A. Genovese

Indium Field Emission Electric Propulsion Microthruster Experimental Characterization

Indium liquid metal ion sources have been flying for more than 10 years on a variety of spacecraft for spacecraft potential control and as the core element of mass spectrometers. Since 1995, a dedicated indium field emission electric propulsion (In-FEEP) thruster has been under development and recently passed a 2000-h endurance test. The In-FEEP thruster is a micropropulsion device for the 1-100 μN thrust range with low thrust noise and high resolution. The latest performance characteristics including direct thrust measurements and beam profiles are summarized. This information is very important for many upcoming missions that require ultraprecise drag-free control such as the Gravity Field and Steady-State Ocean Circulation Mission, LISA, Terrestrial Planet Finder/Darwin, or SMART-2.

2000-Hour Endurance Test of Indium Field Emission Electric Propulsion Microthruster Cluster

An indium field emission electric propulsion (In-FEEP) thruster was recently selected as a micropropulsion candidate for ESA’s Gravity Field and Steady-State Ocean Circulation (GOCE) mission. Within a pre-verification phase, a cluster of two In-FEEP thrusters successfully completed a 24 h endurance test. This is the longest endurance test ever performed for field emission thrusters. Although the test did not include a neutralizer, the results show stable and good performance and suggest that lifetime is mostly limited by the propellant reservoir tank size. This information is very important for many upcoming missions that require ultraprecise drag-free control such as GOCE, LISA, Terrestrial Planet Finder/Darwin or SMART-2.

Indium FEEP Cluster Development

During the GOCE microthruster program and follow-up endurance tests, ARCS has collected more than 3800 operating hours at a mean thrust of 10 μN with one microthruster, and 18000 hours of cumulative operation with several thrusters. The lessons learned during this intensive test campaign allowed to greatly improve the performance of the indium FEEP emitters: higher mass efficiency, lower erosion, higher lifetime. These emitters have a maximum continuous thrust level of 10-12 μN. In order to fulfill the thrust requirements of missions like LISA Pathfinder (100 μN), it is necessary to cluster a certain number of emitters. This cluster must be operated with a single high voltage power supply, in order to reduce costs and system complexity. In this case, the ion emitters must fulfill stringent clustering requirements on mass efficiency, electrical impedance and threshold voltage. In this paper we report the experimental investigation on new emitter concepts suitable for clustering. Four emitters of the selected concept are now running in the first In-FEEP Cluster Endurance Test, which is the longest reported endurance test of a FEEP microthruster, with 3100 hours of continuous operation at maximum thrust (26-30 μN) collected till 4 of July; this corresponds to almost 40% of the LISA Pathfinder total impulse. We also report about the design of a 4x4 Indium FEEP Cluster Breadboard, which can operate at a maximum continuous thrust of 150 μN. This cluster includes a new focusing system, that reduces the ion beam divergence to less than 30°, within the LPF requirements.

Indium FEEP Thruster Beam Diagnostics, Analysis and Simulation

Space Propulsion Austrian Research Centers Seibersdorf, A-2444 Seibersdorf, Austria Indium FEEP thrusters based on space proven needle type liquid metal ion emitters are presently considered for a variety of missions which need ultraprecise drag-free capabilities such as LISA, SMART-2, GOCE or DIVA. One of the requirements of these missions is to investigate possible thrust vector variations, therefore the ion beam profile needs to be measured. A dedicated beam diagnostic assembly consisting of a wire and a Langmuir probe was installed in the laboratory to investigate single and multi-emitter configurations in the full thrust level of 1 – 100 µN per emitter. The derived thrust coefficient, thrust vector angle and beam divergence were within the expected limits. Multi-emitter configurations did not show any significant ion beam interaction. This analysis was supported by numerical simulations that could obtain similar results for emission in a pure vacuum environment without ambient electrons that can provide neutralization.

Liquid-metal-ion source development for space propulsion at ARC

The Austrian Research Centers have a long history of developing indium Liquid-Metal-Ion Source (LMIS) for space applications including spacecraft charging compensators, SIMS and propulsion. Specifically the application as a thruster requires long-term operation as well as high-current operation which is very challenging. Recently, we demonstrated the operation of a cluster of single LMIS at an average current of 100muA each for more than 4800h and developed models for tip erosion and droplet deposition suggesting that such a LMIS can operate up to 20,000h or more. In order to drastically increase the current, a porous multi-tip source that allows operation up to several mA was developed. Our paper will highlight the problem areas and challenges from our LMIS development focusing on space propulsion applications.

