Onset Phenomena in MPD Thruster
The MPD before (left) and after (right) intensive firing above the “onset parameter”. The outer copper ring electrode (the anode) has been pitted, scarred, and blackened. Strange things happen to the electrical characteristics as well. Why?
Documentation for Onset in MPD thrusters
Background and History
|Onset 101: What is this Onset Phenomenon, and why does it matter? This introduction will walk the reader through the history of onset, why it is a concern, the experimental knowledge surrounding it, and previous attempts at explaining what is going on.
|A History of
|The two primary onset symptoms, voltage hash and anode damage, are not unique to the MPD thruster; their observation in other arc discharges dates back to at least the 1950's. This overview reviews what has been discovered about "onset" phenomena in the wider arc community.
|The voltage drop within the thin sheath region surrounding the anode is dependent upon the electric field at the anode surface and the properties of the adjacent plasma. Can the behavior of the sheath voltage be related to the behavior of the external power supply?
|Voltage Hash and
|When the voltage across the MPD thruster oscillates wildly (which we call "hash"), what part of the voltage drop is fluctuating? Taking the fluctuation to be happening in the anode sheath provides a good strategy to correlate the voltage hash with that other stalwart of onset: anode damage.
|Many of the arguments about current conduction to anodes depend upon equating the current density there to the random thermal current density, as though the electron gas is a collision-dominated fluid. But what if it's dominated by a magnetic field? [Delayed]
|What does the MPD anode look like under the scrutiny of a high-speed video camera? It looks different, depending upon whether the thruster is being fired below or above onset. Read the PDF linked to the left for a description of what the videos show. [Video: Below Onset] [Video: Above Onset]