Instrument Information
IDENTIFIER urn:nasa:pds:context:instrument:caps.co::1.1
NAME CASSINI PLASMA SPECTROMETER
TYPE SPECTROMETER
DESCRIPTION The content from this description has been adapted from the CAPS instrument description paper [YOUNGETAL2004]. Instrument Overview =================== The CAPS instrument was comprised of three sensors: the Electron Spectrometer (ELS), the Ion Beam Spectrometer (IBS), and the Ion Mass Spectrometer (IMS). The ELS sensor measured the velocity distribution of electrons from 0.7 eV to 30 keV, a range that permited coverage of thermal electrons found at Titan and near the ring plane as well as more energetic trapped electrons and auroral particles. The IBS sensor measured ion velocity distributions with very high angular and energy resolution from 1 eV to 50 keV. It was specially designed to measure sharply defined ion beams expected in the solar wind at 9.5 AU, highly directional rammed ion fluxes encountered in Titan's ionosphere, and anticipated field-aligned auroral fluxes. The IMS sensor was designed to measure the composition of hot, diffuse magnetospheric plasmas and low-concentration ion species with an atomic resolution M/delta M ~70 and, for certain molecules, (such as N2+ and CO+), effective resolution as high as ~2500. The three sensors were mounted on a motor-driven actuator that rotates the entire instrument over approximately one-half of the sky every 3 minutes. Scientific Objectives ===================== The Cassini Plasma Spectrometer (CAPS) had many complimentary science objectives during the Cassini mission. The CAPS objectives were as follows: - Perform in-situ studies of dust-plasma interactions in Saturn's magnetosphere - Study Saturnian auroral phenomena - Observe Saturn Kilometric radiation (SKR) generation - Study magnetospheric external and internal boundaries, structures and plasma domains (including flux tubes) - Study microphysics of the bow shock, magnetosheath, magnetopause and magnetotail - Study magnetospheric dynamics (driven by solar wind, internal rotation, substorms, radial transport, and mass loading) - Study the composition of the ionosphere, exposphere and atmosphere at Titan - Study the interaction of Titan's upper atmosphere and ionosphere with Saturn's magnetosphere (precipitation, ion pickup) - Study the structure of upstream, bow shock, wake, and flux tube regions around Titan - Determine composition of ionized molecules originating from the ionosphere and the icy satellite's expospheres - Study the composition of the exposphere and surfaces around icy satellite - Look at the interaction of the magnetosphere (precipitation, ion pickup) with icy satellite exosphere - Study the structure of upstream, bow shock, wake, and flux tube regions around the icy satellites - Study the composition of ring exposphere and ring particle surfaces, characterize ring/magnetoshere interactions, dust-plasma interactions, ring particle dynamics and erosion, and study the interactions of ring plasma with Saturn's ionosphere Operational Considerations ========================== The ELS measured differential electron velocity distributions at densities as low as 10**(-3) cm**(-3). The measurement range extended from 1 eV to 28 keV. The lower end of this range, allowed detailed studies of secondary electron fluxes that contribute to ionization and chemical processes taking place at Titan and elsewhere. At tens of keV ELS contributed to studies of trapped electrons and those associated with saturnian aurora. Throughout its energy range ELS provided a global survey of plasma density, temperature and electron pitch angle distributions that were needed to derive a comprehensive view of plasma dynamics within the magnetosphere and, for roughly 50% of the mission, in the solar wind and magnetosheath. The large amount of time that Cassini spent in the solar wind during the Cassini tour presented the opportunity to study both the solar wind's intrinsic characteristics and its interactions with the magnetosphere of Saturn, and the comet-like magnetosphere of Titan. The IBS energy and angular resolution (few percent and ~2 degrees respectively) provided the capability of making solar wind measurements, and for observing ion ram fluxes at Titan and auroral ion beams. The IBS energy range extends from 1 eV up to 50 keV. The IMS was designed to provide comprehensive measurements in all regions of the magnetosphere. IMS relies on time-focused optics combined with carbon foil technology. It was designed to separate atomic species with high resolution, and to identify isobaric molecular species such as CH4+, NH2+, and O+ (all with M/Q = 16, where M/Q is the mass/charge ratio) or N2+ and CO+ (M/Q = 28) that would otherwise have required a very large conventional instrument to achieve. Because of its 1 eV to 50 keV energy range IMS was also used to study the composition of Titan's ionosphere at a few eV, complementing INMS [WAITEETAL2004], and to study energetic trapped ions, complementing the MIMI/CHEMS investigation [KRIMIGISETAL2004]. Detectors ========= ELS --- The ELS sensor was a hemispherical top-hat electrostatic analyzer (ESA) similar to that described by [CARLSONETAL1983]. The detector consisted of a chevron microchannel plate (MCP) pair with a gold-coated copper spacer 66 microns thick positioned between the two plates. At operating voltage, the measured full width half max (FWHM) pulse height distribution was 130%. The resistivity of the glass in the MCP was low enough to allow the plate to respond to count-rates up to 1 x 104 