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Unformatted text preview: 16.522, Space Propulsion Prof. Manuel Martinez-Sanchez Lecture 17: NOTES ON HALL THRUSTERS NOTES ON HALL THRUSTERS 1. Introduction Hall thrusters are electrostatic ion accelerators in which the grid system (which serves in classical ion engines to anchor the negative charges used to accelerate the ions) is replaced with a relatively strong magnetic field perpendicular to the flow. This magnetic field impedes the counterflow of electrons in the accelerating field, and, as will be shown, does away with the space-charge limitation which restricts the flow and thrust of ion engines. Hall thrusters have had a curious and long history. Discovered in the early 1960s in the U.S. (1) , and possibly independently in the USSR (2) , they were later abandoned in the West when it became apparent that there were strong instabilities which could not be completely eliminated, and when some additional work in Germany (3) indicated higher effective plasma collisionality than had been expected. Contributing to the demise was the simultaneous successful development of gridded ion engines of the Kaufmann type, which appeared to satisfy most high V mission equirements then envisioned, and the promise of efficient hydrogen arcjets for the low and intermediate V ranges. It later developed that many important missions optimize in the Isp range of 1000-2000 sec., not well covered by either of these types of engines, and this remained an unmet requirement for many years, at least in the West. Development continued, however, in the USSR, particularly at the Kurchatov Institute in Moscow (2) , under A.I. Morozovs leadership. Progressively more efficient configurations were evolved there, and it was realized that the instabilities, while present and annoying, did not materially interfere with performance. The Soviet generic name for this type of engine is Thruster with a Closed Electron Drift, and two competing implementations have been developed, known respectively as the Stationary Plasma Thruster (SPT) and the Thruster with an Anode Layer (TAL). The general principles are the same in both, and will be discussed below. By the early 1980s these engines had achieved operational status in the USSR, and have since flow in over 50 missions, which, however, were until recently limited to relatively small total impulses. Starting in 1991, with the complete removal of the earlier communications barrier, development has re-started in the West, in the form of collaborative efforts with Russian teams aimed at improved life, lighter electronics and flight qualification to the generally more stringent Western standards. The strong interest shown by the user community stems from the ability of Hall thrusters to operate with fairly good efficiency ( 50%) in the hitherto difficult specific impulse range around 1500 sec., and also from their relative simplicity compared to ion engines....
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- Fall '05