Overview of the Plasma Instrument

A pictorial view of the plasma instrument (PLS) is shown in Figure 1. The instrument is divided into two analyzers, A and B. Each electrostatic analyzer comprises three 70° spherical-segment plates. The outer and inner plates are grounded and the center plate is supplied with a programmed series of voltages to effect analyses of the energy spectra of electrons (E) and positive ions (P). The inner and outer channels between the plates are the positive ion and electron analyzers, respectively. A charged particle successfully passes through the channel on the basis of its energy-per-unit charge (E/Q). Continuous-channel electron multipliers, or Spiraltrons, are employed as sensors and are positioned at the exit apertures of the electrostatic analyzers. Charged particles arrive at positions at the exit aperture according to their direction of arrival at the entrance aperture. The analyzers are mounted on the science (magnetometer) boom of the spacecraft such that charged particles moving perpendicular to the spacecraft spin axis arrive at sensors 4E and 4P, and particles generally moving parallel and antiparallel to the spin axis are detected with sensor pairs 7E, 7P and 1E, 1P, respectively. Thus the fan-shaped fields-of-view are divided into segments by the use of multiple sensors. The instantaneous fields-of-view for the seven ion sensors as projected onto the unit sphere for velocity vectors are shown in Figure 2. Rotation of the spacecraft spinning section allows coverage of almost the entire unit sphere and angular distributions are obtained by electronically sectoring the sensor responses as a function of spacecraft rotation angle. The angular sampling of electron velocity distributions is similar. The instrument is placed at a sufficient distance out along the boom to avoid obstruction of the fields-of-view by the large dish antenna of the spacecraft.

Three miniature mass spectrometers are included in the instrument for determining the composition, i.e., mass-per-unit charge (M/Q), of the positive ion plasmas. Two of these mass spectrometers are positioned at the exit aperture of electrostatic analyzer B, the third spectrometer is in analyzer A. Each of these mass spectrometers is equipped with two Spiraltrons as sensors and an electromagnet. One of these sensors is placed behind the electromagnet such that it accepts ions not deflected by the gap magnetic field. These 'integral flux' sensors are shown as 1MI, 2MI, and 3MI. The second sensor in each mass spectrometer is displaced from the undeflected path and accepts ions with M/Q values that are a function of the gap magnetic field. These 'differential flux' sensors are 1MD, 2MD, and 3MD. A programmed series of currents is fed to the electromagnet. If the polar angle is taken as 0° in the direction of the spacecraft spin axis, then the fields-of-view are 11°-38°, 87°-93°, and 142°-169° for spectrometers 1, 2, and 3, respectively. The instantaneous fields-of-view of the three mass spectrometers as mapped onto the unit sphere are shown in Figure 3.

The aperture cover serves two purposes. Prior to and during launch the cover in its closed position prevents contamination of the sensors from dust and condensable vapors. After the launch sequence, the cover is opened and is employed to tailor the fields-of-view of the sensors viewing at small angles to the spin axis of the spacecraft. The corresponding obstructions are identified as shapers in Figure 1.

A photograph of the Galileo plasma instrument in shown in Figure 4, along with a 6-inch ruler. The aperture cover has been opened to show the two arc-shaped opening that are the entrance apertures for the two analyzers, A and B. A thermal blanket is yet to be installed around the instrument.