mm**(-2) s**(-1) or approximately 106 electrons per anode per second, without saturation causing significant gain degradation. The MCP high voltage could be varied from 0 to 3.5 kV in steps of approximately 60V. This allowed the MCP bias to be increased throughout the mission to recover possible gain loss. The bias voltage at the input to the MCP was maintained at +150 V to ensure all electrons have sufficient energy to be detected. During calibration, the operational voltage on the MCP was approximately +2.4 kV. Electrons leaving the rear of the MCP traversed a gap of 500 microns before striking the anode. A voltage of +82 V applied between the anode surface and the back surface of the MCP optimized spreading of the charge cloud leaving the MCP. The anode had eight discrete 20 degree-wide electrodes separated by 150 microns. The active anode area was formed by 10 micron thick gold on a Deranox 975 Alumina substrate. The area of the separator contacting the MCP was coated with 10 microns of gold. A signal ground plane incorporated into the bottom layer of the multilayer ceramic provided electromagnetic screening of the anode from the analyzer structure. IBS --- The IBS sensor was similar to ELS, as it was also based on the principles of a curved-electrode electrostatic analyzer. The primary difference, aside from its larger radius, was that the spherical IBS electrodes extend 178 degrees from the entrance aperture to channel-electron multiplier detectors located at the exit. A unique aspect of IBS was the method used to obtain high angular resolution 3-D velocity space measurements. Based on the crossed-fan field-of-view concept employed in an earlier solar wind ion instrument [BAMEETAL1978], it was possible to obtain the required angular resolution by tilting the acceptance fans of each aperture 30 degrees relative to the others. With that in mind, there were three curved 2.5 x 15 mm apertures in the IBS faceplate, each with a nominal acceptance fan of +/- 1.5 degrees FWFM in azimuth (set by the ESA characteristics) and +/- 75 degrees FWFM in elevation angle (set by the apertures) from the normal to the plane of the aperture. If we define the middle aperture as being along the 0 degree radius from the center of the instrument faceplate, the other two apertures were located at +/- 30 degrees relative to it. There were three CEM detectors located 180 degrees around the faceplate from each of the apertures, i.e. in the position where ions entering the apertures from any transmitted direction come to a focus. The FOV of the middle aperture was oriented such that its long (polar) dimension was parallel to the azimuthal (Z) axis of the CAPS actuator. The FOV of the other two apertures were therefore 'crossed' with inclinations of +/- 30 degrees with respect to that of the middle aperture. The three matched detectors used in IBS were custom-built CEMs chosen for their rugged construction out of solid ceramic, their high gain characteristics (typical plateau ~10^8), and their availability in nearly any desired form factor (the manufacturer was Dr. Sjuts Optotechnik GmBH of Germany). The input funnel interior dimensions are 5 x 20 mm, which was more than adequate to intercept all of the ions converging at each of the sensor's three focal points. Ninety percent transmission grids were stood off from the front of the CEM funnels and biased negatively with respect to the funnels in order to minimize secondary electron loss. The input assembly (grid and funnel) was biased at negative high voltage so that all transmitted ions received post-acceleration equivalent to the CEM bias. A CEM responded to an incoming ion by emitting an electron charge pulse at its exit, which was biased at about -100 V. IMS --- The IMS sensor approach was to choose a novel technique based on isochronous (time-focusing) mass spectrometry based on carbon foils. In order to achieve IMS measurement goals the properties of a toroidal ESA were matched to those of the TOF analyzer. The combination of an ESA with carbon-foil based TOF measurements had been previously developed and flown in space by Gloeckler and colleagues [GLOECKLER&HSIEH1976, GLOECKLERETAL1995, GLOECKLERETAL1998, HAMILTONETAL1990] and Young [YOUNGETAL1992, YOUNGETAL1998B]. Early development of the LEF concept [GLOECKLERETAL1995], resulted in a high-resolution 1-D (in energy) isochronous instrument used to measure the collimated flow of the solar wind. However, [MCCOMASETAL1990] and [MCCOMAS&NORDHOLT1990] were the first to describe the principles and operation of a cylindrically symmetric LEF analyzer capable of making 2-D energy-angle measurements. [MCCOMASETAL1998] and [NORDHOLTETAL1998] further describe the LEF application to IMS. Ions initially enter the top-hat portion of the ESA through a grounded collimator (the top-hat was similar to the geometry used in the ELS). A flat circular plate truncates the inner ESA toroid at the point where it joins the top-hat section. There are two advantages to toroidal geometry: (1) Toroids have two radii of curvature that independently control ion focusing within the TOF section (in spheres the two radii are degenerate and equal, in cylinders one radius is infinite). (2) The top-hat entrance aperture of a toroid like the one used in IMS was larger per unit of ESA plate surface area than for a comparably sized spherical top-hat, giving higher sensitivity per unit of sensor weight [WOLLNIK1971, YOUNGETAL1988]. In the elevation plane, ions are formed into a beam by a combination of vanes in the entrance collimator and