Experimental Study for Electrode Configuration Problem in an MPD Thruster
- Paper number
IAC-06-C4.P.4.13
- Author
Mr. Daisuke Nakata, University of Tokyo, Japan
- Year
2006
- Abstract
NASA declared that they would pay the primary attention on large-scale interplanetary missions using Nuclear Electric Propulsion (NEP) after retirement of space shuttles. For such a NEP system, MPD MagnetoPlasmaDynamic) thruster will be a hopeful candidate in future because of its very small specific weight (<0.5kg/kW) and the reliability under the large input power (>100kW). In Addition, MPD thruster can use variety of gases or liquids as a propellant. However, the most significant problem of MPD thruster is its low propulsive efficiency. That discourages the motivation of practical use, and then, other electric propulsion (Hall or Ion thrusters) is widely accepted. On the other hand, varieties of electrode configuration are adopted in each organization in the world (Princeton University, University of Stuttgart, MAI, Centrospazio, Osaka University and so on) even considering the difference of the operating power range or operating condition (steady or pulsed). In the recent research, we pointed out the optimum configuration decided from quasi-one dimensional analyses, that was, the coupling of the converging-diverging anode and the long cathode. The reason why such converging-diverging geometry was efficient is as follows. It is well known that the discharge current tends to concentrate the root and tip of the cathode in “Straight type” electrode. But the current distribution pattern should be moderately uniform along the axial direction of the channel since any local concentration of discharge current causes extreme joule heating and the dissipation of the energy. In the view of electrothermal acceleration mechanism, this is acceptable when the heat energy is converted into the kinetic energy again, but if we want to use the electromagnetic force as a primary acceleration mechanism, this is nothing but a waste. The rate of energy converted into the heat energy should be minimized. converging-diverging geometry is preferable because it makes the discharge current easily drops at the “middle” region in the discharge chamber and arrange the entire current distribution to be uniform. Resultant “optimum” geometry was led under very simple assumption; i.e., particle pressure, ionization, pumping force and electrical sheath and other a lot of effects were ignored, and in addition, electrical conductivity was set as a constant value. Therefore, it might not be appropriate to apply this “optimum solution” to a real thruster. It is necessary to confirm it in the more precise simulation or direct experiment. Based on the above background, in this research, 12 types of configurations were prepared and their thrust performances were compared experimentally. The experiment was conducted under self-field, pulsed condition and argon was selected as a propellant because it was easy to compare the theoretical result. We made an effort to reduce the error of thrust measurement and mass shot measurement. So, we could identify the advantage of one configuration to the others clearly. As a result, converging-diverging electrode led 1.2 times higher efficiency than straight configuration. The degree was too close to the forecasted one in the theoretical analysis. On the other hand, the efficiency of the best configuration, which was still under 30 percent, was not enough to consider practical use. We think it will be necessary to use hydrogen or ammonia propellant. From the viewpoint of long-term storage, the latter was preferable. So we conducted the experiment in the case of ammonia propellant additionally. And then, we tried new cathode material including Y2O3 or La2O3 in above experiment. Such a series of experiment will be reported in the congress.
- Abstract document
- Manuscript document
IAC-06-C4.P.4.13.pdf (🔒 authorized access only).
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