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 an open ion source, a closed ion source, a quadrupole deflector and lens system, a quadrupole mass analyzer, and a dual detector system. For information on these components, click on their names.
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.