field-correcting slits (one per 20 degree pixel) at the exit of the ESA. A voltage applied to the inner ESA electrode created an electric field in the top-hat that deflects ions into the toroid. As with ELS and IBS, only particles within a particular range of E/Q and direction of arrival were transmitted through the ESA to the TOF analyzer. Ions that successfully exited the ESA were accelerated by +/- 14.56 kV into one of eight ultra-thin (~100 angstrom) carbon foils distributed around the entrance to the TOF analyzer (one foil per angular 'pixel'). Ions falling through the accelerating field, gained sufficient velocity perpendicular to the foil so that they would penetrate the foils even at the lowest energies (~ 1eV). Higher post-acceleration voltages would naturally have been desirable, but would have driven up instrument size, mass, and power in addition to risk. Electronics =========== ELS sensor electronics ---------------------- The electronics were accommodated on four circuit boards integrated to a single motherboard consisting of flexible and rigid sections. This design eliminated the need for an internal cable harness, and at the same time coupled ELS to the CAPS/DPU interface connector. MCP pulses collected on eight anodes were passed to an equal number of Amptek(R) A111F charge amplifier/discriminators that converted raw signals above a predetermined threshold into 5 V, 300 ns logic pulses. Thresholds were set in hardware to 3.4 x 105 electrons, which yielded an equivalent level of 25 mV (into 2.3 pF), giving good rejection of electronic noise. A decrease in the threshold level by 2.5 mV increased spurious electronic noise counts by a factor of ten. (This relationship held over a wide range of thresholds. MCP dark counts and penetrating radiation were the main remaining contributors to background.). Front and rear MCP bias voltages were provided by Zener diodes, which required filtering at these low currents (around 10 microAmperes). The MCP anodes were biased at high voltage so signal pulses had to be decoupled by high voltage capacitors before the signal goes to the amplifiers which shared the same circuit board with the HV bias/anode coupling circuitry. The HV section was carefully designed and laid out to support a maximum field of 800 V/mm. The SMU (sensor management unit) received and interpreted sensor commands sent by the CAPS DPU and accumulated and transmited ELS data back to the DPU. It stored the sequence of high voltage steps to be applied to the analyzer, the grid voltage setting, and the MCP voltage table. SMU circuitry supplied stimulation test pulses of variable amplitude and frequency to the amplifier/discriminator channels. Under control of the CAPS DPU, the SMU clock speed could be successively halved to lengthen the data acquisition period from 31.25 to 1000 ms/step, creating progressively longer energy sweeps. Furthermore, the sample deadtime could be varied between 25% and 12.5% of the sample period to increase counting rate capability at high rates. IBS sensor electronics ---------------------- For IBS, all of the electronics were mounted on two circular circuit boards housed in the ESA hemispherical cavity. The upper board contained both the ESA and the CEM high voltage power supplies, while the low-voltage amplifier/discriminators, test pulser, and digital-to-analog converters (DACs) were contained on the lower board. The electrical interface from the IBS to the CAPS DPU was a single 62-pin connector. The CEM HV supply, which provided a negative bias voltage to the three IBS detectors in parallel, was commanded by an 8-bit word over a range of 0 to -4.0 kV. The ESA supplied biases to the inner ESA hemisphere with a stepped negative potential that determined the instantaneous energy of transmitted ions. For IBS to be able to function as desired, the ESA HVPS had to cover the voltage range -0.05 to -2600 V in 1.284% increments with a time between steps of 7.8125 ms. In order to have sufficient resolution over the entire voltage range, the ESA supply was designed with three ranges (-0.05 to -1.85 V, -1.85 to -69.6 V, and -69.6 to -2600 V) each of which was 12-bit controllable. This results in a total of 12,288 possible voltage steps although only 852 logarithmically spaced steps were required for full coverage of the voltage range with 1.284% spacing. During operation a 14-bit data word controlling the ESA stepper supply was written to a latch in the IBS electronics every 7.8125 ms (one-eighth of the IMS stepping interval of 62.5 ms). The basic operational mode consisted of a 255-step voltage scan (corresponding to an energy scan) lasting 2 s. IMS sensor electronics ---------------------- This section describes the IMS electronics subsystems by tracing the order in which signals are acquired and processed, namely FEE (front-end-electronics) to TDC (time-to-digital converter) to SAM (spectrum analyzer module), and then to CPU2. Front-End Electronics ---------------------- Once charge from an MCP detector was collected on an anode, it enters a series of amplifiers, discriminators, and logic that identifies the event and begins processing. In this discussion the signals are identified as 'START' and 'STOP'. Both event types are handled in essentially identical ways. When a START event is detected the signal is split into two parts. One is used to identify the location of the event in one of eight elevation angles or one of the two stop channels (LEF or ST). This part of the signal (termed the identification or ID signal) is processed through fast (0.2 