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.