XENON GAS INJECTION IN SPT THRUSTERS

The present development of 2D hybrid code simulation of Closed Electron Drift (Hall) Thrusters (CEDT) makes possible a better defined comparison between experimental characterizations and code predictions. Various inputs are required in order to represent in a realistic way the physical conditions used in simulation codes. The present paper is focused on the work developed at GREMI in order to obtain a confident definition of xenon injection in the channel of a Stationary Plasma Thruster (SPT) and to examine the impact of various injection schemes on the thruster itself. This task has been achieved in three main steps. The first one was to develop simulations of gas injection structures, the second one was to obtain experimental data on gas distribution in the channel, the last one was to characterize the impact of the injector structure on the thruster characteristics. The simulation codes have been developed on the base of particle in cell (PIC) approach, with non trivial assumptions on initial velocity distributions of gas beamlets leaving the injector holes. The atom-walls conditions have been represented in the two extreme cases of specular and perfectly diffusive ones. An electron gun able to deliver a focused beam (10 keV, 2 mm, 10μA) has been developed in order to measure a local density distribution across and along the SPT channel, by using the local gas fluorescence signal recorded through a focusing lens and fibre optic system. The calibration of light emission as function of neutral density was done by using stationary xenon pressure conditions. A 2D translation unit is used to scan the thruster channel cross section (r,z) and the derived neutral density distributions are compared to code predictions for various injector structures. Finally the impact of various neutral gas injection structures on thruster SPT100ML is presented and discussed. For similar operating parameters (gas flow, magnetic field, thruster voltage) data show that the tested gas injection system have not a strong impact on overall performance (thrust, Isp, efficiency) but they suggest the thruster plume is significantly modified. Introduction SPT devices have been studied for several years and their design is mainly resulting from pragmatic developments based on physical understanding. Facing the fast increasing interest of the CEDT’s systems a better basic knowledge of the discharge and the plasma dynamics is required. In the frame of the cooperative research program GDR n°2232 (CNES, SNECMA, ONERA) several academic research teams are involved for experimental (Aérothermique Orleans, LPGP Orsay, GREMI Orleans) and modelling studies (CPAT Toulouse, CPHT Palaiseau) /1,2,3/ In spite of such long term efforts, simulations able to give confident results on lifetime and up-scaling are not presently available due to the complexity of the physical processes in particular electron transport. The anode of a stationary plasma thruster which is located in the bottom of an annular channel also plays the role of the gas (xenon) distributor. Holes are shared out azimuthally on a grid to enable an homogeneous gas distribution. Their position is optimised according to two criteria : providing the most homogeneous distribution and minimising the influence of the discharge. Previous numerical simulations and experimental (optical) investigations have shown that the dynamics of the ionisation zone is linked to the gas distribution in the channel. The successive filling-depletion of neutrals in the channel leads to low frequency axial movements and spread of the ionisation zone /9/. Results of the numerical studies also showed that the gas ionisation takes places preferentially in the centre of the channel /4/. These physical processes influence the angular distribution of the ejected ions, an important parameter in the processes of the channel ceramic sputtering /5/ and interactions with the solar panels of the satellite /12/. Due