It has been clear for many years that the low-energy Hohmann transfer orbits that have been used for planetary exploration by a generation of robotic spacecraft are unsuitable for manned planetary explorations because of the length of the transit times. One alternative that has been studied for almost as long as this problem has existed is the use of some form of continuous thrust. The continuous thrust approach requires the development of thrusters capable of operating in space for periods of months to years. The logistic requirements of such systems have dictated that serious consideration be given to various types of electrically powered rockets that derive at least part of their kinetic energy from the radiant energy of sunlight.
One of the advanced technology rocket propulsion systems being considered is the Variable Specific Impulse Plasma Rocket that is now in the early stages of prototype development and testing at JSC. This rocket is a plasma (or ionized gas) magnetic mirror machine that has a plasma source at one end and is open at the other end. The exhaust is a neutral but highly ionized plasma. One of the studies that remains to be done on this rocket is a careful characterization of the exhaust plume that the rocket produces in various operating configurations. These studies will require the use of a device that is capable of measuring the kinetic energy distribution function of the ions in the plasma as a functions of position in the plane perpendicular to the nozzle axis and of ion species.
The best method for making the required measurements is to use a multi-grid planar plasma probe or ion trap of the type known as a retarding potential analyzer (RPA). An RPA measures the ion current that is collected on a negative electrode or anode as a function of the positive retading potential on one of the grids. In a flowing plasma, it is possible to infer the kinetic energy distributions of each ion species from such data. The use of an array of small ( i cm diameter) RPA's with multi-channel sensing electronics will provide the necessary spatial information. We propose here to design, build and test an RPA array with an operating range suitable for use in the VSIPR exhaust plume.
A related publication is ``A Sounding Rocket Observation of an Apparent Wake Generated Parallel Electric Field," E. A. Bering III, J. Geophys. Res., 88, 961-979 [1983].