The application of ion thrusters to high thrust, high specific impulse nuclear-electric missions
- Paper number
IAC-05-C3.5-C4.7.04
- Author
Dr. David G. Fearn, EP Solutions, United Kingdom
- Year
2005
- Abstract
This report forms one part of a study by members of the IAA to examine the merits of nuclear electric propulsion (NEP) for challenging deep space missions requiring a high value of velocity increment, ΔV. The need for a large value of ΔV, typically of many km/s or even tens of km/s, eliminates the use of chemical propulsion owing to the large mass penalty incurred by employing this traditional technology. This mass penalty exists because the effectiveness of space propulsion depends critically upon the exhaust velocity achieved by the engine utilised, or its specific impulse (SI); basically, the higher the SI the smaller the propellant load required to complete the mission. Whereas the best conventional chemical system, using liquid oxygen and liquid hydrogen, is limited to an SI of about 470 s, a typical gridded ion engine (GIE) can readily yield 3500 to 6000 s, and much higher values are being realised in preparation for future interplanetary missions. In experimental work, values approaching 30,000 s have been reported. It is significant that many missions which have been studied in depth require values of SI of between 5000 and 10,000 s. These are clearly within the range that can be provided by GIEs, but are currently well beyond those appropriate to Hall-effect thrusters (HETs) and arcjets. Moreover, it is unlikely that typical existing magnetoplasmadynamic (MPD) thrusters can reach the higher of these values. It is also worth noting that the exhaust velocity of a GIE is determined only by the characteristics of the ion extraction grid system used, and that, in principle, any desired value can be achieved merely by altering the applied voltages. Thus the GIE has the potential to be tuned exactly to provide the required SI, thereby optimising the mission characteristics. No other relatively high thrust EP system has this ability. It should also be noted in passing that no electric propulsion system can match the high thrust provided by almost any chemical engine. Thus chemical systems cannot be disregarded in designing the missions and spacecraft of interest, since they will still be necessary for manoeuvres in which high thrust is needed for specific purposes. An example would be to achieve planetary capture from a situation in which the approach velocity is substantial and there is little remaining time in which to conduct this manoeuvre. This report therefore reviews in some depth the ways in which GIE systems might cover the power range from a few tens of kW to several MW. By consideration of the relevant scaling relationships, which have been validated up to about 30 kW, it is concluded that the same basic concepts will suffice for this entire range of power levels. This conclusion is certainly valid for the Kaufman-type of direct current (DC) discharge thruster, for the 1 MHz-type of radiofrequency ionisation thruster (RIT), and for electron cyclotron resonance (ECR) ionisation thrusters which do not use permanent magnets. Unfortunately, the severe temperature constraints on high field permanent magnets suggest that the cusp-field type of DC thruster cannot operate at the very high power levels considered here. It is shown that the design process is aided considerably by the separation of the ion production and extraction/acceleration regions in gridded thrusters. Thus the ion beam parameters can be deduced without reference to the ionisation mechanism employed to produce the plasma from which the ions are extracted. Consequently, the two regions of the thruster can be designed separately, which is a simplifying benefit only available to gridded devices. It is concluded that the required power density can be achieved and exceeded using GIEs, by operating their grid systems at very high perveance and by raising the SI to values which are significantly above those commonly employed at present. The latter ensures that most of the energy supplied is used in accelerating the beam ions, thereby allowing extremely high values of electrical efficiency to be achieved; in the limit, this parameter exceeds 99For the lower power applications, normal twin- or triple-grid configurations are entirely adequate if higher perveance operation is adopted and if the SI can be increased above current values. However, the limit here is due to the penetration of the plasma sheath at the innermost grid into the discharge chamber plasma, which increases as the electric field becomes larger or the plasma density lower. This greater curvature of the sheath, which effectively emits the beam ions, then causes direct impingement of high energy ions onto the outer grids, severely limiting lifetime. With this configuration, the maximum ion extraction potential is likely to be about 5 kV. If Xe propellant is used, this gives an SI of approximately 7000 s, depending upon the propellant utilisation efficiency achieved. A total of about 100 kW power consumption can then be realised with a small array of thrusters. As the applied potentials increase, a more complex but satisfactory alternative is to employ a 4-grid system with both a greater perveance and providing an increased SI. In all cases, additional research and development are needed, since this regime has not been properly explored to date. The lifetime implications are of particular interest and may dictate the way forward. Thus the operational envelope can be massively extended by use of the 4-grid configuration. There is considerable documentation concerning such systems in the controlled thermonuclear research (CTR) community, where ion accelerators of the size required, or smaller, have been constructed which produce MW beams at 70 or 80 keV. This is possible because the 4-grid arrangement permits the ion extraction process to be separated from the main acceleration region, and the limitation of the sheath penetration no longer applies. Thus the extraction part of the system can be designed to operate at near maximum perveance, and the subsequent further acceleration of the ions can be dealt with independently in the design process. An additional advantage of this configuration is the very low beam divergence typically found. This can be less than 1° at an acceleration potential of 70 kV. It is thus concluded that well understood thruster technology, when combined with the 4-grid configuration based on that utilised in CTR ion injection machines, will permit MW power levels to be achieved. Thus a relatively small array of thrusters, with beam diameters not exceeding 40 cm, will be able to consume the several MW, although the SI, using Xe propellant, is likely to be somewhat above 10,000 s. With this arrangement, the power density can reach 4.5 kW/cm2 and the thrust density 30 mN/cm2 and, if required, the specific impulse can attain 30,000 s with Xe. If higher values of SI are required, the utilisation of lower atomic mass propellants will permit this to be achieved, with an ultimate limit using hydrogen compounds of about 150,000 s. As a specific example, an array of nine 40 cm beam diameter thrusters using Xe propellant and operating at 10 kV, but with an ion extraction potential of 5 kV, will consume 7.4 MW if the perveance is limited to 50 To summarise, the design for a 20 to 50 kW thruster can be extrapolated directly from current capabilities, taking advantage of the higher perveance values made possible by introducing improved grid technologies. However, at the higher power level it would be advantageous to utilise a 4-grid configuration. Thus an array of selected existing devices could probably consume up to 100 kW or so, although none of these thrusters are space qualified and they all require life-testing. The work required to meet a multi-100 kW level, utilising the 4-grid configuration, would be much more extensive, so would require more time and greater funding. A multi-MW system can be implemented using extended scaling relationships, but there is no precedent for such an exercise. Thus this complete process, encompassing design, development, performance testing, environmental testing and life-testing, would require to be organised and funded. Bearing in mind the need to co-ordinate such a development very closely with that of the power source, work on both systems should be conducted in parallel, with very close collaboration at all times. This co-operation is desirable because it is envisaged that the bulk of the power consumed by the thrusters would be supplied directly from the source, with no further power conditioning. Finally, it should be noted that the performance- and life-testing of large, high power thrusters requires the use of major facilities with substantial pumping speeds, which are costly to build and to operate. The provision of such facilities must precede the development of the thrusters.
- Abstract document
- Manuscript document
IAC-05-C3.5-C4.7.04.pdf (🔒 authorized access only).
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