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1 1. Introduction The ability to measure the energy distribution of particles near the Earth has been and will continue to be of great importance to experimental space physics. Historically, near-Earth particle distributions have been measured at single locations by individual platforms. This has lead to an understanding of only the local particle distribution function. Global distributions have had to be inferred from statistical averages of these single-point measurements. It is not known, however, if these statistical models are representative of the state at any instant of time. Future measurements need to be made simultaneously from multiple locations in order to experimentally determine the global particle distribution to a much higher precision. This requires a new class of spacecraft instrumentation which can be manufactured at low cost so that it can be carried on numerous small satellites, yet maintain a high quality of scientific data collection. 1.1. Scientific Objectives Of fundamental importance to studies of the aurora is the phase space distribution of the electrons and various ions as they flow along magnetic field lines to the upper atmosphere. By measuring the energy and direction of plasma along a spacecraft's flight path, the velocity distribution along that path can be determined. A single instrument, however, can measure the velocity distribution function only at a single point at each given time, so the complete distribution f (v, r, t) cannot be determined. Because the distribution changes with space and time, it is currently impossible to get a global high 2 time resolution picture of the particle distribution. Current global particle distribution models rely mostly on many successive passes of a single instrument, so that different local times are only sampled as the spacecraft precesses in its orbit. It is unclear if current global models built from repeated observations represent what the actual distribution may look like at any given time. Making many simultaneous observations of the velocity distribution at different locations would help eliminate this ambiguity. As few as six satellites in near polar low Earth orbit could give two-hour local time resolution with each pass, and reduce the global coverage time from a couple of months to a couple of weeks. Also, local structure of the distributions can only be deciphered with a close formation of instruments sampling different areas of a region in three dimensions. Three or four satellites orbiting in formation would be needed for this type of study. The ability to place many detectors in orbit is the first step to being able to make these simultaneous observations. Following the NASA philosophy of "faster, better, cheaper," attention must be paid to the cost, both in dollars and in satellite resources, of any detector system that is proposed. By having a detector that follows this philosophy, it can be included on many of the smaller satellite missions that might otherwise lack the ability to make these measurements. As the population of similarly instrumented satellites in orbit increases, so does the spatial resolution that can be obtained by correlating their data. Thus, a detector is needed which can provide the energy (velocity) resolution required, while being plentiful enough in the sky to provide adequate spatial coverage. 3 1.2. The Astrid-2 Mission The Swedish microsatellite program is well suited for inexpensive multipoint auroral in situ measurements. The second in this series, Astrid-2 (Figure 1-1) is a very small (35kg, less than 0.1 cubic meters in volume) microsatellite which is in orbit (launched 10 Dec, 1998) at an altitude of 1000 km and an inclination from the equator of about 83º (Blomberg et al., 1997). It will cover all local times every 3.5 months. This high latitude orbit will take Astrid-2 over the auroral regions in the north and south, providing an excellent view of the precipitating particles that cause the phenomena. The data collected by the scientific instruments it carries will contribute to our understanding of the space environment around the Earth and the interaction between Earth and its surroundings. The dual objectives of this mission are to study various physical processes associated with aurora, and to demonstrate several new technologies being employed for the first time. Among the scientific topics to be addressed are the electrodynamics of aurora and black aurora, a follow-up on a discovery made by the Swedish Freja spacecraft (Marklund et al., 1994; 1997). Freja found an anti-symmetry between the upward field-aligned currents that accelerate auroral particles and downward currents, leading to upward electron acceleration in the downward current region and strong transverse fields. Many other field-particle phenomena will also be investigated, such as the physics of transverse ion heating, and sources of the cross-polar potential drop. In addition, electric and magnetic field mapping and structure studies will be conducted, along with investigations of field waves and wave-particle interactions. 