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The Retarding Ion Mass Spectrometer (RIMS) experiment onboard the DE-1 satellite was designed to perform energy and mass-per-charge analysis on low-energy ions (<50 eV) with mass/charge ranging from 1 to 40 amu/Z. The instrument is fully described by Chappell et al. (The Retarding Ion Mass Spectrometer on Dynamics Explorer-A, Space Sci. Instrum., 4, 477, 1981). RIMS has significantly improved capabilities over previous retarding potential analyzer instruments (RPA) by providing ion mass/charge separation so that RPA and spin curves are obtained separately for each programmed species. These enhanced instrument capabilities, combined with the DE-1 orbit, produced a unique opportunity to investigate the variable dynamics and composition of the low-energy ion population in the near-Earth space environment.
DE-1/RIMS consists of four instrument assemblies interconnected to form one experiment. Three of the assemblies are sensor heads and one is the central electronics assembly. The three heads are labeled according to the mounting axis on the DE-1 spacecraft: Radial, +Z, and -Z. The radial sensor views perpendicular to the spacecraft spin axis, while the ±Z sensors on the ends of the spacecraft view parallel and anti-parallel to the spin axis. The central electronics assembly (CEA) provides the spacecraft interface, all data processing, command decoding, and complete timing control of the entire RIMS experiment.
Each sensor head consists of an RPA followed by a magnetic mass analyzer with two separate exit slits corresponding to two mass/charge ranges in the ratio 1:4. The total mass/charge range covered is 1 to 40 amu/Z. Surrounding and attached to the entrance of the sensor head is a 20 cm circular aperture plate. The plate is connected to a relay which can, on command, connect to either spacecraft chassis ground or to the aperture potential power supply output. The aperture potential power supply can be set by spacecraft minor mode A command to 0, -2, -4, or -8 V to partially compensate for a non-zero positive spacecraft potential.
The retarding grid of the RPA is connected to the retarding potential power supply through a shielded conductor. The retarding potential may be set to any one of 1024 linearly spaced steps from 0 to +51.125 V with a resolution of 0.05 V. The RPA collector plate is connected through a co-axial cable to a 5-decade logarithmic amplifier. The reference voltage for the front end of the log amp is the output of the aperture potential power supply. The output of the log amp is converted, by command from the CEA, to a 10-bit digital word using an analog/digital (A/D) converter. The digital word is held in a holding register until the CEA is ready to read and process the data.
Those ions passing into the Ion Mass Spectrometer (IMS) are sorted according to their mass to charge ratio. The proper combination of ion accelerating voltage, applied magnetic field strength, and ion beam gyro radius in the applied magnetic field determines the mass/charge of the ion focused on each collector slit. Varying the ion accelerating voltage varies the ion mass/charge detected. Ions of mass/charge 1 to 8 amu/Z and 4 to 32 amu/Z can be focused on the low and high mass slits, respectively. Ions exiting the collector slits are counted by channel electron multiplier (CEM) detectors. The ion mass/charge range is also programmable by a minor mode command. Any 32 of 4096 voltage steps may be selected. All 32 steps may be the same, in which case the mass/charge analyzer will be locked onto a given pair of mass/charge peaks having the ratio 1:4. The two-channel IMS uses CEMs as detectors. The two CEMs in each sensor head are powered by a single multiplier high-voltage power supply which can be set, by spacecraft minor mode A command, to any one of four voltages, -1200, -2100, -2400, and -2800 V. The IMS accelerating voltage comes from a swept high voltage power supply that can be addressed to any one of 4096 linearly spaced steps between 0 and -2250 V.
The output of each detector is connected to a charge sensitive pulse amplifier whose output is sent to a level detector. The discrimination level is set by spacecraft minor mode A command. Pulses from the discriminator are coupled to an 18-stage binary counter. Four bits of the counter are located in each head, and the other 14 bits of each accumulator are located in the CEA. The CEA contains the circuitry for compressing each accumulator output into a 10-bit base 2-floating point number (6-bit mantissa and 4-bit exponent) for output into the telemetry buffer.
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