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Cassini Plasma Spectrometer (CAPS)

Three independent sensors, a data processing unit, and an actuator make up CAPS. The Ion Mass Spectrometer (IMS), together with its High-Voltage Units (HVU-1 and -2), is the largest and most complex of the three sensors. The Electron Spectrometer performs measurements analogous to those of IMS but for electrons, while the Ion Beam Spectrometer is dedicated to measuring the very narrow distributions of ions expected at Titan and in the solar wind and auroral zones.

Each of the sensors is self-contained. High-voltage supplies (up to ±15 kilovolts in HVU-1) are located within or immediately adjacent to their respective sensors, and all low-voltage analog signals generated by particle detectors are converted to high-level digital signals within each sensor. This modular approach reduces engineering risk and allows the far-flung instrument team (nine institutions in six countries contribute to CAPS hardware) to efficiently build, test, and deliver the sensors for final integration at Southwest Research Institute.

All three sensors are cylindrically symmetric and are based on curved-plate electrostatic analyzers (ESA) that perform angular and energy/charge selection of incoming ion or electron fluxes. Each sensor produces a counting range proportional to particle flux within a narrow passband of energy/charge and angle of arrival. Different particle energies are selected by varying the voltage on the ESA plates every few milliseconds or tens of milliseconds, depending on the sensor. The IMS is an entirely new mass spectrometer design based on measuring ion time-of-flight that yields an unprecedented mass resolution of 80 at energies as high as 16 kiloelectronvolts.

Published in the Fall 1997 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

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