The Next-Generation Gravity Mission (NGGM) concept, under study at the European Space Agency (ESA), will make use of the Low-Low Satellite-to-Satellite Tracking technique to monitor the temporal variations of the Earth gravity field over a long time span. One of the enabling technologies for the realization of the NGGM is the micro propulsion system. Granted of an ESA contract within the basic Technology Research Programme (TRP), an engineering model of such a thruster was designed, constructed and tested in TransMIT GmbH in Germany with subcontracts to the University of Southampton and CEC GmbH. The paper discusses the design specifications and delivers the test results.

Since a couple of years, scientific European Space Agency missions like «Post Goce», «NGGM», and «LISA» are being prepared which need a very precise micro-thrusting in the range of 50 to 2500 μN. Thus, in 2004, Giessen University started a scaling-down program of the standard RIT-10 engine. Three mini-thrusters have been built and tested, whereby the 2.5 cm device, called μN RIT2.5, reached the status of an advanced breadboard model. Following extensive optimization tests at Giessen, the thruster was operated at the Nanobalance Facility of Thales Alenia, Torino/Italy under ESA/ESTEC contract. Thrust range and linearity, thrust resolution, thrust noise, and response time have been measured showing that the μN RIT-type would be a good candidate for micro thrusting on the above mentioned satellites and spacecrafts.

The paper deals with the results of two test campaigns carried out at Alta and at Giessen University respectively on Snecma’s PPS1350-TSD Hall thruster and on RIT-10 ion engine for performance characterization with nitrogen and oxygen as propellants. The activity sponsored by the European Space Agency is of particular interest for RAM-EP concept. Both thrusters have operated in stable mode with both propellants, although as expected a degradation of performance has been observed, mainly due to the low ionization efficiency.

A feasibility study of implementing a MicroPropulsion Subsystem (MPS) based on the RIT technology in place of the FEEP for the LISA Pathfinder mission was performed. The mission imposes stringent requirements on the MPS thrusters in terms of thrust range and dynamics, thrust accuracy, thrust noise and stability. In addition, the mission application brings significant constraints for integration on a spacecraft that is reaching the end of its development, not only in terms of mass and power, but also in terms of mechanical and electrical integration without impacting what has already been build and verified at system level. Previous tests performed on a miniRIT thruster operating at higher thrust levels showed the capability to comply with those requirements and constraints. A number of RIT thrusters were designed, built and tested to verify compliance to the major LISA Pathfinder MPS specifications. The system implications of using a RIT MPS have been studied. Restrictions on power and mass budgets, use of existing interfaces had to be taken into account. Existing FEEP MPS equipment and units, e.g. PCUs and Neutralizers, were re‐used as far as possible. Different RIT MPS architectures were considered. Finally, to minimise the impact on the already built platform, the RIT MPS was designed as a form fit replacement of the Slit FEEP MPS. 1 Electric Propulsion Engineer, Directorate of Technical and Quality Management, Mechanical Department, Propulsion and Aerothermodynamics, TEC-MPE, davina.maria.di.cara@esa.int 2 Lisa Pathfinder Senior System Engineer, Directorate of Science and Robotic Exploration, SRE-PNS, stein.strandmoe@esa.int 3 Lisa Pathfinder Thermal Engineer, SRE-PNS, jose.antonio.romera.perez@esa.int@esa.int 4 Lisa Pathfinder Spacecraft and AIV Manager, Directorate of Science and Robotic Exploration, SRE-PNS, luca.stagnaro@esa.int 5 Electric Propulsion Team Leader, Hans.Leiter@astrium.eads.net 6 System Engineering Team Leader, Rainer.Killinger@astrium.eads.net 7 Head of EP-Group, University of Giessen, davar.feili@uni-giessen.de 8 Physicist, EP-Group, University of Giessen, benjamin.lotz@physik.uni-giessen.de 9 LISA Pathfinder Micropropulsion S/S Program Manager, aldo.polli@selexgalileo.com 10 LISA Pathfinder Micropropulsion S/S System Engineer, luca.ceruti@selexgalileo.com

It is known that the presence of residual gases during on-ground testing of electric propulsion thrusters can significantly influence the plasma parameters and the quantity of charge-exchange ions obtained in the backflow of the ion source. In the frame of an ESA study dedicated to the "Assessment of Interactions between Spacecraft and Electric Propulsion Systems" (AISEPS) a miniaturized μN-RIT thruster (RIT-4), developed by Giessen University, was tested in the Corona vacuum facility at the ESA Propulsion Laboratory (EPL), ESA-ESTEC. A single filament neutralizer was used for beam neutralization. The main ion beam and backflow properties of the thruster were investigated by means of Faraday probes (FP) and retarding potential analyzers (RPA). The Cathode Reference Potential (CRP) was also investigated. The background pressure in the vacuum facility was increased to assess the influence of the xenon residual neutral density on the plume of the thruster. Different electrical coupling configurations between the thruster and the ground were also studied. The μN-RIT was operated at three different thrust levels ranging from 100 to 500 μN while the neutralizer was operated with constant heating voltage during the entire test campaign to allow emission up to 12 mA depending on the electrical grounding configuration. The background pressure was increased in the main vessel of the Corona facility by injecting an auxiliary xenon flow ranging from 10 to 50 sccm. The influence of the neutralizer was clearly observed on the RPA measurements in the main ion beam. The divergence was also clearly correlated to the grounding configuration of the thruster. However, even if the xenon background pressure increased the backflow ion current it did not have a clear influence on the divergence of the thruster.

