1 A 4-Gridded Ion Engine for High Impulse Mission M. Coletti University of Southampton, Southampton, SO17 1BJ, United Kingdom coletti@soton.ac.uk P. Gessini Mars Space Ltd, Southampton, SO16 3QD, United Kingdom rodrigo.marques@mars-space.co.uk S. B. Gabriel University of Southampton, Southampton, SO171BJ, United Kingdom sbg2@soton.ac.uk Abstract— At the end of 2008 a European Union sponsored project called HiPER (High Power Electric propulsion: a Roadmap for the future) started in Europe with the aim of defining the roadmap for the future of electric propulsion. Within this project several different propulsive systems are under study and among those the Dual Stage 4 Grid Ion Engine (DS4G). In this paper the DS4G concept will be investigated defining the performance range in which is application is useful and producing a preliminary design of the ion optics for two different specific impulse levels. 1. INTRODUCTION The aim of the HiPER project is to define the roadmap for the future of electric propulsion [1]. To do so several missions have been studied, including a Mars sample return mission, rendezvous with Near Earth Objects, building of infrastructure on the Moon-Earth L1 point and building of infrastructure on Mars. Different electric propulsion subsystems are under study for these missions including Magneto Plasma Dynamic Thrusters (MPDTs), Hall Effect Thrusters (HETs) and Gridded Ion Engines (GIEs). Mars Space Ltd and the University of Southampton are working on the design of GIEs and in particular of a new concept of GIE called Dual Stage 4 Grid Ion Engine (DS4G) originally developed by the late D. G. Fearn [2] based on tetrode extraction system for high energy neutrals and ions injection systems used in tokamaks [3-6]. In this paper the concept of the DS4G thruster and its advantages over a common GIE will be presented; the limit of applicability of the DS4G will be investigated and a preliminary design of the ion optics presented. 2. THE DS4G THRUSTER Existing 2 and 3 grid GIEs can provide specific impulses in the 2000-10000s range [7] with an experimental GIE able to achieve 19300s [8]. However, GIEs present an intrinsic compromise between maximum current density and specific impulse, which limits the maximum achievable thrust for a given specific impulse. This is because, using 2 or 3 grid extraction systems, the extraction of ions from the discharge chamber and their acceleration process are deeply interconnected, with both processes taking place in the gap between the first and the second grid. If we express the thrust, the specific impulse and the extracted current as a function of the grid voltages assuming to be in space charge limited conditions (as is normally the case in GIEs) we have   2 2 / 3 2 1 0 2 9 4 d V V m q J i i    (1) i dec m V V q Isp ) ( 2 cos 1    (2) dec g g i i V V T A J q m T   1 cos 2  (3) Where T g and A g are the grid transparency (%) and the grid area,  is the beam divergence angle, d is the gap between the first and second grid, V 1 and V 2 are the voltages of the first and second grid respectively, where 2 1 12 V V V   , and where V dec is the voltage of the deceleration grid that is the third grid in a 3 grid system and the fourth one in a 4 grid engine. The value of the electric field between the first two grids cannot be arbitrarily large since arcing between the grids must be avoided and since a too large electric field will produce an excessive curvature of the plasma sheath in the discharge chamber, causing the extracted ions to impact on the first grid, thus reducing the thruster performance and lifetime. For this reasons the value of V 12 / d is normally limited to a value of the order of 1kV/mm. If we rewrite Eq. (1) and (3) to isolate the V 12 / d term we obtain