4 Figure 1-1 The Astrid-2 Satellite 5 The technologies demonstrated will show that a low-budget, small platform satellite system can still carry a comprehensive set of scientific instruments for auroral research. The wire booms for the electric field probes will be deployed using a new system that is highly integrated with the satellite structure. The booms are wound about the spacecraft body and released with a step motor located inside one of the legs separating the satellite platforms. This results in a weight savings of almost half over conventional mechanisms. Spacecraft attitude will be determined by the Advanced Stellar Compass (ASC), a star imager with an accuracy of 1 arc second for the spin axis and 20 arc seconds for the spinning axes. Another new feature of this mission is the high degree of autonomy of the ground stations, which will be able to operate for extended periods without manual intervention. The remaining payload consists of a magnetometer, a Langmuir probe, two spin-scanning UV photometers for Lyman and oxygen emissions and the particle detector that is the subject of this paper. 1.3. The Munin Mission Munin is the model for a new class of "nanosatellites". Designed, managed and built at the Swedish Institute for Space Physics (IRF) in Kiruna, Sweden, it promises to set a new standard for very small, high tech scientific satellites (Figure 1-2). With its incredibly small size (5 kg, 20x20x25 cm) and passive attitude control, this is the ideal platform for multiple observations of the near-Earth particle environment (Norberg, et al., 1998). This largely student-designed spacecraft contains no moving parts, aside from the 6 Figure 1-2 The Munin Satellite 7 simple separation system, and solar panels on all six sides for power. The Attitude Control System (ACS) consists of permanent magnets. Such a simple attitude system is possible because of the small mass and relatively low orbit of Munin. It works by aligning itself with the Earth's magnetic field, which is well known at low altitudes. The satellite makes two revolutions during each approximately polar orbit, "rolling over" as it passes over each pole. Smaller magnets act as oscillation dampers, so that the satellite axis is always aligned with the Earth's magnetic field to within 5º (Munin web site 1997). The required particle detector is oriented such that its field of view contains the local magnetic field and thus makes a complete pitch angle scan. The expected orbit for Munin is near polar, with an apogee of 2000 km, a perigee of 705 km, and an inclination of 95°. With this orbit, which is similar to Astrid-2, but having an abbreviated payload, the Munin data should complement nicely that of its larger cousin. In addition to a particle detector, Munin will also carry two solid-state ion and neutral detectors, which will extend the detection of particles up to energies of 150 KeV. The final part of the scientific payload is a slightly modified off-the-shelf digital CCD camera for auroral imaging. If successful, this satellite will demonstrate that very small, inexpensive satellite systems can be flown without sacrificing the quality of scientific data collected. It is hoped that many such nanosatellites will eventually fill the skies, providing the global instrument coverage needed to fully understand and model the nearEarth particle environment. 8 1.4. Background In answer to this need for a small, relatively inexpensive and easy to build plasma detector, Southwest Research Institute (SwRI) has developed a compact ion/electron energy spectrometer, the Miniaturized Electrostatic DUal-top-hat Spherical Analyzer (MEDUSA). This instrument combines proven detector technologies with the latest miniature off-the-shelf components to create a package which satisfies NASA, DoD, and ESA requirements. The “top hat” type detector was chosen over other particle instrument designs for several reasons. First, the desired energy range for the study of auroral plasmas is most suited to an electrostatic analyzer (EsA) type instrument. In general, these instruments use an electric field to bend a particle trajectory into a desired path. Particles that pass between the field plates are selected and counted. The most common plate geometries are cylindrical, spherical, and toroidal (Young, 1997). A 90º spherical plate detector also has a nice focusing advantage. A plane parallel input beam can cover the entire entrance aperture, but since the particle trajectories follow great circles (a circle along the sphere with the same radius as the sphere), after 90º the particles from the beam converge to a single point (Theodoridis, 1969). A special case of the spherical plate analyzer is the top hat, named for the cap over the entrance aperture (Figure 1-3). The main advantage of this configuration over the other geometries is its 360º field of view within the plane perpendicular to the entrance aperture. An instrument with 22.5º azimuthal resolution 9 can do the work of 16 conventional 90º spherical plate analyzers. Thus, this design is able to return a greater amount of data for its size and mass. 1.5. The Top Hat Analyzer A typical top hat analyzer consists of a collimator, a pair of concentric hemispherical deflection plates, and some form of particle counter (see Figure 1-4). The outer deflection plate has a hole at the top for particles to enter. Above this hole is the top hat plate, often a part of the collimator, that limits the entrance of particles into the deflection plates. The opening angle of the collimator, and its length, help determine the elevation angle field of view of the instrument. Because of the detector's cylindrical symmetry, it can simultaneously accept particles from a full 360º in azimuth within this elevation acceptance band. These analyzers determine particle energy per unit charge by applying a voltage to the inner