microsecond deadtime) preamp and level-discriminators and can be used to correct for deadtime in the slower (2.2 microsecond) circuits. The other 'half' of the START signal is sent to the TDC where it triggers the timing circuitry. Similar processes apply to the STOP signals. Time-To-Digital Converter ---------------------- Once a START pulse triggers the TDC, the state of the ID discriminators is captured giving the identity of the event origin. Subsequent START events are ignored for timing purposes but are counted as ID events. Once initiated, the TDC converts time directly into a digital word using a clock and vernier technique in the following manner. A valid START enables a gated 80 MHz clock that increments a counter. A subsequent STOP that occurs later than 40 ns (the pulse pair resolution of the TDC) after the START event, inhibits that counter, thus providing a coarse time measurement with a resolution of 12.5 ns. If no STOP event was recorded the counter times out at 1600 ns, corresponding to the longest time required for the heaviest, lowest energy ions to cross the TOF optics as discussed above. To obtain finer time resolution, the phase of the clock relative to the clock edge is measured at the instant the STOP was received by means of the delay line vernier implemented using a 12.5 ns delay line with 16 taps. As the STOP pulse propagates down the delay line, a changing pattern of 1's and 0's appears on the delay line taps. This pattern is latched at the end of every clock period. When a STOP occurs the resulting digital pattern represents the time of the STOP signal relative to the clock edge. The FEE logic then encodes it into a binary word that becomes the fine time measurement. Thus the TDC resolution is effectively 12.5 ns/16 = 0.781 ns, corresponding to 1600 ns/0.781 ns = 2048 channels. If the TDC is instructed to identify molecular events then the gated clock is fed to two independent counters. The first STOP causes one counter to be inhibited and the vernier pattern to be read out and stored. The second STOP inhibits the second counter and causes a second vernier pattern to be read out. There are several possible outcomes of events detected by the TDC logic in addition to the ideal outcome (false events). The rate of accidental coincidence events is dependent in a complicated non-linear way on the total event rates. False events and background must be removed from IMS data before they are usable. After a TOF measurement is completed, the TDC encodes an 11-bit TOF value, a 3-bit elevation sector value, a resolution bit signaling ST or LEF data, and a 'continuation' bit indicating a molecular event. Each 16-bit TOF word is then sent to a first-in-first-out (FIFO) buffer read out by the spectrum analyzer module (SAM). The TDC is then reset in preparation for the next measurement. During a single IMS sampling period (62.5 ms) the TDC also accumulates the number of ID events corresponding to the eight start anodes, two stop anodes, two timing discriminator channels, number of times when no coincident stop TOF event was detected, total number of TOF measurements completed, and the number of events recorded in two other configurable channels. These configurable channels can be set to be read out into the data stream. To achieve the best possible performance at high- event rates, the TDC operates as a non-paralyzeable counter with a fixed deadtime of 2.187 microseconds for TOF measurements. ID measurements (collected in 'SINGLES' counters), on the other hand, have a 0.2 microsecond deadtime making them suitable for correcting the slower timing measurement rates. Finally, the FEE/TDC system has built-in test pulsers that can stimulate any START or STOP pair with one of 24 selectable time delays at periodic event rates up to 1 MHz. Spectrum Analyzer Module ---------------------- The SAM processing method allows a high level of TOF data compression (~1000:1) to be achieved with little sacrifice in accuracy. The key to carrying out this process at high-event rates and within the short IMS sampling interval (62.5ms) is a high-speed, deterministic deconvolution technique that we will refer to as the SAM algorithm. The hardware that supports this algorithm has a distributed, pipelined architecture based on four loosely coupled, reconfigurable modules. Once SAM processes a single data sample (i.e., data gathered in 62.5 ms) they are transferred to the CPU2 processor and the cycle repeated. Although data transfers from FEE to TDC and TDC to SAM occur at random rates, control of all other processing, data transfers and command and control functions are synchronized, taking place during the final 12.5% (7.81 ms) of the sampling interval. Dual memory banks allow raw data acquisition to proceed while CPU2 and SAM processors both access data from the previous 4.0s acquisition interval. At the end of each interval, spectra that were processed during the previous interval are read from SAM by CPU2. In addition, new, selectively binned TOF spectra, are read into the SAM processor. There, during the next measurement interval, the SAM algorithm deconvolves TOF spectra, extracting selected ion M/Q values. Once analysis is complete, SAM interrupts CPU2, which then reads the M/Q-sorted data. SAM is then ready for the next acquisition and processing cycle. SAM operates as a CPU2 slave. The analysis algorithm is coded in the Ada language and executed in the 1750A microprocessor module. Its address space is accessible by CPU2, and program and local data are loaded into the module by