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Instrument: CEP : Cylindrical Electrostatic Probe
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The Cylindrical Electrostatic Probe (CEP) consisted of two
identical instruments designed to measure electron temperatures,
electron and ion concentrations, ion mass, and spacecraft
potential. One probe was oriented along the spin axis of the
spacecraft (usually perpendicular to the orbit plane), and the
other radially, so that it could observe in the direction of the
velocity vector once each 15-s spin period. Each instrument was
a retarding-potential Langmuir-probe device that produced a
current-voltage (I-V) curve for a known voltage pattern placed
on the collector. Electrometers were used to measure the
current. There were two systems of operation (one with two modes
and another with three modes) using collector voltage patterns
between plus and minus 5 volts. Most modes involved an
automatic or fixed adjustment of collector voltage limits
(and/or electrometer output) such that the region of interest on
the I-V profile provided high resolution. Each system was
designed for use with only one of the probes, but they could be
inter-switched to provide backup redundancy. The best
measurements in the most favorable modes provided 1-s time
resolution; electron temperature between 300 and 1.E4 deg K (10%
accuracy); ion density between 1.E4 and 1.E7 ions/cc (10-20%
accuracy); electron density between 50 and 1.E6 electrons/cc;
and ion mass at ion densities above 1.E4. Each probe had a
collector electrode extending from the central axis of a
cylindrical guard ring. The 2.5-cm-long guard ring was at the
end of a 25-cm boom, and the collector extended another 7.5 cm
beyond the guard ring. The boom, guard, and collector was 0.2 cm
in diameter.

[Source: NASA]