Ion Mass Spectrometer (IMS)

Description

The objective of the investigation is to measure the ion composition, energy, and angular distribution in the coma of Comet Halley. The instrument consists of two sensors: the High-Energy Range Spectrometer (HERS) specialized for the outer coma and the High-Intensity Spectrometer (HIS) specialized for the inner coma. The HERS sensor has (1) an electrostatic mirror for deflecting ions from the spacecraft ram direction into the sensor, instead of having the solar wind ions enter; (2) cylindrical acceleration grids which change the energy of the ions so they can pass through the magnetic analyzer; (3) a sector magnet that acts as a momentum/charge filter; (4) an electrostatic deflection plate that sorts the ions by energy/charge; and (5) particle detectors consisting of a two-dimensional microchannel plate (MCP) and channel-electron-multipliers (CEM) for measuring mass/charge and elevation angle. The energy range is from 20 eV to nearly 16 keV, depending on the M/Q, and the mass is determined in 3 or 4 mass groups (e.g., 1 to 4, 4 to 16, 16 to 64 u) with a mass resolution of M/delta M equal to about 20. The elevation angle, the polar angle relative to the spin axis, covers 30 deg and is measured in 4 bins, giving a resolution of 7.5 deg. The azimuth angle is spin-scanned and has a resolution of 4 deg. The MCP allows the simultaneous measurement of mass and elevation angle for a given ion momentum per charge (8 kV). A complete scan over mass, energy, and angle takes about 12 s, but other scans can be selected. The HIS sensor is used to measure the relatively cold, low-energy cometary species. This sensor has (1) a set of deflection plates above the spacecraft skin; (2) a quadrispherical lens followed by (3) a set of acceleration grids; (4) a permanent magnet; (5) a second quadrispherical lens, or analyzer; (6) a block of glass with conductive surfaces with holes in four directions that serves as a particle distributor and amplifier; and (7) 16 CEMs to detect the emergent particles. HIS measures energies from 300 to 1625 eV over a M/Q of 12 - 65 with a resolution (M/Delta M) of about 20, in an elevation angular range of 27 deg. The azimuth angle is spin-scanned and the time resolution for a set of measurements is about 4 s. The density range covered by HERS is 1.E-3 to 1.E2/cc, while that for HIS is 1.E-2 to 1.E4/cc. The instrument uses a microprocessor for operation and control. More details about this instrument can be found in "The Giotto Mass Spectrometer" by Balsiger et al., ESA SP-169, June 1981.