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Ion-probe positioning system

Participants: CSU
Status: completed

Boeing has been developing ion thrusters for space vehicle applications for a number of years. These engines use a gas (typically xenon) which is ionized (i.e., an electron is knocked off) and then accelerated through an electric potential and out into space, creating a small thrust. The engines require electric power to run and use up the xenon propellant — but very slowly. They can be left on continuously, and, although their thrust is quite small, they can eventually build up quite a lot of speed. In comparison, a chemical rocket consumes propellant rapidly, giving a short burst of large thrust.

Tuning ion thrusters requires careful measurements of the ion plume coming out of the engine while the whole assembly is operating in a vacuum chamber. The ion detectors will rapidly overheat and ablade or erode due to the ionic radiation (think of the radiation as very small BBs blasting into the surface with enough force to remove even the hardest of materials).

We were contracted to build a rapid and accurate system for positioning the ion probe behind the engine. The probe has to move and settle quickly, so that measurements (which are subject to even small electrical noises associated with rubbing cables) can be made rapidly. The probe is repositioned in two dimensions (laterally and angularly) for a subsequent measurement. Important in this project was the ability to “point” the probe at a constant position on the back of the engine while scanning the probe past the point. This, it is necessary for the head of the probe to rotate to keep “looking” at the same point. Additionally, the back of the engine is not flat, so the curvature of the engine must be compensated for in the code regulating the positioning system to measure multiple points accurately.

The senior design group responsible for the design and construction of the positioning system did an excellent job, and Boeing now has the unit in their lab in Los Angeles.

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