrusters based on the results of previous experimental testing. Experiments were conducted on a T5, T6 and ¼ inch cathode. An indirect pendulum micro-thrust-balance was built for the measurement of thrust production in the T5, T6 cathodes while a hemispherical energy analyzer was used for ion energy measurements in the ¼ inch hollow cathode. The data suggest that the thrust mechanism of hollow cathodes is composed primarily of gas dynamic mechanisms, particularly as a result of an intense electron pressure at the cathode exit, and magneto-hydro-dynamic (MHD) forces at high currents and very low flow rates arising from the self-induced azimuthal magnetic field within the orifice. While this initial characterization can only loosely attribute the relative magnitude each mechanism plays it does show evidence for each. Smaller cathodes, such as the T5, operate at lower currents in a more arcjet like mode and show the necessary performance and efficiency to compete with conventional propulsion systems at a mission level
A dual Stage ion engine for high impulse missions
No full textIn this paper, the applicability of dual stage ion optics and in particular of the so-called dual stage ion engine to high power, high specific impulse missions will be evaluated. First, the performance limits of conventional two gridded ion engines (GIE) will be discussed and the advantages provided by dual stage ion engines reported. The limits of applicability of a dual stage ion engine will be analyzed analytically and the results confirmed numerically. The lifetime and performance of a three gridded dual stage ion engine (DS3G) will be numerically investigated and compared to those of a conventional GIE assessing for the first time in the open literature under what condition dual stage ion optics provide performance improvements over conventional GIEs and what is its impact on the thruster lifetime. Dual stage ion engines have been found to be capable of providing higher thrust density and longer lifetime with respect to conventional gridded ion engines
Investigation of the discharge characteristics of the T6 hollow cathode operating on several inert gases and Kr/Xe mixture
No full textXenon is currently the propellant of choice for gridded ion thrusters. But in order to make deep space missions feasible, an increase in the Specific Impulse (SI) that these thrusters can achieve is necessary. One method of achieving this is to use a propellant with a lower atomic mass (e.g. argon). However, the feasibility of operating the hollow cathode using these alternative propellants has to be demonstrated. Moreover, interest in decreasing the propellant cost in missions and ground testing (especially life tests) have led to the comprehensive discharge characterisation of several gases that will be presented in this paper. The tests were carried out in diode configuration using a T6 hollow cathode with an enclosed keeper design employing xenon, krypton, argon and a Kr/Xe mix. The discharge initiation tests were undertaken with a view to investigate some of the factors thought to influence the starting potential such as mass flow rate and tip temperature. It was found that, for mass flow rates ranging from 0.2–1.1 mg/s and cathode tip temperatures ranging from 900–1300°C, the breakdown potential was less than 50V for argon, less than 25V for krypton, less than 21V for xenon and less than 35V for the Kr/Xe mix. The discharge initiation results were then compared to those obtained by Fearn et al. [8] with a T5 cathode operating on mercury and with a T6 cathode utilising an open keeper design using xenon propellant [3]. Steady state discharge behaviour was also investigated under a range of operating conditions. Spot to plume mode transitions were observed in argon, krypton and Kr/Xe discharges
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