A total of seven sensors are used for the two electron analyzers, and seven for the two positive ion analyzers. These continuous channel multipliers are Spiraltrons, model SEM 4211 with 1-mm diameter apertures and model SEM 4213 with 3-mm diameter apertures, manufactured by Galileo Electro-Optics Corporation. Entrance apertures of these sensors are positioned at a distance 16 mm from the exit aperture of their respective electrostatic analyzers. The Spiraltrons with larger apertures are used for the two ion sensors that view closest to the spin axis of the spacecraft, i.e., the polar sensors, in order to offset the reduced projected area of the entrance aperture. The sensors are screened for stability by operation for ~2 × 109 accumulated counts at a gain > 108. Grounded mesh screens are mounted in front of the entrance apertures of the sensors to shield the sensor post-acceleration electric fields for the prevention of the collection of secondary charged particles produced in the interior of the instrument. The post-acceleration voltage for the ion sensors is approximately the bias voltage, and about +150 V for the electron sensors. The nominal gain of the Spiraltrons is 5 × 107 to 3 × 108 in the saturated pulse counting mode. The output charge is collected by small plates and the collection efficiency is improved by a potential difference of about 120 V for the electron sensors and 200 V for the ion sensors. This charge is received by hybrid amplifiers and discriminators manufactured by AMPTEK Inc., model A101. The threshold for these amplifiers was conservatively set at 4 × 106 electrons. The high voltage for sensor bias is programmable by ground command in 32 increments spanning the range 2200 V to 3800 V in order to maximize the operating lifetime of the sensors against degradation by using the minimum charge per pulse. The pulse pair resolution of the amplifier/discriminator is nominally 250 ns (4 mHz), and about 1.4 microseconds (700 kHz) after modification for use in the instrument.
An example of the mass spectrometer performance taken from laboratory calibrations of the flight instrument is shown in Figure 6. The value for E/Q is 947 V for the two ions in the beam, H2+ and OH+. The sweeping of the ions from the integral sensor as a function of M/Q and mass channel (magnet current) is clearly evident. At higher mass channels (larger current) these ions are deflected sufficiently to be detected with the differential sensor. A summary of the measured performance of the miniature mass spectrometer as functions of M/Q, E/Q, and electromagnet current is given in Figure 7. The M/Q value for the integral sensor is taken at a fraction 0.5 of the undeflected responses. For a given current step of the mass spectrometer, the averaged FWHM for the three mass spectrometers in terms of ion energy is dE/E = 0.06. In general the differential channel is used for the detection of trace fluxes of light ions and the integral channel for abundant heavy ions in the Jovian magnetosphere. The mass resolutions of the mass spectrometers are M/dM = 4.2 at full-width at 50% responses (FWHM) for the differential sensors (MD) and M/dM approximately equal to 2 for the integral sensors (MI). This resolution has been chosen to allow identification of the species H+, H2+ (He++), He+, O++, O+, Na+, S+ and K+ with the MD sensors and H+, H2+ (He++), O++, O+, S+, and SO2+ with the MI sensors. The E/Q ranges vary with the M/Q of the ion species, e.g., for the MD sensors, 0.9 V to 20 kV for H+, 0.9 V to 3 kV for O+, and 0.9 V to 800 V for S+. For the MI sensors, these ranges are 10 V to 52 kV for H+, 0.9 V to 52 kV for O+, and 0.9 V to 14 kV for S+. The mass spectrometers cannot distinguish between two ions with the same M/Q, e.g., O+ and S++. The mass spectrometers are designed in part with the criterion that corotating SO2+ (M/Q = 64 amu, E/Q approximately equal to 2 kV) can be identified at Io's orbit.