This paper deals with the up to now rf plasma simulation using PIC-MCC method for N-RITs. Beside the RF generator and coil parameters a main input parameter for the plasma discharge modeling is the neutral gas density prole inside the thruster. At rst, we investigated these proles depending on the gas inlet position, grid conguration, and amount of gas ow. To handle this task we used the software Flowsim which had been developed at the I. Physikalisches Institut of the University of Giessen. We will present results for a simplied N-RIT with a cylindrical discharge chamber. Further, we will show the dependence of plasma and neutral gas distributions within the ionization chamber. As a consequence a self-consistent combination of the plasma and neutral gas modeling is not necessary.

RIT-μX (Elegant BB) is a radio frequency ion thruster especially designed for the demands of high precision formation flying missions and the needs of fine and ultrafine drag control. The thruster design bases on the experience in more than 40 years radio frequency ion thruster development. The RIT-μX thruster and system development has been performed in the frame of ESA's GST program. This publication is focused on the results of the thruster functional test program. During the functional test campaign the basis performance (Isp, Thrust, Power consumption) as well as advanced parameters (Thrust resolution, Controllability, Thrust Noise) have been investigated.

Many of ESA’s and NASA’s future missions are based on Formation flying, Fine attitude control and (fine) Drag compensation. As prominent examples one could mention LISA, DARWIN, PROBA-III, etc. For all these applications there is a need for propulsion system with: - High thrust accuracy and - High thrust controllability Taking into account, that up to 16 of these thrusters are needed for every space craft, there would be other requirements necessary: - Low mass, - High ISP, - Low power consumption All these requirements combined with high mission durations of up to 10 years, which enforce a life time of over 20,000hrs show the challenges, which the thruster and system designers are confronted with. University of Giessen started 2004 the works on micro Newton ion thrusters based on Radio frequency discharge. The works leaded to a miniaturized RF ion source with very low power consumption and mass, which is under industrialization to higher TR Levels under ESA contracts. The performance of such a small thruster will be presented in this paper. From the other side the whole system architecture for such a propulsion system will be discussed.

Since the early 60ies, gridded ion thrusters with rf-ionizaton of the propellant (first mercury, lateron xenon) have been designed, built, investigated, optimized, and tested at Giessen University. Since 1970, the industry (now EADS Space Transportation) carried out related D & Q programs. The joint work was at first focused on the 10-cm NSSK engine RIT-10, which was flown onboard "EURECA", lifetime tsted over more than 20,000 hrs, and successfully applied in the "ARTEMIS" rescue mission. Since the 70ies, rf-engines have been scaled up and tested by the Giessen team or by EADS with 15, 20, 22, 26, and 35 cm of ionizer diam. In addition, spin-off engines for material processing and fusion plasma heating have been developed. Moreover, discharge and beam diagnostic work, thruster modelling, and mission analysis have been performed. Recently, the Giessen EP-team is engaged in supporting the EADS-tests of the RIT-22 thruster, in investigating of an insertless rf-electron source, in SEP-mission analysis, and especially in scaling-down the standard RIT-10 engine. For microthrusting applications, 4-cm and 2-cm diam RIT-prototypes are under R & D programs.

Be-like Ag43+, Sn46+, and Xe50+ ions were produced by beam-foil excitation at the UNILAC accelerator of the GSI. The wavelengths of the 2s2 1S0–2s2p 3P1 intercombination lines were measured using the GSI 5 m grazing incidence spectrometer. The projectile spectra were calibrated in situ with well known ionic lines of a VUV Penning discharge lamp. A precise determination of the Doppler shift was necessary. An experimental precision of 10-4 was realized. Our medium-Z experimental results represent the heaviest Be-like ions studied. They are compared with published MCDF and MBPT calculations along the isoelectronic Be-like series. Excellent agreement exists between our Xe-value and a recent result from an electron beam ion trap.

In this paper we present a precise and reliable measurement of the 1s22p 2P3/2 lifetime for Li-like Ni. The measurement was done using the technique of beam-foil spectroscopy employing a beam of 5.9MeV/u Ni from the UNILAC accelerator at GSI. Special attention was taken concerning satellite blending and foil-ageing problems to justify the reliability of the lifetime value extracted from the measured decay curve. The experimental value for the 2p 2P3/2 lifetime, 0.168±0.006ns, is found to be in very good agreement with theoretical results.

Be-like Ag43+, Sn46+, and Xe50+ ions were produced by beam-foil excitation at the UNILAC accelerator of the GSI. The wavelengths of the 2s(2) S-1(0)-2s2p 3p(1) intercombination lines were measured using the GSI 5 m grazing incidence spectrometer. The projectile spectra were calibrated in situ with well known ionic lines of a VUV Penning discharge lamp. A precise determination of the Doppler shift was necessary. An experimental precision of 10(-4) was realized. Our medium-Z experimental results represent the heaviest Be-like ions studied. They are compared with published MCDF and MBPT calculations along the isoelectronic Be-like series. Excellent agreement exists between our Xe-value and a recent result from an electron beam ion trap.

Be-like Ag43+, Sn46+, and Xe50+ ions were produced by beam-foil excitation at the UNILAC accelerator of the GSI. The wavelengths of the 2s2 1S0–2s2p 3P1 intercombination lines were measured using the GSI 5 m grazing incidence spectrometer. The projectile spectra were calibrated in situ with well known ionic lines of a VUV Penning discharge lamp. A precise determination of the Doppler shift was necessary. An experimental precision of 10-4 was realized. Our medium-Z experimental results represent the heaviest Be-like ions studied. They are compared with published MCDF and MBPT calculations along the isoelectronic Be-like series. Excellent agreement exists between our Xe-value and a recent result from an electron beam ion trap.