NASA ATMOSPHERIC RESEARCH

VENTED BENNETT POSITIVE ION MASS SPECTROMETER

circa 1969

  
   When the team I was working with at the NASA Goddard Space Flight Center in Greenbelt, MD was accepted to fly their positive-ion mass spectrometer on the Atmospheric Explorer C mission, I realized that the past design of the instrument had a serious limitation.  The closed tube design would allow neutral particles to build up to such a degree during the lower altitude passes planned for the satellite that no charged particles would reach the collector plate at the rear of the tube, the plate that was the "sensor" for the spectra to be analyzed.
   I went to the library and found the formulae that applied, modeled the situation in FORTRAN, and predicted the altitude where this "gumming up" would occur.  That altitude was higher than the mission targets. Then, remembering a concept from my graduate course in electrical engineering (a course which I had to repeat), I suggested that the back end of the spectrometer be opened up to allow the neutral particles to "flow through", and the collector plate be replaced by the same type of grid that was used in the rest of the tube.  Because with sufficient voltage any of our ions would be drawn to the grid wires because each has a "capture radius", and the instrument should still work.
   Thus, with the help of our talented mechanical engineer, James Burcham, the "Vented Bennett Ion Mass Spectrometer" was born!  With a reorganization going on, I had switched to the new programming-support branch for the division, and that branch chief, the late John Quann, invited me to help write a paper on the new design, a paper which was published in the United Kingdom.  I remember taking a royalty check made out for a few British pounds to the bank one day as a result.
NOTE: Mr. Quann rose to the position of Deputy Director of the Goddard Space Flight Center.  I now wish I had remained at Goddard longer (promotions were frozen, I had a family to support, and I took a position at the next GS level at the NOAA Environmental Satellite Service doing systems planning).
   The success of this mission, both the software and the new design, propelled my supervisor, the late Henry Brinton, into a career at NASA headquarters and my radical design was flown to Venus where it orbits to this day. That mission was the "Pioneer Venus" project, and it helped scientists understand for the first time the nature of the Venus outer atmosphere and its interaction with the solar wind.
   Below is a photo of the spectrometer. I left NASA before it flew, but my fellow scientists did include me as one of the authors in the papers cited below.

Brinton, H.C., L.R. Scott, M.W. Pharo, III, and J.T.C. Coulson, The Bennett ion-mass spectrometer on Atmosphere Explorer-C and -E, Radio Sci., vol. 8, 323, 1973. 

Leighton's "Vented Bennett In-mass Spectrometer"

design idea executed in the prototype

The Bennett ion-mass spectrometer on Atmosphere Explorer-C and -E

Radio Science

Volume 8, Issue 4, April 1973, Pages: 323–332, H. C. Brinton, L. R. Scott, M. W. Pharo III, J. T. Coulson

   The Bennett spectrometer to be flown on Atmosphere Explorer-C and -E (AE-C and AE-E) is designed to measure, throughout the 120 to 4000-km orbit, the concentrations of all thermal positive ions in the mass range 1 to 72 amu and number density range 5 to 5 × 10⁶ ions cm⁻³. To reduce the buildup of ram pressure and facilitate measurements at low altitude, the analyzer is vented, and a multi-grid ion-current collector is employed. An extensive command capability permits optimization of instrument parameters for particular measurement objectives; commandable functions include mass-scan range and period, the sensitivity-resolution characteristic of the analyzer, orifice potential, and in-flight calibration. Any combination of three mass ranges (1 to 4, 2 to 18, 8 to 72 amu) may be selected as the mass-scan mode; each range is normally scanned in 1.6 sec, corresponding to a distance of 12 km along the orbit. Ion spectra will be simultaneously telemetered in both analog and digital form; the digital data result from on-board processing of the analog spectra and consist only of ion-peak coordinates. The three overlapping mass ranges will enable in-flight evaluation of mass discrimination within the ion analyzer. Verification of absolute instrument sensitivity will require correlation of the spectrometer data with results from the companion electrostatic probe and retarding-potential analyzer. Such correlations should permit individual ion concentrations to be determined with an accuracy of ±10%.