CPU2. However, SAM itself cannot read or write to CPU2 memory. The three remaining modules (TOF, TOFACC, and EVENT) are each autonomously controlled by separate gate arrays. In order to speed up TOF data processing, dedicated paths support concurrent data flow from the input FIFO to the appropriate processing modules. In addition, each module manages its own local processing and external access to data. SAM acquires data from the TDC via a FIFO buffer at a maximum rate of 5 x 105 periodic events/s. The FIFO can be configured by command to enable dual-stop operation (for molecules) and data acquisition from selected elevation ID channels. Spectrum Analyzer Module ---------------------- The IMS high voltage (HV) system was made up of five supplies controlled independently by CPU1. The supplies are contained in two separate units: 1. nominally +/- 15 kV in HVU1 2. the two MCP supplies and ESA stepping supply in HVU2 All supplies feature current-limiting and breakdown protection, as well as analog monitoring (digitized in the data stream). HVU1 is powered by filtered +30 V from spacecraft primary power whereas the HVU2 supplies use +15 V generated by CAPS. High-voltage cables rated at >= 2.5 times the respective supply voltages connect HVU1 and HVU2 to IMS sensor electrodes (40 kV cables are used for HVU1 connections). All HV connectors were custom designed and fabricated from low-void ceramic to prevent high-electric field concentrations that might lead to HV breakdown over the course of the mission. Metal covers placed over the HV terminals reduce electric stresses and provide mechanical as well as contamination protection. The HVU1 supply is composed of independent positive and negative voltage converters whose output is maintained at +/- 1% over all line and temperature variations with very low ripple. The primary purpose of HVU1 is to provide the negative and positive high voltages that together create the LEF by correctly biasing a string of 30-high ohmic resistors running the length of the TOF ring stack. HVU1 also generates regulated 1200 V referenced to -15 kV that is used to bias the first plate in the LEF MCP stack. Because power for the LEF MCP regulator is derived from the -15 kV supply the latter must reach a minimum of approx. -12 kV in order for the LEF MCP to reach ~950 V and operate correctly. A voltage drop of -100 V is placed on a grid in front of the MCP to return secondary electrons to the MCP surface, thus maintaining high efficiency. To make a compact but reliable unit, electric fields in the HVU1 supplies were kept below 1 kV/mm relative to the grounded housing. Transformer and high voltage assemblies were not encapsulated in order to avoid multi-material composite structures that could introduce mechanical stresses that might in turn lead to dielectric faults and eventual breakdown. Instead, after fabrication and cleaning, the interior surfaces and components were coated with Parylene-C, a tough, low-outgasing polymer with high-dielectric strength. Both of the MCP supplies are located in the HVU2 as is the ESA supply. The MCP supplies are virtually identical in construction and operation, relying on a pulse-width modulator control circuit and a resonant converter operating at a switching frequency of 100 kHz. The capacitor networks provide rf filtering. A resistor divider biases the field-correcting aperture located at the ESA exit to approximately one half the ESA plate potential. Another function of the MCP supplies, in addition to powering the two detectors, is supplying a suppression bias voltage to high transmission grids to return secondary electrons to the MCP (the same function provided by HVU1 for the LEF MCP). Similar potentials at the back of the third stages of both detectors accelerate electron charge clouds toward their respective anodes. Calibration =========== ELS calibration --------------- ELS was calibrated in the Mullard Space Science Laboratory (MSSL) electron calibration facility developed for Cluster [JOHNSTONEETAL1997]. A mercury lamp generated UV that struck a gold layer deposited on a quartz disk. From this, photoelectrons were extracted by applying a bias potential to the gold surface, creating an electron beam 15 cm in diameter with divergence less than 1 degree (at 1 keV) and good uniformity over the ELS aperture. During calibration ELS was mounted on a two-axis rotary table and turned to allow electrons from defined directions to enter (additional information regarding the calibration theory can be found in [YOUNGETAL2004]). A micron-metal shield inside the vacuum chamber shielded the calibration volume by reducing the residual magnetic field to less than 10% that of the Earth. Electron beams with energies above ~30 eV showed minimal directional deviation. Beam current measurements that provide absolute calibration were made with a faraday cup and picoammeter. During calibration sequences beam stability was monitored with a CEM. A tritium source provided a cross check after each sensor re-configuration to maintain consistency during calibration. Calibration of the flight model was made at ten electron energies between 2.3 and 16260 eV (see [YOUNGETAL2004] for details). Two basic types of data were taken: First a finely stepped elevation angular scan was made at constant energy and beam azimuth angle. Second, a full three-dimensional calibration (energy, elevation, azimuth) was obtained at defined resolutions in the three dimensions. The most detailed calibrations were made at 125 and 960 eV. In each case some 150,000 data points, corrected