INSTRUMENT_HOST_ID = "MESS" INSTRUMENT_ID = "EPPS" INSTRUMENT_NAME = "ENERGETIC PARTICLE AND PLASMA SPECTROMETER" INSTRUMENT_TYPE = "ENERGETIC PARTICLE AND PLASMA SPECTROMETER" INSTRUMENT_REFERENCE_INFO: PDS3_REFERENCE_KEY_ID = "ANDREWSETAL2007" INSTRUMENT DESCRIPTION: 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]."