Indium FEEP Micropropulsion Subsystem for LISA Pathfinder

ARC-sr together with EADS Astrium and EADS Space Transportation is jointly developing a flight design for an Indium FEEP Microthruster suitable for LISA Pathfinder. The paper reviews all basic design elements and performance parameters and shows that the Indium FEEP thruster is capable of meeting all key requirements for ultra-precise attitude and orbit control.

5000h Endurance Test of an Indium FEEP 2x2 Cluster

In the framework of the GOCE Micro-thruster Program, ARC Seibersdorf research (ARC-sr) has collected more than 18000 hours of cumulative operation with several Indium FEEP emitters. The lessons learned during this intensive test campaign allowed to greatly improve the performance of the emitters: higher mass efficiency, lower erosion, higher lifetime. However, they have a maximum continuous thrust level of 10-12 μN. In order to fulfill the thrust requirements of the LISA Pathfinder mission (100 μN), it is necessary to cluster a certain number of emitters. ARC-sr has proposed a Thruster Flight Model based on a cluster of 9 indium FEEP emitters. Probably, the most demanding LISA Pathfinder (LPF) thruster requirement is the total impulse (nominal mission impulse of 1100 Ns for one thruster). In order to demonstrate it, on the 23 of February 2005, ARCS started an endurance test of the first Indium FEEP Cluster prototype (4 emitters), with a flightrepresentative experimental set-up. The cluster has been operated for 5000 hours, without breaking vacuum, at a thrust (25-30 μN) higher than the maximum thrust foreseen for LPF science operations (< 20 μN). This is the longest lifetime ever demonstrated for a thruster at μN level. In spite of a degradation caused by a facility effect, i.e. indium contamination due to aluminum particles back-sputtered from the collector, emitters 2 and 3 could demonstrate a total impulse of 175 and 167 Ns respectively, which is already more than the LPF total impulse per emitter (122 Ns).

Indium FEEP Multiemitter Development and Test Results

Based on the Indium FEEP emitters developed at ARC Seibersdorf research (ARCS) in the framework of the GOCE Microthruster Program, a new generation of indium FEEP multiemitter configurations are currently under development and testing. They promise the possibility of producing thrust levels > 100 µN and increased total impulse capability. In order to achieve homogenous firing of an Indium FEEP emitter array powered by only one power supply, the emitter design was enhanced to increase the electrical impedance and manufacturing reproducibility. In this paper we report endurance testing of this new emitter generation, the cluster design approach, and preliminary results on the first cluster prototypes from 0.4 to 100 µN, a thrust range that can easily fulfill the requirements of future missions like LISA Pathfinder and DARWIN.

Development and Test of an Indium FEEP Micropropulsion Subsystem for LISA Pathfinder

The Laser Interferometer Space Antenna project (LISA) is a co-operative program between ESA and NASA to detect gravitational waves by measuring distortions in the spacetime fabric. LISA Pathfinder is the precursor mission to LISA designed to validate the core technologies intended for LISA. One of the enabling technologies is the micro-propulsion system necessary to achieve the uniquely stringent propulsion requirements.

Development of a Focus Electrode for an Indium FEEP Thruster

The geometry of the electrode array in a field emission electric propulsion (FEEP) thruster is extraordinarily important, not only for thrust-relevant parameters, but also to ensure long-term reliability. Ion beams require good focusing because stray ions and charged droplets cause damage to the device in multiple ways. The aim of this paper is to investigate the correlation of initial spray angle at the Taylor-cone to the emitter current for an indium Liquid Metal Ion Source (LMIS) and to design an improved focusing electrode which guides the ion beam in such a manner that even very divergent beams caused by large emitter currents are focused and expelled without contact to the extractor, the focus electrodes or the plume shield. To that goal, the electrode array of an Indium-based thruster being developed by the Austrian Research Center Seibersdorf (ARC-sr) is simulated in an ion-trajectory program and various geometries tested for their focusing qualities. Finally, a focus electrode is designed which shapes the ion beam in such a manner that the expelled ion beam conforms to the specifications imposed by the requirements for the LISA mission.