INMS Engineering Technical Write-up

TL: Dr. J. Hunter Waite

INMS General Description:

The Ion and Neutral Mass Spectrometer (INMS) is intended to measure positive ion and neutral species composition and structure in the upper atmosphere of Titan and magnetosphere of Saturn, and to measure the positive ion and neutral environments of Saturn's icy satellites and rings.

INMS Scientific Objectives:

  • To measure ion and neutral species composition and structure in the upper atmosphere of Titan.
  • To study Titan atmospheric chemistry.
  • To investigate the interaction of Titan upper atmosphere with the magnetosphere and solar wind.
  • To measure ion and neutral species compositions during ring plane crossings and icy satellite flybys.

INMS Sensing Instruments:

  • Closed source (neutrals only) and open source (ion and neutrals)
  • Lens System
  • Quadrupole Mass Analyzer and Detector, 1-99 amu

INMS Instrument Characteristics:

  • Mass (current best estimate) = 9.25 kg
  • Average Operating Power (current best estimate) = 27.70 W
  • Average Data Rate (current best estimate) = 1.50 kilobits/s

The Ion and Neutral Mass Spectrometer Subsystem (INMS) will be used to study the neutral gases and positive ions in the atmospheres of Saturn and Titan and gases in the vicinities of the Saturnian rings and the icy satellites. It will also study the magnetosphere of Saturn.

The major functional components of the INMS Subsystem are:

Open ion source
Closed ion source
Quadrupole deflector and lens system
Quadrupole mass analyzer
Dual detector system

The open ion source produces ions by ionizing neutral gases. It includes an ion trap/deflector that forms trapped ions into a beam. This minimizes interaction effects between the gas environment and the open source surface as the source directly samples the gaseous species.

The closed ion source also produces ions by ionizing neutral gases. It uses ram density enhancement to provide measurements of higher accuracy and sensitivity for the more inert atomic and molecular species than provided by the open ion source. This is achieved by maintaining a high input flux to an enclosed antechamber and then limiting the gas conductance or output from the antechamber by the use of an orifice.

Ions are directed to the mass analyzer from the selected ion source by changing the potentials on a 90-degree quadrupole deflector. This electrostatic device allows both sources of ions to be sequentially switched into a common exit lens system.

The quadrupole mass analyzer consists of four precision ground hyperbolic rods mounted in a rigid mechanical assembly. The transmitted mass, the resolution, and the ion transmission are controlled by variations in RF and d.c. electric fields between adjacent rod pairs, while opposite rod pairs are kept at the same potential.

The ion dual detector system amplifies and detects the input from the mass analyzer by the use of two continuous dynode multipliers.

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