Here we describe in some detail the design, hardware and inner workings of the EIS instrument. Each instrument comprises two subsystems, the sensor head and the main electronics (Fig. 12, upper right). Each sensor head incorporates electron and ion sensors (e.g. Fig. 9 and later discussions), plus detector preamplifiers. The sensor head and main electronics are mechanically integrated together and mounted as a single unit to the spacecraft.
Note that to keep the technical descriptions simple and straight forward, we do not provide many of the technical specifications for the EIS instrument in this section (mass, power, sizes, materials, densities, thicknesses, gaps, voltages, etc.). Those specifications are provided in the Appendix.
Principles of Operation
EIS measures ion energy, directional, and compositional distributions using Time-of-Flight by Energy (TOF×E) and Time-of-Flight by MCP-Pulse-Height (TOF×PH) techniques. EIS measures electron energy and directional distributions using collimated solid-state-detector (SSD) energy measurements (these electron SSD’s, as opposed to the ion SSD’s, have 2 microns of aluminum flashing deposited on them to keep out protons with energies less than about 250 keV). EIS combines multidirectional viewing into individual compact sensor heads (Fig. 12). The sensor heads include time-of-flight (TOF) sections about 6 cm across feeding a solid-state silicon detector (SSD) array. Secondary electrons, generated by ions passing through the entry and exit foils (Fig. 12 left), are detected by the microchannel plate (MCP) stimulated timing anodes and their associated preamps to measure ion TOF. Event energy (E) and TOF measurements are combined to derive ion mass and to identify particle species.
The EIS acceptance angle is fan-like and measures 160° by 12° with six ∼26.7∘ look directions (this pattern is quantitatively disrupted for the two central pixels due to the solar blockage—Fig. 10). Particle direction is determined by the particular look direction in which it is detected (six different view directions for each species, labeled v0, v1, v2, v3, v4 and v5). That directionality is determined by the active SSD in the case of electrons, and by the determination of the entrance position on the MCP-stimulated time-delay anode nearest to the start foil in the case of ions (time delay along a chain of 12 “start” anode pads connected by inductors is used to determine entrance position). Ions that pass through the sensor encounter three separate thin foils mounted on ∼90 % transmission grids. The first one, the “collimator foil,” mounted within the collimator, is a 350 Å aluminum foil. The next foil, the “start” foil, is a 50 Å carbon/350 Å polyimide/50 Å carbon foil. These 2 foils reduce the UV (e.g. primarily Lyman alpha) photon background. The exit apertures are covered by the third or “stop” foil of 50 Å carbon/350 Å polyimide/50 Å carbon/200 Å aluminum. All foils are mounted on high-transmittance (90 %) metal grids supported on a metal frames.
Before an electron passes through the TOF head, it is first decelerated by a 2.6 kV potential (part of the TOF optics for measuring ions); it is later reaccelerated by 2.6 kV after exiting the stop foil prior to reaching the SSD detectors. Energetic electrons from 25 keV to 1000 keV are measured by the electron SSD detectors. The electron detectors are covered with 2 μm aluminum metal flashing to keep out protons and ion particles with energies less than about 250 keV. No TOF criterion is applied to the electron measurements. The sensitivity to particles can be adjusted by a factor of 20 by selecting large or small SSD pixels (discussed below).
As described above, an ion encounters 3 foils on its way to the SSD (Fig. 12 left). Secondary electrons from the start and stop foils are electrostatically separated from the primary particle path and diverted on the microchannel plate (MCP), providing start and stop signals for TOF measurements. The segmented MCP anodes, with two start and two stop anodes for each of the six angular segments, determine the direction of travel. A 500-volt accelerating potential between the foil and the MCP surface, combined with the cone-like electrostatic mirror (labeled “electron deflector” in the upper left of Fig. 12), controls the electrostatic steering of secondary electrons. The dispersion in secondary electron transit time is less than 1 nsec. As an aside we should note that after penetrating any foil, the ion may emerge as an ion or as a neutral. If it emerges as an ion from the collimator foil it is subject to the acceleration and/or deceleration potentials (2.6 kV) associated with the secondary electron optics
Ion energy measurements using the ion detectors are combined with coincident TOF measurements to derive particle mass and identify particle species (the TOF×E method). With the TOF×E method the incoming particles are measured from 50 keV to above 1 MeV; they are discriminated in the energy system above 50 keV for protons and above about 150 keV for heavy ions (such as the CNO group). An example of a TOF×E matrix and how it separates different mass species is shown in the left panel of Fig. 13 from the New Horizons PEPSSI instrument at Jupiter (McNutt et al. 2008). Lower-energy ion fluxes are measured using TOF-only measurements (the TOF×Pulse Height method); detection of MCP pulse height provides a coarse indication of low-energy particle mass. An example of how a TOF×PH spectrum crudely separates different ion mass species at Earth, from the IMAGE HENA instrument, is shown in Fig. 13 (right; Mitchell et al. 2003). Sensitivity to higher energy ions (those with energies above the SSD channel thresholds) can be adjusted by selecting large or small SSD pixels. However, this capability is a hold-over from the Juno JEDI instrument. It is unlikely that the small pixels will be needed in the primary MMS target regions.