for dead time and beam monitor readings, were summed to produce a plot. The 8 anodes showed a nearly uniform response with some loss of transmission at the two end anodes. This was to be expected because the grid holder cuts off incident trajectories at +/- 80 degree elevation. A summary of 125 and 960 eV calibration data can be found in [YOUNGETAL2004]. Energy-angle scans with a 125eV electron beam were made at the azimuthal center of each of the eight anodes. The plots generated showed the analyzer peformance in 3-dimensions was consistent from one anode to the next and deviated little from instrument simulations. IBS calibration ---------- Initial modeling of the IBS was chiefly concerned with determining manufacturing tolerances required to achieve the desired energy-angle resolution and transmission efficiency of the ESA [VILPPOLAETAL1993]. Analysis of simulation results showed that it was necessary to align the two ESA hemispheres relative to one another with an accuracy of better than ~25 microns in order to obtain the desired energy resolution of delta E/E = 0.015. Furthermore, deviations of the ESA plate surfaces could not depart from perfect sphericity by more than 300 microns (0.3% of plate radius) if >90% transmission efficiency was to be maintained. Initial calibration results showed that IBS responded as expected except for the presence of a few small but unusual features. Of these, a double-bend in the energy-polar angle response was the most notable. To address these findings, a more accurate simulation of the sensor was developed and used to investigate IBS response in more detail [VILPPOLAETAL1996]. Improvements made to the model included addition of curved apertures at the correct standoff distance from the ESA plates, introduction of fringing fields, and a realistic description of the ion beam that matched that used in calibration. These upgrades to the model did not, however, account for the bend feature seen. A further refinement of the model allowed the introduction of slight asymmetries in ESA plate geometry [VILPPOLAETAL2001]. The resulting simulations with asymmetric hemispheres and a slight (few tens of microns) misalignment of the two hemispheres produced good agreement with laboratory results. While the flight sensor's ESA plates may indeed have been slightly misaligned as suggested by simulations, the response of the engineering model IBS was almost identical to the flight model, which would be surprising if a random misalignment occurred. Moreover, the responses of the three individual fan apertures in both IBS models were also very similar. This suggests that the unexpected calibration response was due to a small systematic error in alignment or was inherent in the overall electro-optic design and not a function of alignment accuracy. As of 2004, the latest simulations suggested that a small systematic manufacturing fault might be to blame. Calibration of IBS took place in ion beam facilities at Los Alamos and SwRI (the same facilities that were used to calibrate IMS). The work at Los Alamos concentrated on angle-angle and angle- energy responses while absolute energy and sensitivity calibrations took place at SwRI. At Los Alamos ions were produced in a radio-frequency discharge ion source and then accelerated down a 3 m flight tube into the calibration chamber where IBS was located. Both external supplies and the IBS internal power supplies were used in calibration. A nitrogen beam was typically used which was accelerated to between 0.3 keV and 60 keV. Beam location and uniformity were measured but not absolute ion current. Typical operating pressures were in the low 10-8 Torr range. The ion current extracted from the Los Alamos source was quite stable but not easily varied over a wide dynamic range. Therefore a series of slits were employed to adjust the current delivered to the target chamber. IBS was mounted on a stand whose orientation could be adjusted in one translational axis (across the beam) and two rotational axes. Thus the incident ion beam could be made to impinge on the IBS aperture at any desired combination of elevation and azimuthal angles. All of the diagnostic, motion control and data acquisition systems were computer-controlled. During a typical calibration run, three separate 1-D scans in energy, elevation and azimuthal angle would be taken across the center of the response function. Three central 2-D cuts through the response function would then be taken, followed by a series of energy-azimuth cuts along the elevation axis at 10 degree intervals. Finally, data were corrected for deadtime losses and variation in the beam current during the calibration run. Angular data were transformed from laboratory to spacecraft coordinates. After calibration at Los Alamos, the IBS was integrated with the CAPS flight instrument and underwent checks of calibration in the ion beam at SwRI. The latter was similar to that Los Alamos used for IMS as described below. Typical beam spread at SwRI had delta theta was approx. 0.2 degrees and delta E/E was approx. 