These values are computed by comprehensive ray tracing of trajectories through the electrostatic and magnetic analyzers and with the nominal entrance area of the sensor. In practice both the efficiency and this area vary with individual sensors and final values of the geometric factors are derived from laboratory measurements and inflight responses in an isotropic plasma such as that in the plasma sheet during Earth1 encounter. These geometric factors are tailored to provide effective measurements of both the dense plasmas in the torus and the sparse plasmas of the outer Jovian magnetosphere.
The sensitivities for detecting these plasmas are summarized in Figure 8. The maximum responses of a single sensor to several representative plasmas are shown as functions of the plasma temperature, bulk flow speed V, and species. The bulk speed of 100 km/s has been chosen as scale-wise representative for the corotational speeds in the torus. The densities of all the plasmas are each assumed to be cm-3. For example, if the density of S+ ions is 1000 cm-3, V is 100 km/s, and the temperature kT is 100 eV, the maximum responses of the ion sensors of the electrostatic analyzer (P) and of the ion sensors of the mass spectrometers (M) are 4 × 106 and 2 × 105 counts/s, respectively, when viewing in the bulk flow direction. The geometric factor of the ion sensor (P) is sized such that these responses are somewhat above the saturation values for the sensor/amplifier, ~106 counts/s. The ion sensors in the mass spectrometers are employed to extend the dynamic range of theses ion measurements to the larger ion densities by means of their lesser geometric factors. On the other hand, the large geometric factor of the ion sensors for the electrostatic analyzers provides the capability of the determining densities of hot (~tens of keV), isotropic ions as low as 10-3 to 10-2 cm-3 in the outer regions of the magnetosphere. Thus the combined geometric factors of the electrostatic analyzers and mass spectrometers accommodate a large range of ion densities. If the electron densities in the center of the plasma torus are 3000 cm-3, the the maximum responses for the electron sensors are ~2 × 105 and 6 × 105 counts/s for electron temperatures kT = 1 and 10 eV, respectively. For an electron temperature of 10 keV in the outer magnetosphere, densities as low as 10-4 to 10-3 cm-3 can be well determined.
Considerable attention in the design of the instrument was directed toward minimizing the sensor responses to the intense fluxes of energetic electrons in the inner Jovian magnetosphere. The Spiraltrons are shielded in all directions by a minimum of 2.5 g cm-2 equivalent of aluminum. This corresponds to an electron range of ~5 MeV. In addition the Spiraltrons used for ion sensors are operated at a sufficiently low voltage that two or more initial secondary electrons at their entrance apertures are necessary to yield an electron pulse above the discriminator level of the amplifiers. This mode of operation reduces the sensor efficiency for the detection of ions by 50% (plus or minus 10%), with a corresponding decrease in the geometric factors cited in Table 1. Such operation of the sensors at bias voltages ~2400 V allows discrimination against detection of penetrating electrons. The omnidirectional geometric factors for detection of penetrating, > 5 MeV electrons are ~10-4 cm2 for the ion sensors with 1-mm apertures, and ~10-3 cm2 for the 3-mm ion sensors (see Table 1). The corresponding geometric factors for the Spiraltrons used in the electron analyzers are ~10-3 cm2. At the orbit of Io the electron intensities with E > 5 MeV are ~2 × 107 cm-2 s-1 (Van Allen, 1976). Thus the background counting rates are ~2 × 103, 2 × 104, and 2 × 104 counts/s for the 1-mm ion sensors, the 3-mm ion sensors, and the 1-mm electron sensors, respectively. For comparison, the sensor responses in the direction of flow (S+, 1000 cm-3, 50 eV, 100 km sec-1) are ~5 × 106 counts/s for the ion channels of the electrostatic analyzer and ~3 × 105 counts/s for the sensors in the mass spectrometer. The analyzer responses to electrons (e-, 1000 cm-3, 50 eV) are expected to be ~6 × 105 counts/s. The corresponding S/N ratios are 2500, 150 (I) and 15 (D), and 30 for the ion sensors, mass spectrometer sensors and electron sensors, respectively.