The Johns Hopkins APL has generated and flown numerous TOF×E instruments, generally including SSD-based sensors, on numerous spacecraft. The list includes the Earth-orbiting AMPTE CCE MEPA instrument (McEntire et al. 1985) and Geotail EPIC instrument (Williams et al. 1994), the Jupiter-orbiting Galileo EPD instrument (Williams et al. 1992), and the New Horizons PEPSSI instrument now on its way to Pluto (McNutt et al. 2008). Instruments that have used the TOF×PH method include the Earth-orbiting IMAGE HENA instrument (Mitchell et al. 2003) and the Saturn-orbiting Cassini MIMI INCA instrument (Krimigis et al. 2004). The EIS design is heavily based on the JUNO JEDI instrument design (Mauk et al. 2013) which draws its heritage from the New Horizons PEPSSI instrument. The RBSPICE instrument on the Van Allen Probes mission (Mitchell et al. 2013) was also based closely on JEDI.
EIS Block Diagram and Details of the Electronic Design
The electronics box contains all the electronics to run the instrument other than the energy and timing preamps which are located in the sensor head. The box comprises three 10×15 cm boards mounted into 2.5 mm thick metal frames. The boards stack one on top of the other, with a stacking connector providing electrical interconnects between the boards.
The EIS Block Diagram is shown in Fig. 14. On the left, the sensor generates analog representations of particle timing signals that go into the determination of Time-of-Flight (TOF), and energy E from the SSD’s. Each SSD has both electron and ion pixels. There is only one analog electronics processing chain per SSD. Consequently, to collect both electrons and ions, the hardware is time-multiplexed between the electron and ion detectors. The hardware is in fact time-multiplexed between three possible modes: electron energy, ion energy (with no TOF constraint), and ion species. An event trigger selects what combination of TOF and SSD pulses defines an event. With energy trigger, an SSD energy (E) pulse defines an event. With TOF trigger, a TOF pulse, with or without an E pulse defines an event. For EIS, it will be typical to cycle through 2 different species modes (3/4 of the time on ion species and 1/4 of the time on electrons) every ∼0.67 seconds (exactly 1/32 of a spin). The EIS hardware passes valid particle event data to the software for further analysis using a First-In First-Out (FIFO) device.
Each ion species event consists of several parameters, which are shown in Table 4. For energy events, only SSD data are valid. For events that trigger the Time-of-Flight system, TOF1, TOF2, and TOF3 are the raw values produced by the three TOF chips. TOF chips are used for determining both the times of flights of the particle through the system and the arrival directions of the particles using time-delay anodes on the start and the stop portions of the anodes. TOF1 and TOF2 provide redundant measurements of the particle’s time-of-flight. TOF1 measures the time between the Start0 and Stop0 pulses (0 being one end of the time-delay anodes); TOF2 measures the time between the Start5 and Stop5 pulses (5 being the other end of the time-delay anodes). TOF3 measures the time between the Start0 and Start5 pulses; this provides the particle’s position on the start anode. The “TOF” parameter in Table 4 provides the corrected TOF value, the average of TOF1 and TOF2. The start position is measured by TOF3. The stop position, i.e. the time between Stop0 and Stop5, is calculated in the FPGA as TOF2+TOF3−TOF1; the result is not reported in the event data. The start position and the stop position are used to calculate a start direction and a stop direction, respectively. Note that various levels of agreement between the start, stop positions, and SSD can be enforced by EIS firmware and software to restrict the ranges of ion path lengths for any one view direction; for example one may set the parameter “n”, in the equation “Stop_Position=Start_Position±n”, where n can be 0, 1, or 2.