0.005. One difference in the two calibration systems was important: the integrated CAPS unit was positioned to calibrate mainly IMS. Thus the rotation axes of the calibration goniometer were centered on the IMS FOV. Since the plane of the IBS apertures was offset 32.0 cm from the central axis of the IMS and ELS sensors, the IBS aperture plane was neither coplanar nor co-aligned with the other two sensors: Any rotation of CAPS tended to move the IBS FOV out of the ion beam. The displacement of the aperture through the small IBS maximum angular acceptance of approximately +/- 2 degrees was less than 1 cm, well within the diameter of the calibration beam as seen from IBS. Thus only calibrations of the energy response at azimuth = elevation = 0 degrees and of absolute sensitivity were possible. IMS calibration ---------- Calibration was performed with a Von Ardenne plasma discharge source at SwRI and a radio frequency source at LANL [ALTON1993]. Beam intensity was fairly stable over short time periods (~1 hour) but could be controlled only by pressure feedback in the source itself. The beam used for calibration at SwRI was monochromatic and plane parallel to a high degree. It was verified to have an angular spread of 0.2 degrees, about 2.5% of the width of IMS acceptance in angle. Beam width in energy was (delta E)/E = 0.005, about 3% of the IMS passband. Only the mass resolution of the beam separator was below expectations (M/(delta M))beam approximately equal to 40. During calibration a known ion species was selected, the energy of the beam set, and J0 measured. Because it was far simpler than varying the energy of the ion beam, the voltage on the ESA was micro-stepped at increments of ~0.3% of the passband. Angular measurements were made by placing the IMS on a three-axis goniometer (two angles plus translation) and rotating it about the center of the IMS FOV. Because of the considerable time required to obtain the full set of data needed to calibrate the geometric factor (G) for one energy and mass, only a few selected points were chosen for full calibration of G. On the other hand, calibration of the IMS response to mass, energy, and J0 could be achieved relatively quickly. A wide range of energies was calibrated using N+ as a standard. Similarly, a wide range of ion species was calibrated using a few specific ESA energies as standards. At every opportunity the value of J0 was measured to quantify absolute calibration. Correctly associating the center of TOF peaks with corresponding M/Q values for known gasses established TOF calibration, the correction factor sigma(E, M) for energy losses in the foil, and the 'effective spring constant'. The mass-analyzed ion beam at LANL was used initially to test IMS energy-angle response and TOF characteristics. From the latter, initial coefficients for the Spectrum Analyzer Module (SAM) algorithm could be calculated for the purpose of simulations. During these tests cross-talk between elevation-angle channels caused by internal particle scattering was discovered. Coating the inner plate of the ESA with copper sulfide black, an anti-scattering agent, eliminated most corss-talk, but some remained. Final calibration of the IMS in flight configurations was carried out at SwRI. Only about 30% of planned activities were completed. Fortunately that work covered nearly all aspects of IMS performance, although not to the desired depth, particularly in the area of energy-angular models of IMS (both refurbished to be as similar as possible to the flight model). Mounting ======== One particular concern regarding location was obtaining good separation from the main Cassini engines and thrusters (potential sources of chemical contamination), separation from the radioisotope thermoelectricgenerators (potential source of penetrating background radiation), and separation from any sources of electrostatic charging. With all these considerations in mind, the best location for CAPS turned out to be on the underside of the fields-and-particles pallet adjacent to the MIMI/CHEMS instrument [KRIMIGISETAL2004] and just below the INMS [WAITEETAL2004]. Although meeting all of the above criteria for location, CAPS still did not have an acceptable field-of view because it was fixed to the spacecraft body and thus could only view in directions constrained by spacecraft orientation. In order to counteract this limitation, the CAPS sensors were mounted on a rotating platform driven by a motor actuator capable of sweeping the CAPS instrument by ~180 degrees around an axis parallel to the spacecraft Z-axis. In this way nearly 2pi steradians of sky could be swept approximately every 3 minutes regardless of spacecraft motion or lack thereof. While not ideal for plasma measurements under all circumstances (e.g. when the spacecraft body blocked the direction looking into a plasma flow), careful design of observing periods permited effective performance under most conditions. The ELS and IMS axis of symmetry and the IBS field of view were all perpendicular to the sense of rotation for the CAPS instrument, which was about an axis parallel to the Z-axis. When the actuator was at position 0, the field of view (FOV) was boresighted with the imaging subsystems out the -Y axis. Further, azimuths were defined in the spacecraft X-Y plane and elevation was defined as being parallel to the spacecraft Z-axis. Even though adding a rotational platform provided a means to turn the instrument, the spacecraft still occluded parts of the CAPS field of view. At ~ +80 degrees azimuth, parts of the fields and particles pallet (FPP), the neighboring MIMI/LEMMS instrument, and the RTG shielding obscured the CAPS field of view. A visual description of this discussion can be found in the CAPS instrument paper [YOUNGETAL2004]. Operational Modes ================= Depending on mission phase and spacecraft power resources, CAPS transitioned to a lower-power state. This sleep state was preferred to being powered-off, as it did not require power-cycling the high voltages and the time to