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1 Iac-06-C4.4.7 the Innovative Dual-Stage 4-Grid Ion IAC-06-C4.4.7 THE INNOVATIVE DUAL-STAGE 4-GRID ION THRUSTER CONCEPT – THEORY AND EXPERIMENTAL RESULTS Cristina Bramanti, Roger Walker, ESA-ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands [email protected], Roger. Walker @esa.int Orson Sutherland, Rod Boswell, Christine Charles Plasma Research Laboratory, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200, Australia [email protected], [email protected], [email protected]. David Fearn EP Solutions, 23 Bowenhurst Road, Church Crookham, Fleet, Hants, GU52 6HS, United Kingdom [email protected] Jose Gonzalez Del Amo, Marika Orlandi ESA-ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands [email protected], [email protected] ABSTRACT A new concept for an advanced “Dual-Stage 4-Grid” (DS4G) ion thruster has been proposed which draws inspiration from Controlled Thermonuclear Reactor (CTR) experiments. The DS4G concept is able to operate at very high specific impulse, power and thrust density values well in excess of conventional 3-grid ion thrusters at the expense of a higher power-to-thrust ratio. A small low-power experimental laboratory model was designed and built under a preliminary research, development and test programme, and its performance was measured during an extensive test campaign, which proved the practical feasibility of the overall concept and demonstrated the performance predicted by analytical and simulation models. In the present paper, the basic concept of the DS4G ion thruster is presented, along with the design, operating parameters and measured performance obtained from the first and second phases of the experimental campaign. Finally, the implications of these findings for future gridded ion thruster developments and future mission applications are addressed. INTRODUCTION velocities of up to 4,000 km/s. One of the maximum cases recorded experimentally was at the Culham For over 20 years, ion sources, coupled with Laboratory, Oxford, UK with neutral atomic beam charge-exchange gas cells, have been used to inject injectors developed for the JET tokomak2. Extremely highly energetic neutral particles (80-100 keV) into high power densities of 6670 W/cm2 were achieved. fusion reactor experiments2-5. These ion sources are A new thruster concept, the Dual-Stage 4-Grid remarkably similar in design to gridded ion thrusters (DS4G) ion thruster, has been proposed by D. Fearn1 (without external neutraliser) with the exception that and drew inspiration from the above mentioned they use 4-grid systems in order to achieve extremely Controlled Thermonuclear Reactor (CTR) high accelerating potentials and hence ion velocities experiments. with light gases such as hydrogen. This 4-grid Due to the high beam potential, the Dual-Stage 4- configuration allows the ion extraction and ion Grid ion thruster concept is theoretically able to acceleration processes to be decoupled, essentially deliver substantial improvements in propulsive creating a dual-stage system. The CTR dual-stage ion performance over the current state-of-the-art gridded sources were only 10-40cm in beam diameter, but ion engines in terms of specific impulse, power and managed to process input powers from 250kW to thrust density, maximum power and beam divergence 4.8MW and accelerated ionised hydrogen to exhaust at the cost of a higher specific power. For example, a single 20 cm diameter 4-grid ion thruster could operate at 250 kW power to produce a 2.5 N thrust Copyright © 2006 by the International Astronautical and a specific impulse of 19,300 s using Xenon Federation. All rights reserved. 1 propellant operating at a 30 kV beam potential. These limited in order to avoid excessive penetration of the high power and thrust densities allow the maximum grid electric field into the discharge chamber attainable power for gridded ion engines to be containing the plasma. The resulting effect of this extended from 40 kW of the present day, to hundreds penetration is a curvature in the plasma sheath (or of kW. This aspect, in combination with the very high meniscus) attached to the holes in the screen (first) specific impulse and a lightweight power system, grid. Some penetration is desirable, since these results makes them especially suitable for large, high delta-V in a curved plasma sheath which both increases the future exploration missions, such as ‘delivery and ion emission area and aids focusing. Unfortunately, return’ cargo transportation of surface modules in low the higher the beam potential, the greater the orbits of the Moon and Mars needed for human curvature, and hence the greater the extracted ion exploration, or even for an advanced human Mars beamlet divergence, until significant direct ion mission