I. Introduction evelopment of efficient low power Hall thrusters for small/micro-satellites, operation in dual mode (Highthrust and low-specific impulse mode and a high-specific impulse and low-thrust mode) 1, and achievement of erosion free operation at low power for deep space missions still remains a major technical challenge. It is because anomalous electron transport and plasma-wall interactions are not completely understood yet. Plasma-wall interactions have substantial effects on discharge characteristics as well as on thruster performance2-4. Power loss to the channel walls is the main power, and thus efficiency, loss source and cause of erosion in conventional Hall thrusters. Fundamentally, it is electric field in the channel should be arranged to prevent plasma contacting channel walls. In general, electric field may be arranged through:(1) magnetic field profile;(2) anode’s relocation at the thruster channel exit. Thrusters employing cusped magnetic field5-7, magnetic shielding8, and peak radial magnetic field outside the channel9 are examples of the first approach, and most of the studies so far have focused on this approach.

The second approach has been investigated under different designs and names so far. In the early 1980s, Kapulkin et al. 10 proposed a third kind of Hall thruster with low-level thrust (~ a few mN) to be used for precise attitude control of spacecraft, and named it “Outside electric field thruster (OEFT). OEFT has two anodes placed in series in an annular cavity made of dielectric material. The first anode ionizes the propellant, and the second anode accelerates the ions. Recently, Mazouffre et al. 11 moved the anode of conventional 200 W-class, and 1.5 kW-class Hall thrusters towards the discharge channel exit, and characterized its effect under the name of ‘Wallless Hall thruster (WL-HT)”. Despite the location of WL-HT’s annular hollow anode at the channel exit, ionization/acceleration regions reach behind the anode, and deep inside the discharge channel serving as neutral distributor. Thus, WL-HT has an effective magnetic field shaped like Gaussian having its peak at the center of the anode in its standard configuration; whereas, peak radial magnetic field is located in front of the anode in its optimized configuration12.