deflection plate, creating an electric field that attracts the correspondingly charged particles. As a particle enters the deflection region, the attraction bends its trajectory towards the same radius of curvature as the plates. Particles that travel along a great circle without hitting either plate over a narrow energy range are counted. By controlling the inner deflection plate voltage, specific phase space characteristics may be determined. The force balance within the deflection region can be seen using the Lorentz force equation and centrifugal acceleration. A particle between the hemispherical plates with inner plate voltage V0 and a grounded outer plate sees a radial electric field 10 Figure 1-3 Top Hat Analyzer Figure 1-4 Top Hat Analyzer 11 E VO R1 R2 , where R1 and R2 are the inner and outer plate radii, respectively, and R is r 2 R the distance between them. This corresponds to an inward force F qE qVO R1 R2 , r 2 R which tends to draw the particle to the inner sphere (assuming a positive voltage V0 for the electron case and negative V0 for the ions). It also, however, feels a centrifugal force mv 2 of F outward as its path is bent into a circle. If these two opposing forces are r equal, then the particle will pass through the plates. If we set these forces equal to each other and solve for the velocity (or in this case kinetic energy), we have F mv 2 qVO R1 R2 r r 2 R or Energy(eV ) VO R1 R2 KVO 2rR where we have defined the “K factor” to be K R1 R2 for a central path between ( R1 R2 )R the plates. Since the charge of an ion is Ze and not just –e as in the electron case, an ambiguity is introduced for an ion detector since the charge cannot be determined. 1.6. The Particle Sensor Once a particle has passed through the deflection plates, it must be counted by a particle sensor and converted into a digital signal for the instrument electronics. Historically, channel electron multipliers (CEM's) were used to amplify a single particle into enough electrons that their charge could be measured by the instrument electronics. 12 CEM's consist of a reduced glass tube with a flared entrance aperture at one end and a charge collector at the other. The incident particle strikes the inner surface of the tube, and electrons are liberated from the impact. These electrons travel down the tube, accelerated by an electric field, and continue to strike the surface. As each electron liberates more electrons, they build up like an avalanche until they reach the bottom with a measurable total charge. To make sure the electrons strike the tube surface enough to build up a sufficient charge, and to prevent ions from flowing backwards along the tube, it is bent into a spiral. A newer, but similar technology is employed in the micro channel plate (MCP). The microscopic holes in the reduced glass plates act like tiny CEM tubes, causing a cascade of electrons to flow downward to an anode connected to a charge amplifier. Manufacturing difficulties prevent the individual tubes from being curved, however this problem can be overcome with multiple plates. All of the tubes, or pores, in a MCP are slanted relative to the plane of the surface, usually by about 8º. This allows the particle path to form a V or Z shape when two or three of these plates are stacked together. "Chevron" or "Z-stack" MCP stacks simulate the curving nature of the CEM, while packing thousands of channels into a small area. This high density of channels allows for greater resolution and a smaller package than a set of CEM sensors. The surface area of the openings, or "open area", is only about 45% of the total surface, so more than half of the incident particles strike the solid glass surface and do not enter a channel. Of those that do make it into a channel, the MCP still has only about a 60% chance of starting the chain reaction that leads to a countable pulse. These and similar factors must be taken 13 into consideration when computing the total amount of incident particles from the raw count rate. A very new product called a micro sphere plate (MSP) was also considered for use. Its size and function is similar to the MCP's, but instead of channels, microscopic spheres are bonded together in a sponge-like configuration. The electron cascade propagates by bouncing down through the spheres and knocking more electrons from the spheres. Although its geometrical open area is much smaller than a MCP (~30%) the curving nature of the surface elements gives it an effective open area that is similar to a MCP (El-Mul literature, 1996). It seemed attractive for space applications due to its high gain and low noise, however lifetime tests in Sweden showed that its characteristics degraded with continuous use. At this time there are no plans to include MSP's in a particle instrument. 14 2. Instrument Design The Miniaturized Electrostatic DUal-top-hat Spherical Analyzer (MEDUSA) instrument was developed as part of the scientific payload of the Swedish Royal Institute of Technology's Astrid-2 satellite which was launched on a Russian Cosmos rocket on December 10, 1998. This instrument combines two-species observations with state-ofthe-art miniaturization to achieve a low-cost, compact instrument capable of making the measurements that will be required in the future. MEDUSA is a particle detector that can measure the energy/charge of electrons and ions simultaneously with a 360 field of view. The engineering, prototype flight and second flight models were constructed at Southwest Research Institute (SwRI) and the Swedish Space Physics Institute (IRF). The prototype, currently in orbit aboard Astrid-2, is well on its way to proving its value as a scientific instrument, and the second flight model will fly in 1999 on the Swedish Munin satellite. 