Neutralizer-free gridded ion thruster

Abstract

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 selfcompensated broad ion-electron beam with same thrust efficiency as traditional gridded ion thrusters.Nomenclature e 0 = electron charge n e = plasma density M i = ion mass m e = electron mass V DC = direct current (DC) voltage V RF = radio frequency (RF) voltage j i = ion current density I tgt = target current U tgt = target voltage ε 0 = vacuum permittivity ω = frequency of RF generator ω pe = electron plasma frequency ω pi = ion plasma frequency

Experimental study of a neutralizer-free gridded ion thruster using radio-frequency self-bias effect

“…The space charge limited current is almost doubled in the RF acceleration compared with the DC acceleration, which is attractive for EP to increase thrust [6,26]. An experimental prototype named Neptune was performed with an RF power supply and the acceleration frequency was 4 MHz [18,19,27,28]. The research indicates that the quasi-neutral beam was successfully extracted in RF acceleration [18][19][20].…”

“…PEGASES II is one of the neutralizerfree grid thrusters whose quasi-neutral beam is obtained by alternately extracting positive and negative ions from the plasma source [6,17]. In recent years, the radio-frequency (RF)-biased grid technology has been applied in gridded ion thruster (GIT) to extract electrons and ions from the same plasma source quasi-simultaneously, which uses RF self-bias effect [18][19][20][21]. In this concept, the screen grid is connected to the RF power supply via a blocking capacitor, and the acceleration grid is grounded.…”

“…This acceleration technology is called the RF acceleration in this paper, corresponding to the direct-current (DC) acceleration in classical GIT. In RF acceleration, ions are continuously accelerated within the grids while electrons are extracted periodically when the sheath collapses [18,19,22,23]. The quasi-neutrality of the beam is obtained by extracting positive ions and negative electrons from the same plasma source.…”

“…The experimental results showed that this design has many disadvantages and limitations when applied to EP, such as low ion energy (<300 eV), ion sputtering, and low propellant utilization efficiency [6,22,24]. In order to solve these problems, an alternative method with a double-grid accelerator using the RF self-bias effect was proposed to extract a quasi-neutral plasma beam [18,20,25]. In the single-grid system, the RF voltage drop occurs mainly between the plasma and the grid.…”

Numerical study of the radio-frequency biased accelerating system in ion thrusters

“…In recent years, a novel accelerating grid system with self-neutralization, which uses radio-frequency (RF) self-bias grids instead of DC biasing, was proposed for the GIT [8][9][10][11].…”