itself. impingement occurs on the acceleration (second) grid The Advanced Concepts Team of ESA, after and deceleration (third) grid. having envisaged the potential and assessed possible Direct impingement of energetic ions causes applications of this concept, awarded a contract to sputtering of the grid material, leading to excessive Australian National University for the design and grid erosion (essentially hole widening), performance construction of a small experimental laboratory model degradation and eventually limitation of thruster thruster to be tested at the ESA-ESTEC Propulsion lifetime. Moreover, the plasma number density, n, in Laboratory. the discharge chamber must also be taken into This paper is aimed at the presentation of the account, since the penetration of the sheath increases results of the proof-of-concept experimental research as n falls. This problem is relevant to the use of a project on the Dual-Stage Four Grids (DS4G) ion wide throttling range6, or if the radial plasma density thruster. The experimental test campaign comprised distribution is strongly peaked. However, in the latter two successful test phases which were conducted in case the dimensions of the screen grid apertures can be the CORONA vacuum facility at ESTEC during matched to this distribution to alleviate adverse November 2005 and May 2006, with the aim of effects7. Thus it is clear that high values of E and/or demonstrating the practical feasibility of the 4-grids low values of n allow the sheath to penetrate deeply concept, verifying the high performance predicted by into the plasma. The resulting curvature of the surface the analytical and simulation models, and from which the ions are extracted causes many of the investigating critical design issues and technological trajectories to diverge from the desired paraxial challenges. direction, and to impact upon the accel or decel grids. In the following sections, the basic concept of the The erosion that this causes severely limits lifetime8. DS4G ion thruster will be presented; in particular, the This situation is depicted in Fig 1, in which the design, operating parameters as well as the performance obtained from the experimental test campaigns. The measurements performed by the diagnostics tools will then be illustrated. Finally, the implications of these findings for future gridded ion thruster developments and future missions will be addressed. DUAL-STAGE 4-GRID (DS4G) CONCEPT Electrostatic grid systems used in conventional state-of-the-art ion thrusters perform ion extraction & acceleration simultaneously in a single-stage process, Figure 1. Triple grid configuration, indicating the typically using three permeable grids closely spaced in effects of the sheath shape on ion trajectories series. In this scheme, the thruster body and screen sheath positions for a moderate and a high electric grid are at a high enough beam accelerating potential field, E, are shown. necessary to achieve the ion beam velocity required. In order to avoid beamlet divergence, impingement To raise the ion velocity, and thus the specific and sputtering, grid accelerating potential is limited to impulse, it is merely necessary to increase the beam a maximum of about 5 kV in present systems (usually potential as appropriate. This ultimately limits much less). Practically, this limits the achievable achievable performance in terms of power density, specific impulse to <10,000 s, thrust density to <0.5 thrust density, exhaust velocity, and specific impulse mN/cm2, and power density to <20 W/cm2 with Xenon because the inter-grid accelerating potential must be propellant. Due to these performance limits and 2 thermal/mechanical constraints placed on the thin, A further potential benefit of the dual-stage system closely spaced grids, present gridded ion thrusters is a very low beam divergence of about 2-6 deg only operate at powers of up to 40-50 kW maximum with due to the strong acceleration of the second stage, thus beam apertures of up to order 50 cm diameter. reducing further any possibility of adverse thruster- This problem is avoided by the use of 4 grids, spacecraft interactions, and also attitude control since the extraction field is defined by the first two of demands when compared to other thrusters. Present these (Fig 2). As previously mentioned, this gridded ion thrusters have typical divergence of 12-15 configuration was originated in the controlled deg, and Hall effect thrusters about 40 deg. Fig 3 thermonuclear reaction (CTR) research community in shows a schematic of the 4-grid ion thruster concept. order to produce very high energy particles for injection into fusion machines. Plasma sources fitted with such grid systems are used to accelerate ions, usually hydrogen,
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