Development of a Simple Instrument for Measuring Ionospheric Low Energy Electrons using a Delta-Doped CCD as Electron Detector (LEES), 15-R8053
Principal Investigators
Raymond Goldstein
Bill Tomlinson
Nick Alexander
Rudy Frahm
Inclusive Dates: 04/01/09 – 04/01/11
Background — SwRI has a long and successful history of designing and building instruments using specialized detectors for a variety of applications. In space applications, it is desirable to reduce the power, weight and complexity of the detector and supporting electronics. Reducing weight and power aids in meeting the spacecraft resource limitations for the instrument. For instrument operation, the less complicated the detection electronics, construction is easier and fewer things can go wrong. In a previous project, SwRI researchers successfully demonstrated that a new type of detector known as a delta-doped CCD developed by the Jet Propulsion Laboratory is capable of detecting charged particles. The chief advantage of this type of detector is that it does not require high voltages for its operation, in contrast to conventional particle detectors that require typically a few thousand volts. Eliminating this requirement will help simplify instrument design and fabrication, and reduce cost. This feature also allows its use under conditions (such as certain atmospheric/plasma pressure regimes) in which the presence of high voltages would result in dangerous arcing.
Approach — This project seeks to design, build and test a simple instrument that could be used for measuring electrons in a planet's ionosphere. Upon successful completion of this project, a valid science instrument will result. The basic design of the instrument is a standard retarding potential analyzer (RPA) combined with a Faraday cup. The RPA uses a set of grids, one of which has a variable negative voltage applied that then repels electrons of energy lower than the voltage at a given time. The Faraday cup houses the detector to measure the electron current passing through the grids. The instrument covers the energy range from 10 to 350 electron volts. This potential is swept over its range on the repeller grid.
Accomplishments — The LEES (Low Energy Electrons) instrument has been designed and built. A photograph of LEES without its outer aluminum shell is shown in Figure 1. (The entrance aperture faces through the aluminum support plate to the bottom of the assembly. Most of the structure consists of the electronic circuit boards.) LEES is approximately 2.5 in. x 2.5 in. x 2.5 in., consumes 2.5 W electrical power, and has a mass of approximately 650 g. The CCD detector was delivered by the supplier and has been installed in LEES. The project team has also designed, built and tested the electrical control system, based on a laptop computer and commercial control software. The setup, arranged on a table-top, is shown in Figure 2. The LEES instrument is designated by the red circle in the figure. Preliminary tests of the instrument were conducted in the SwRI electron test chamber. LEES appears to be operating as designed, but the output has a high level of noise, apparently the result of some internally generated interference. Because the project has terminated, researchers are unable to continue the development. However, NASA funding has been received that will allow researchers to test LEES on a suborbital rocket flight in early 2012.