The event board digitizes the TOF, the MCP Pulse Height (PH), and the SSD energy E; and reads the events into a Field Programmable Gate Array (FPGA). The FPGA contains event processing logic and a processor. Some events are passed to software running on the processor for further analysis and science processing. The Event board directly processes the sensor SSD and anode preamp output signals, and contains all the necessary analog and digital circuitry to process and store event information on an event-by-event basis. The energy signals from the six SSD preamplifiers and the MCP anode pulse height are processed in parallel peak-detect/discriminator circuit and multiplexed into a single 12 bit ADC. The MCP anode signals are processed via constant-fraction discriminators (CFDs) and time-to-digital (TDC) circuitry; these measured time differences are converted into event look direction and particle velocity in the FPGA. FPGA-based event logic also determines which signals comprise valid ion and electron events and coordinates all event hardware processing timing. An APL-developed soft-core processor, the Scalable Configurable Instrument Processor (SCIP) is also embedded in the FPGA to provide all command, control, telemetry, and data processing functions of the instrument (Hayes 2005). SRAM memory storage is provided on the board to support this processor. EEPROM and boot PROM support is provided on the Support Board. The Event board plugs into the Support and Power boards.
The Support board provides a variety of support functions for the instrument. It also contains EEPROM and boot PROM accessible to the FPGA on the Event Board. The command and telemetry interface to the spacecraft is provided here. The board includes the high voltage power supply, which generates the necessary high voltage outputs for the sensor MCP and electron optics; maximum voltage is 3300 V. The Support board plugs into the Event and Power boards.
The Power board contains both the low and SSD bias voltage power supplies. The low voltage portion takes spacecraft primary power on a single 9 pin connector and generates 1.5 V (for FPGA core), 3.3 V (primarily for digital interface logic, memories, and TDCs), and 5 V (primarily for analog functions). A 15 V output powers the high voltage electronics on the support board. The board also switches power to the sensor cover actuator mechanism and generates and filters 100 V bias for the SSD detectors. The board plugs into both the Event and Support boards.
EIS utilizes five different APL-developed rad-hard ASICs in its electronics (Paschalidis et al. 2002; Paschalidis 2006). It has three APL TOF-C ASICs to measure the “Start” (entrance) and “Stop” (exit) positions on the sensor timing anodes and the time-of-flight for ions traveling between the Start and Stop foils. The TOF ASICs, which also incorporate very fast constant fraction discriminator front ends, are configured to measure times between 0 and 32 nsec with 50 psec resolution (anode positions) and between 0 and 160 nsec with 200 psec resolution (time-of-flight). Each of EIS’s six look direction utilizes a Quad Energy Chip (preamp/shaper) ASIC followed by a peak detector/discriminator ASIC to process the 4-pixel solid state detector (SSD) arrays. EIS’s control circuitry utilizes a 16-channel TRIO ASIC to multiplex and perform 10-bit analog to digital conversion of analog status information, and a number of Quad 8-bit DACs to set thresholds and control high voltage and SSD bias levels; the TOF ASICs communicate with the instrument FPGA via a parallel interface, while the Quad DAC and TRIO use serial I2C interfaces. The ASICs each require between 5 and 25 mW, and all inherently meet performance requirements beyond 100 krad radiation dose.
The sensor head is electrically connected to the electronics box via coaxial cables and twisted wire interfaces. These lines are fairly short in length (typically less than 10 cm), and are covered by a thin EMI shield to extend the faraday cage between the sensor and electronics housings. The instrument is electrically interfaced to the spacecraft and CIDP via dedicated spacecraft-provided power and data cables.
Table 5 documents the numerous JEDI hardware modes and parameter settings that can be set by command. Several “standard” settings (setting all of the various parameters shown in the table) are generated by running one of several internal instrument “macros” during the EIS turn-on sequence or by external command at other times. As an example, optimum threshold settings have some sensitivity to temperature, and onboard macros are embedded within the instrument for several temperatures over the operational range.
EIS Mechanical Configuration
External Instrument and Mounting
The external mechanical configurations of the four EIS instrument configurations are shown in Fig. 15. The schematic shows the instrument with its one-time deploy acoustic doors deployed, whereas the photographs show the doors closed. EIS, like most other MMS instruments, is mounted to the MMS payload deck (Fig. 2). EIS is mounted from the bottom side of the instrument deck (Fig. 16; on the spacecraft interior side of the payload deck), such that the instrument “look direction 5” is towards the mounting deck (and thus the +Z Spacecraft axis; aligned with the +Y EIS axis), and the instrument “look direction 0” is towards the −Z axis (Fig. 16; aligned with the −Y EIS axis). The instrument coordinate system is shown on Fig. 16 in red.
