Insights on Physics of Hall Thrusters through Fast Current Interruptions and Discharge Transients.

Spontaneous fluctuations or oscillations, with characteristic time scales of few tens μs are well known as one of the most evident instationary phenomena in closed electron drift thrusters (CEDT). They have been already characterised in many experimental studies and rather well reproduced by simulation studies. When looking at these spontaneous phenomena, time and space resolved data on discharge plasma and plume revealed interesting insights on the dynamics of these instabilities. As in various physical systems, the response to a controlled transient perturbation of a “stationary” state would be helpful for an insight on basic properties of the system in this stationary state. Such approach was developed for CEDT’s by using fast current interruptions and looking at the various physical transients induced in plasma and plume. Most of these studies were done on SPT100 type CEDT. The design and test of the current switch able to perform “instantaneous” ON-OFF transitions and the time and/or space resolved diagnostics are key points for such approach and are described or refereed. Results show that such approach opens a convenient way for physical study of these thrusters. New insights have been obtained on collisional-radiative processes, electron transport phenomena and connections between channel plasma and plume. Moreover, the development of wide frequency band diagnostics lead, for the first time to our knowledge, to a clear evidence of the connection between microinstabilities and macroscopic behaviour of the thruster current. These data emphasise the importance of refinements in modelling/simulation and seems to be useful for such progresses.