The first proof-of-concept of a neutralizer-free gridded thruster called NEPTUNE which emits simultaneously ions and electrons producing thrust and high Isp is presented. The NEPTUNE prototype is built and tested in a 1m long beam propagation chamber. A broad quasi-neutral ion-electron beam with the ion energy up to 400 eV is successfully generated without the use of a neutralizer. This is achieved by the possibility of an efficient DC self-bias generation within the RF-powered multi-grid extraction system, which is in contact with plasma. A single RF power source is used for the gas ionization, ion acceleration and electron extraction; the NEPTUNE thruster prototype is therefore free of any additional DC power supplies. The beam extraction is investigated by measuring the ion and electron energies, fluxes and densities using a retarding field energy analyzer (RFEA) and different electrostatic probes. Comparison of the main beam parameters and efficiencies of the NEPTUNE source with a traditional gridded source equipped with a neutralizer is presented. It is concluded that the NEPTUNE concept allows generating a well-localized self-compensated broad ion-electron beam with same thrust efficiency as traditional gridded ion thrusters.
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