The internal structure of each EIS instrument is shown with the cutaway diagram in Fig. 17. There is a TiNi actuated pin puller that releases the spring-loaded doors. The figure shows some of the internal structure of the sensor, and the positioning of the three main electronics boards. Photographs of selected elements of the sensor are shown in Fig. 18. The upper left hand portion shows the anode board with the energy system mounted on to it. The metalized anode itself in the center shows 12 anode pads in the “start” portion (bottom) and 12 anode pads in the “stop” region. The anode pads are paired to generate 6 positions in the processing of the time delay along the string of anode pads. The TOF/MCP assembly in the upper right of Fig. 18 has top and bottom Macor ceramic pieces (white in the figure) that sandwich together to hold the start and the stop frames and foils (Fig. 18 lower right). Technical specifications of the foils are given in the Appendix.
The collimator (Fig. 10, one blade of which is shown in Fig. 19) fits into the gap that is apparent in the bottom portion of the sensor assembly shown in the upper right of Fig. 18. The collimator consists of 5 blades of aluminum, each with an array of square slots (Fig. 19). The middle blade holds the collimator foil, which can be seen through the holes in Fig. 19. The sizes of the holes on each blade are graded according to distance of the blade to the center of the symmetry axis of the cylindrical sensor volume (Fig. 10). Further technical specifications are given in the Appendix.
Solid State Detectors (SSD’s)
One of 6 of the SSD holders per EIS instrument is shown in Fig. 20. The side of the holder that is shown holds a single SSD, manufactured by Canbarra, with 4 pixels, 2 electron pixels and 2 ion pixels. Each large pixel is about 0.40 cm2 and each small pixel is about 0.02 cm2, yielding sensitivity ratio of about 20. However, once again, for the primary MMS target regions, the small pixels will likely not be needed or utilized. The electron pixels are covered with an aluminum flashing 2 microns thick. GEANT4 simulations show that, with 20 keV discrimination on the SSD output, electrons with energy starting at about 25 keV and above can be measured, whereas protons with energy of 250 keV and above can to be detected. The solid state detector is 500 microns thick with a dead layer, relevant for the ion side, of about 500 Angstroms. The hanger itself is made of Aluminum and is 0.25 cm thick. It represents one part of the effort to shield the SSD’s from most directions with 0.5 cm Aluminum for background control. On the back side of the hanger is a small board that contains the Energy ASICS described in Sect. 4.3. How the SSD hangers are mounted into the array that is needed for the EIS sensor is shown in Fig. 18 (upper left).
Microchannel Plates (MCP)
The single microchannel plate stack (MCP) within each EIS (Fig. 18, lower left) comprises two 5 cm diameter circular plates mounted, with a small gap between them, in the chevron configuration. The stack is operated with a total potential drop of between 1900 and 2400 volts, and is used with a gain of several ×106. The cloud of electrons coming out of the stack is collected by a segmented anode (Fig. 18 upper left), with 12 segments in the “start” region (connected with time-delay inductors), and 12 segments in the “stop region (similarly connected). The gap between the bottom of the MCP stack and the anode is ∼0.25 cm, and the potential difference that collects the electron cloud is ∼100 volts.
EIS Internal Operations, Operational Modes, and Data Products
Although the EIS instrument can be complicated to run, for MMS it is run in a relatively simple manner, utilizing only a few basic modes of operations. When power is first applied to the EIS instrument, its processor runs software based in a fuse-link PROM resulting in the Boot Mode which establishes basic communication with the spacecraft and runs a small subset of the operating software. Other operational modes are the Cover Test Mode, the Application Mode, Cover Release Mode, HV On Mode, HV Air Safe Mode, Safe Mode, and the Science Modes. The primary EIS science modes are Calibration, Fast Survey (during which both Fast Survey and Burst data is generated), Slow Survey, and Slow Survey Electron Only.