recover to full science functionality was shorter. In sleep mode, the ESA voltages were set to zero and the IBS, ELS, and IMS voltages were stepped down to safe levels. In addition, the +/- 14.56 kV voltages were lowered to a safe value. The IBS and ELS sensors could be brought out of this state in ~15 minutes. The IMS sensor required ~2 hours. While in sleep the instrument did not produce any science packets, though it did continue to produce houskeeping data. All spacecraft operating modes (OpModes) planned for tour allowed CAPS to remain on and operating. Use of sleep mode was expected to be limited to activities associated with Saturn orbit insertion and the Huygens probe mission. Within the normal science state CAPS was capable of performing a wide range of table-driven programs devoted to science data gathering. As part of the science planning process, observational modes were specified that controlled the range of velocity space covered by the sensors, the rate at which velocity space was scanned, the conditions under which IMS data was taken, and the data products returned from the three sensors. Voltage tables that controlled each sensor's ESA, as well as the range of actuator angles scanned, could be independently programmed thus controlling the ranges of velocity space covered by CAPS. The range of actuator motion was a compromise between time resolution and angular coverage. Sweeping through 180 degrees (close to the maximum range) required approximately 200 seconds, while sweeping through a 24 degree range took only 48 seconds. The actuator could also be held at a constant position, producing a two-dimensional cut of velocity space at the 2 or 4 second energy sweep period of the sensors. This was the planned mode of operation during icy satellite encounters, which were very rapid (e.g. during Enceladus encounters, the spacecraft crossed the satellite's geometric wake in under 50 seconds). Solid-state recorder (SSR) memory on the spacecraft was allocated on a downlink pass or obervation period basis based on the detailed Cassini science plan. Depending on the CAPS allocation and particular measurement objectives during the day, one or more data rates and their duration were chosen to produce the required volume. In addition to the normal science mode, CAPS had an engineering mode in which multiplier gains were checked, pulser stimulation signals were used to test and calibrate detector signal chains, and detector background was measured for an extended period. Before reaching Saturn the engineering mode was operated as frequently as cruise data volume and pointing restrictions allowed. After reaching Saturn this mode was part of a periodic instrument calibration occurring approximately once every 50 days. Measured Parameters =================== ------------------------------------------------------------------------ Table I. CAPS sensor performance summary ------------------------------------------------------------------------ Parameter IMS ELS IBS ------------------------------------------------------------------------ Med. Res. High Res. --------------------- Energy/charge response Range(eV/e) 1-50,280 0.6-28,750 1-49,800 Resolution(delta E/E) 0.17 0.17 0.014 Angular response Elevation sectors (number) 8 8 3 Instantaneous FOV 8.3x160deg 5.2x160deg 1.4x150 deg Angular resolution 8.3x20deg 5.2x20deg 1.4x1.5 deg Mass/charge response Range (amu/e) 1 ~ 400 1 ~ 100 --- --- Resolution(M/delta M) 8 60 --- --- Energy-geometric factor* (cm^2 sr eV/eV) 5x10^-3 5x10^-4 1.4x10^-2 4.7x10^-5 Temporal response Per sample (s) 6.25x10^-2 3.125x10^-2 7.813x10^-3 Energy-elevation (s) 4.0 2.0 2.0 Energy-elevation-azimuth (s) 180 ------------------------------------------------------------------------ *Applies to total field-of-view and includes efficiency factors References: =========== [YOUNGETAL2004] D.T. Young, J.J. Berthelier, M. Blanc, J.L. Burch, A.J. Coates, R. Goldstein, M. Grande, T.W. Hill, R.E. Johnson, V. Kelha, D.J. McComas, E.C. Sittler, K.R. Svenes, K. Szego, P. Tanskanen, K. Ahola, D. Anderson, S. Bakshi, R.A. Baragiola, B.L. Barraclough, R.K. Black, S. Bolton, T. Booker, R. Bowman, P. Casey, F.J. Crary, D. Delapp , G. Dirks, N. Eaker, H. Funsten, J.D. Furman, J.T. Gosling, H. Hannu la, C. Holmlund, H. Huomo, J.M. Illiano, P. Jensen, M.A. Johnson, D.R. Linder, T. Luntama, S. Maurice, K. P. McCabe, K. Mursula, B.T. Narheim, J.E. Nordholt, A. Preece, J. Rudzki, A. Ruitberg, K. Smith, S. Szalai, M.F. Thomsen, K. Viherkanto, J. Vilppola, T. V ollmer, T.E. Wahl, M. Wuest, T. Ylikorpi, C. Zinsmeyer, Cassini Plasma Spectrometer Investigation, Space Sci. Rev. 114, 1-112, 2004. [KRIMIGISETAL2004] Krimigis, S.M., D.G. Mitchell, D.C. Hamilton, S. Livi, J. Dandouras, S. Jaskulek, T.P. Armstrong, J.D. Boldt, A.F. Cheng, G. Gloeckler, J.R. Hayes, K.C. Hsieh, W.-H. Ip, E.P. Keath, E. Kirsch, N. Krupp, L.J. Lanzerotti, R. Lundgren, B.H. Mauk, R.W. McEntire, E.C. Roelof, C.E. Schlemm, B.E. Tossman, B. Wilken, and D.J. Williams, Magnetosphere Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan, Space Sci Rev. 114, 233-329, 2004. [WAITEETAL2004] Waite, J.H., W.S. Lewis, W.T. Kasprzak, V.G. Anicich, B.P. Block, T.E. Cravens, G.G. Fletcher, W.-H. Ip, J.G. Luhmann, R.L. McNutt, H.B. Niemann, J.K. Parejko, J.E. Richards, R.L. Thorpe, E.M. Walter, and R.V. Yelle, The Cassini Ion and Neutral Mass Spectrometer (INMS) Investigation, Space Sci. Rev. 114, 113-231, 2004.
MODEL IDENTIFIER
NAIF INSTRUMENT IDENTIFIER
SERIAL NUMBER not applicable
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