At larger radial distances, > 20 RJ, the intensities of electrons with E > 5 MeV are typically < 103 - 104 cm-2 s-1 within and near the plasma sheet (Baker and Van Allen, 1976). The corresponding maximum background rates are then < 1 and 10 counts/s for the 1-mm positive ion and electron sensors, respectively. For these maximum rates, the densities for which S/N = 1 for an isotropic, H+ plasma are 3 × 10-3 cm-3 at kT = 10 keV and 5 × 10-3 cm-3 for electrons at 1 keV. The corresponding densities for the mass spectrometer sensors are ~0.1 cm-3 (I) and 1 cm-3 (D). These above examples for H+ give the most pessimistic values because we have assumed worst-case background rates and because the ion plasmas are partially corotating. The S/N ratios will be typically larger by factors of ~10 to 100.
The spacecraft potential is expected to be important at the lower energy range of the analyzer. A quantitative assessment of anticipated spacecraft potentials is given by the Voyager plasma measurements. In the outer magnetosphere, typical Voyager spacecraft potentials were positive in the range of several volts to 10 V (Scudder et al., 1981). Because the plasmas are generally hot, temperatures ~keV, in the outer magnetosphere the plasma measurements should not be greatly impaired. On the other hand, in the highest density regions of the Io torus, Voyager spacecraft potentials were negative with magnitudes up to 25 V (Sittler and Strobel, 1987). In this region electron temperatures are tens of eV or less and the observations of thermal electron plasmas may be precluded if the Galileo spacecraft potential is similar. The energy range of the Galileo plasma instrument is sufficient to determine this spacecraft potential. Determination of the magnitude of the potential will have to await the in-situ observations. The potentials along the boom on which the plasma instrument is mounted and those of the spacecraft body will also affect the trajectories of low-energy particles as viewed by the plasma analyzer. This effect will have to be modeled in detail in order to determine the deflections of the observed angular distributions as a function of particle energy.
The reduction of single-point failures of the instrument proved to be considerably more difficult for the data handling and control sub-system (DHCS). The configuration that was chosen for the microprocessors and associated electronics is shown in Figure 10. There are two separate buses, A and B, that can singly operate the two analyzers. Similarly there are two RCA 1802 microprocessors, 1 and 2, that are each equipped with 4 kbytes of read-only memory (ROM) and 4 kbytes of read/write memory (RAM). Two bus adapters, alpha and beta, couple the microprocessors with the command data system (CDS) of the spacecraft. The instrument is operated with one bus adapter, one microprocessor, and one bus. The bus separator/selector allows the use of any combination of these electronic elements, e.g., bus adapter alpha, processor 2, and instrument bus A. This configuration for the DHCS is set via a hardware bus command (HBC) that transfers the necessary information in the address portion of the packet header from the spacecraft CDS. The HBC is executed regardless of which processor and bus adapter are currently selected. If the currently selected bus adapter fails, the HBC can be used to select the other bus adapter.
Each of the two microprocessors is provided with identical I/O electronics that include an analog-to-digital (ADC) converter (model AD571, Analog Devices, Inc.), three digital-to-analog (DAC) converters, and a digital status input port. A 16-input multiplexor is used with the ADC to monitor voltages within the instrument. The DACs provide the control voltages for the programmable high voltage (plate and bias) and current (electromagnet) supplies.
Two low voltage power supplies, A and B, are included within the plasma instrument. By means of a power distribution system, failure of a single low voltage supply does not result in the loss of the DHCS or instrument bus. Analyzer A or B becomes inoperable with the failure of one of the low voltage power supplies, A or B. A power switching circuit that is controlled by ground command is used to select the analyzer to be operated with the functioning low voltage power supply. The replacement and supplemental heaters shown in Figure 10 are used for thermal control during the mission. The latch for releasing the protective cover over the instrument aperture is a one-shot redundant device with two electrically fired, black powder Unidynamics bellows actuators.