The EIS software divides each spacecraft spin into 32 evenly spaced sectors, which corresponds roughly to 2/3 second, given the ∼20 second spin period of each MMS spacecraft (note that most of the MMS instruments are “time based” whereas both EIS and FEEPS are “spin based”). As the spin rate varies, the duration of a sector varies accordingly. The CIDP sends a sun pulse to EIS every spin. Between sun pulses, the CIDP sends 5759 delta phi timing pulses. The EIS software counts delta phi pulses to space out the sectors. The spin starts, i.e. sector 0 starts, at an offset from the sun pulse. The offset, specified in delta phi units, is an up-loadable parameter. Each sector is further divided into three subsectors. The first subsector is long, 1/2 of a sector. The last two subsectors are short, 1/4 of a sector each.
The sensor hardware can be placed in a different mode during each subsector. There is a fixed dead-time, ∼3.8 ms, for switching between hardware modes and the pattern of modes in each subsector is commandable (the elements are “ion-species”,” ion spectra—used as a diagnostic mode”, and “electron spectra”). Any subsector may collect data in any mode. Each pattern collects different data in different proportions. For example, setting subsectors 1 and 2 to ion species mode and subsectors 3 to electron spectra mode collects ion species 3/4 of the time and electrons the rest of the time; ion energy mode is not collected at all.
During the Fast Survey Mode, Burst data is sampled at the full sectoring cadence of the instrument (32 sectors per spin, ∼2/3 sec accumulation), and Survey Data is averaged to 8 samples per spin, with accumulation periods of order 2.5 sec. Slow Survey products sample in the same way as Fast Survey products, but data is accumulated only once every 10 spins. The various instrument modes, which are invoked via the stored macros, are variations on the main Application Mode; they vary only in which telemetry products are enabled and how often they are downlinked.
The EIS divides the Slow Survey period into two modes; the first, Slow Survey, is basically the same as the Fast Survey, except that we only send down science data from every tenth spin. However, when the spacecraft is inside of a specified Earth radial position (6 to 7 RE) the instrument will start experiencing very high count rates in a region that is outside the mission’s region of interest. The EIS will therefore be operated in the Slow Survey Electron Only mode. In this mode, the high voltages will be stepped down to low levels to conserve MCP lifetime and avoid the very high count rates expected in the inner belts. Only electron data products will be collected during this time.
Onboard Data Products
Table 6 shows the data products that are generated for each of the 3 kinds of sub-sectored data accumulation periods (Electron Energy, Ion Energy, and Ion Species). Most of the data generated by EIS comprises a multiplicity of count accumulation channels. For the electron energy mode and the ion energy mode, the EIS software sorts the SSD energy parameter into a one-dimensional array of numbers that represent the electron or ion energy spectra, with a large number of spectral bins for “high resolution” spectra and a smaller number of spectral bins for “low resolution” spectra. For the ion species mode, one 2-dimensional TOF×E array is used for events that have both a TOF and an energy to sort the events according to mass and energy (which divides the occupied regions of the TOF×Energy space, like that shown in Fig. 13 left, into a large number of discrete boxes to form the different channels; see an example in Mauk et al. 2013, Fig. 25). Another 2-dimensional array, an example of which we show explicitly here in Fig. 21, is used for events that have only TOF information (TOF and Pulse Height) to similarly sort the events according to mass and TOF (or equivalently Energy/nucleon; See Fig. 13, right).
The EIS instrument also generates what are called “Event Words” that comprise Table 4 information (or a subset of Table 4 information) about a small fraction of individual particle events that are detected by the instrument. The raw event data allows us to build (over several hours, given limitations in telemetry) displays on the ground like the two bottom panels of Fig. 13 (EIS examples are shown in Sect. 5—Figs. 23, 25, and 26). In order to diagnose all regions of the TOF×E and TOF×PH arrays, the events that are telemetered to the ground can be selected (by setting a command parameter) according to a rotating priority scheme that cycles through (with highest priority) different broad regions of TOF×E and TOF×PH arrays.
Diagnostic and Test Support
EIS can be commanded into a calibration mode that injects pulses into the preamps of the TOF start, TOF stop, and SSDs to validate the fidelity and stability of the chain of circuits that process the event pulses. Both a start and a stop pulse are generated. The rate and the start-to-stop delay are set by command. The start pulse can be sent to TOF start; the stop pulse can be sent to TOF stop and the SSDs. The TOF start, TOF stop, and SSD pulses can be enabled or disabled individually by command. The EIS hardware can be commanded to measure SSD energy channel or MCP pulse height baseline values instead of doing its normal event processing. The results appear in the event FIFO with the relevant information shown in Table 4.