PDS_VERSION_ID = PDS3
LABEL_REVISION_NOTE = "
2007-08-01 MESS:mick V01 First Draft / Example,
2007-10-07 MESS:chabot V02 Content added"
RECORD_TYPE = STREAM
OBJECT = INSTRUMENT
INSTRUMENT_HOST_ID = "MESS"
INSTRUMENT_ID = "EPPS"
OBJECT = INSTRUMENT_INFORMATION
INSTRUMENT_NAME = "ENERGETIC PARTICLE AND PLASMA SPECTROMETER"
INSTRUMENT_TYPE = "ENERGETIC PARTICLE AND PLASMA"
INSTRUMENT_DESC = "
The MErcury Surface, Space ENvironment, GEochemistry and Ranging
(MESSENGER) mission is designed to orbit Mercury following one Earth
flyby, two flybys of Venus and three of Mercury. It launched in
August 2004 and will use these flybys to achieve an orbit insertion
around Mercury in March 2011. Initial data collection will begin
during the three flybys of Mercury, and will primarily consist of
global mapping and measurements of the surface, atmosphere and
magnetosphere composition. MESSENGER will remain in orbit for the
rest of the nominal mission, which is scheduled to end in March
2012. Once in orbit around Mercury it will begin a series of
observations using multiple instruments. These observations will
provide data to answer questions about the nature and composition of
the crust, tectonic history, the structure of the atmosphere and
magnetosphere, and the nature of the polar caps.
The Energetic Particle and Plasma Spectrometer (EPPS) system
encompasses 2 instrument subsystems - the Energetic Particle
Spectrometer (EPS) and the Fast Imaging Plasma Spectrometer (FIPS).
EPS covers the energy range of 25 to ❯ 500 keV for electrons, and 10
keV/nucleon to ~3 MeV total energy for ions. FIPS covers the
energy/ charge range of ❮ 50 eV/q to 20 keV/q. The desired
throughput for FIPS charged particle and EPS event processing is 5
kHz. The Johns Hopkins University/Applied Physics Laboratory
constructed the EPS instrument, while the FIPS instrument was
constructed by the University of Michigan Space Physics Research
Laboratory.
FIPS Overview
=============
The FIPS sensor is designed to measure the distributions and
composition of magnetosphere ions, as well as to characterize the
nature of the planetary magnetic field of Mercury. It will do this
by measuring the mass per charge, the energy per charge, and
incident angles for particles entering the sensor. The particle
intensity is also calculated from the event rate information. FIPS
generates a single 48-bit raw event packet format, which includes a
1-bit header that identifies the event as a proton event or a
non-proton event; an 11-bit time-of-flight (TOF) value; as well as
Wedge, Strip, and ZigZag values (each 12 bits in size). In addition,
the FIPS system generates counter and housekeeping information that
the EPPS software can access via the I2C bus interface.
The FIPS consists of an electrostatic analyzer (ESA), located at the
entrance to the sensor, a post-acceleration chamber between the
output of the ESA and the carbon foil, and a time-of-flight
telescope. The ESA at the entrance to the FIPS acts as a wide-angle
lens for ions. It only allows ions with a specific energy/charge
band to enter through its output plane. This band is stepped once
per second by changing the deflection voltage of the ESA. A
measurement cycle consists of 64 deflection voltage steps in nominal
mode or 8 in burst mode. Associated with each step in a scan is a
voltage setting, a threshold, a settling time, and a duration or
time interval after which the next voltage step is performed. Ions
exit the output plane of the ESA and are then accelerated in the
post- acceleration chamber. This acceleration is done to boost
low-energy ions to penetrate the carbon foil. The acceleration also
helps to reduce energy straggling and angular scattering - effects
that cause degradation in mass resolution and imaging. The carbon
foil serves as the source for secondary electrons, which are
scattered out by the penetration ions. After penetrating the foil,
the particle resides within the TOF chamber where velocity and
incoming angle are computed. Velocity is determined by the time
difference between the generation of secondary electrons in the
start foil and a stop surface, and angle is determined by spatially
imaging the position of the generation of the start secondary
electrons. From the velocity, energy per charge, and the
post-acceleration potential, it is then possible to calculate the
mass per charge. The measured species for the FIPS range from H to
Fe.
The FIPS instrument provides a single serial stream of event data to
the EPPS system at rates of up to 50000 events per second. The EPPS
software maintains a mass distribution spectrum for the FIPS
instrument. This spectrum consists of a collection of 256 bins (each
24 bits wide) that count the number of events corresponding to mass-
per-charge values. In addition, the software maintains a set of 5
element energy spectra. Each FIPS spectrum corresponds to a
specified mass-per-charge range and consists of 64 24-bit bins. For
events whose mass-per-charge values fall within one of the selected
ranges, an energy value is computed and used to determine which bin
within the corresponding spectrum to increment. The spectra are
accumulated over an integral number of voltage scans, after which
they are compressed and output in telemetry. FIPS also records 5
heavy-ion energy-summed images (called velocity distributions) for
each of the same 5 mass-per-charge values plus one for protons. A
commanded number of raw events will be recorded at each scan level.
EPS Overview
============
The EPS determines the distributions of the higher-energy
magnetospheric ion and electrons, including the composition of the
ions, to characterize the nature of the planetary field of Mercury.
It does this by measuring the energy and velocity of the particles
and then using a look-up table to determine the mass and therefore
the species of particle. The measured species for the EPS include
H, He, CNO, Fe, and electrons. Electrons are measured by
solid-state detectors behind absorbing aluminum flashing.
The EPS sensor consists of a 60-mm diameter, tuna-can-like cylinder,
in which a start foil and stop foil, wrapped around opposite curved
sides of the cylinder, constitute the TOF chamber. An incoming
particle hits the start foil and scatters one or more electron,
which is attracted to the start-anode ground. The particle
continues and hits the stop foil, scattering other electrons, which
are then attracted to the stop-anode ground. The solid-state
detectors outside of, but wrapped around the curved face of, the
stop foil, then detect the particle and measure the energy state.
The detectors are arranged so that each detector senses the events
within a given range of incidence angles. Each of the 6 detector
modules is composed of 4 pixels: large and small ion and large and
small electron. This provides 24 detector elements. At any one
time, 12 of the 24 elements are used (6 ion and 6 electron
detectors). Each of the 6 EPS detector modules also maintains its
own spectrum via 64 16-bit bins. 63 bins will count the
particle/energy combinations of interest, and 1 will count the
remaining background events that do not fall in the particle/energy
combinations of interest. The spectra are accumulated over a time
set by ground command, after which they are compressed and reported
in telemetry.
The EPS system also includes 32 16-bit rate counters and 3 24-bit
rate counters that are read by the EPPS software every n seconds (n
specified by command). EPS status and housekeeping data such as
voltages, currents, and temperatures are also periodically sampled.
The EPPS instrument is described in full detail in
[ANDREWSETAL2007]."
END_OBJECT = INSTRUMENT_INFORMATION
OBJECT = INSTRUMENT_REFERENCE_INFO
REFERENCE_KEY_ID = "ANDREWSETAL2007"
END_OBJECT = INSTRUMENT_REFERENCE_INFO
END_OBJECT = INSTRUMENT
END
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