2.1. Physical and Environmental Constraints One of the first things that must be considered when designing a new particle detector is the actual particle environment that the instrument will encounter when in orbit. MEDUSA is expected to experience an ionized plasma, consisting mostly of electrons and protons, but also oxygen ions, helium ions, and other heavy atomic ions. The energetic electron density at the altitude of Astrid-2's orbit is typically about 10-4 to 15 10 particles per cm3. The ions are in smaller abundances; however, each varies greatly with latitude, local time, season, and solar variations. Unlike many earlier detectors, the determining factor for scaling the deflection plates was in fact not based on the expected environment as much as on available space, mass, and parts. This was done in order to minimize the cost and to fit into an existing mass/volume allotment. The maximum deflection plate size was determined by the size of the standard MCP's to be used. Using a maximum quality area diameter of 33 mm on a 41.5 mm diameter MCP gives an upper bound of the outer plate radius of 16.5 mm. The actual radius chosen was 16 mm, making the inner plate radius 14.8 mm and the top hat radius 17.2 mm (See Appendix B). These dimensions lead to the E/E and geometric factors listed in the following section. There were also size, mass and power constraints which had to be considered in the design. The total weight and power consumption available for the payload on the Astrid-2 satellite, for which the instrument was initially designed, is 9 kg and 16 W, respectively (Marklund, 1997). Being one of five scientific experiments on board, we could not reasonably expect to be allowed more than 20% of the total payload resources. The final calculated values of the MEDUSA and its DPU is 1.68 kg and 3 W, which is 19% of the weight and 19% of the power. The two-platform satellite structure of Astrid-2 has dimensions of 45 x 45 x 30 cm, so we were severely limited in size as well. The sensor head was made to fit through a 11.3 cm diameter circular opening in the top platform, with the small electronics box mounted on the under side (Figure 2-1). 16 Figure 2-1 Astrid-2 Satellite Upper Platform The electric field between the deflection plates and other proximity limits put an upper bound on the maximum potential that can be put on the inner plate, and thus the highest energy particles that would be accepted. In the case of the electrons, the energy range is broken into 31 data steps and one flyback step going from a plate voltage of about 0.16 V, up to a voltage on the inner plate of about 2800 V. Using the plate separation of 1.2 mm, this gives a maximum value of 63 volts/mil across the plates, 17 which is about the largest value recommended by past experience. A similar distance to voltage relation was used to limit the proximity of other items as well, such as the high voltage circuits on the boards, spacing between input anodes and surface traces, and MCP clearances. 2.2. Design Characteristics MEDUSA consists of a pair of hemispherical electrostatic top hat analyzers placed top to top, sharing an entrance aperture and top hat plate (Figure 2-2). The MEDUSA instrument is a modified and scaled down version of the Top-hat For All Species (TFAS) instrument (Sablik et al., 1989). Both instruments are capable of measuring ions and electrons simultaneously with a common aperture, the main difference between them being the larger size, lack of a top hat plate between the sides and flat areas at the tops of the inner plates on TFAS. The common aperture design allows correlative studies of the positive ions and electrons at the same point in time and space. The basic physics and concepts of this instrument are well documented and flight tested (Sablik et al., 1988), so its uniqueness is in its very small size and weight, accomplished by tightly packing the miniature components and in combining ion and electron sensors into a single unit. The characteristics of MEDUSA as originally envisioned are summarized in Table 2.1. 18 Figure 2-2 MEDUSA Diagram 19 Deflection plate radii (mm) Energy resolution (E/E) Geometric factor Energy steps Energy range Sweep time Azimuthal angular resolution Collimator opening angle Collimator length (radius) r1= 14.8, r2= 16.0, r3= 17.2 25% 4x10-4 cm2 sr 32 1 eV - 20 keV 62.5 ms (elec.) 125 ms (ion) 22.5º (16 sectors) 5º 5.7 cm Table 2.1 The collimator for each side is separated by the common top hat for only 29% of the total collimator length. Overall dimensions of the sensor is a cylinder with a diameter of 11.3 cm and a height of 7.45 cm. Energy resolution is a measure of how large a variation of energy may be accepted through the deflection plates at a particular voltage level. It is calculated from a plot of the geometric factor (explained in section 4.2) versus input particle energy, the plate voltage remaining constant. The full width half max (FWHM) of the curve (E) is divided by the energy of the peak value (Eo) and expressed as a percentage. The angular resolution is a measure of how well the direction of the particle in azimuth and elevation can be determined. Azimuthal resolution is determined by the number of sectors that the 360 acceptance plane is broken into (16 in this case). Elevation resolution is limited by the collimator to 10º, but in practice the FWHM of acceptance is significantly smaller (calibration section 4.2). Temporal resolution is determined by the speed of sampling on the instrument. In the case of the electron side, 16 sweeps of 32 samples are made each 20 second. The ion side has a time resolution of half this, making only 8 sweeps per second. This resolution is also related to the spatial resolution by the orbital velocity. Assuming an approximately circular orbit at 1000 km, as with Astrid-2, the orbital speed will be approximately 7.3 km/s. This means that each electron energy sample is taken over 14 meters of the orbit, and a sweep is completed every 459 meters of orbit. The corresponding distances and times for the ion side are all double that of the electrons. Good energy and angle resolutions are necessary for an accurate picture of the instantaneous velocity space distribution of particles. High time (and space) resolution allows a fine-grained analysis of structures within the particle environment. Instantaneous spatial resolution will increase as more and more instruments of this type are in orbit taking measurements. Thus, the cost of each individual instrument becomes an important scientific consideration. 2.3. Design Overview Amy Cotton at SwRI did the initial mechanical design of the MEDUSA instrument under the direction of John Scherrer. Its design was to be simple and practical, using off-the-shelf hardware and technology wherever possible. It also proved very challenging, with a large number of small parts which had to be made to precise tolerances in order to maintain the small size. Following is a description of the mechanical assembly of the sensor as it was originally designed. Each side of the instrument functions independently from the other, except for sharing a common entrance 21 aperture and data processing unit (DPU). This description, except where specified, applies to each side of the instrument. The outer shell, collimator, and outer deflection plate portions of each side were machined as a single piece. The double-sided top hat is attached to the lower (electron) side of the collimator, and the two halves are connected by four hollow standoffs. This forms the basic shell, or body, of the sensor (Figure 2-3). Figure 2-3 MEDUSA Sensor Body 22 The heart of the sensor is the circular amplifier board (Figure 2-4), which not only contains the 16 pre-amps and related circuitry, but also the mechanical assemblies of the MCP and inner deflection plate. The 16 input anodes are printed on the board around the center at a radius that places them between the inner and outer deflection plates. Just outside of this the outer MCP mount, made of Kel-F, is secured with screws from above into nuts pressed into the board called PEM nuts. These screws at the lowest level of the mount also hold beryllium copper "finger" springs which are the outer support for the lower MCP, one of which supplies the lower plate voltage. The orientation, or clocking, of each side is different to allow the high MCP voltage to be closest to the spring for the corresponding (upper or lower) MCP. The inner MCP mount centered on the board is also made of Kel-F and has four "feet" for inner MCP support. The center of the mount which protrudes through the MCP stack to insulate it from the inner deflection plate is "D" shaped, corresponding to the D cutout in the MCP's (Figure 2-5). This is used to align the bias angles of the two chevron-stack MCP's. Upper finger springs on the inner and outer mounts hold the MCP stack in place, while one of the outer springs again supplies the voltage to the upper MCP. Over the MCP's, attached to the highest level of the inner and outer mounts, is the 5 V grid. Its voltage is supplied by one of the outer screws, which go all the way down to the board. In the center of the board, inside the "D" of the inner MCP mount, two screws secure the inner deflection plate to the board (Figure 2-6). These screws are left slightly loose until installation so they can be tightened with the alignment jig in place. The ion board also includes a tiny 37 pin Nanonics connector which mates with one mounted to the ion side instrument body just outside the MCP stack radius. This allows communication from the upper (ion) side to 23 Figure 2-4 MEDUSA Amplifier Board 24 35 Dia. Min. Quality Area 41.25 Dia. 1.0 27 Dia. Max. Quality Area 20.75 Dia. DFi 9.09 Dimensions are in millimeters Direction of Bias Angle Figure 2-5 MCP “D” Cutout Diagram 25 Figure 2-6 MEDUSA Inner Deflection Plate the lower (electron) side of the instrument via the four hollow standoffs between the halves. The high voltage board, made at IRF and shipped to SwRI, was of the same diameter as the amplifier board. It, however, has a keyhole shaped hole in the center so that when placed on top of the first board it clears the MCP stack and the Nanonics connector (Figure 2-4). Eight hollow standoffs separate the two boards at the points where they are screwed to the instrument body. In addition, three threaded standoffs are screwed to each board from the outside to secure them together. Two of these extra standoffs carry the MCP and deflection high voltages to the amp board while the third is 26 used for alignment and grounding. Low voltage and communication is supplied to the high voltage board with a 9-pin surface mount connector which is designed to mate when the boards are connected together. Each board assembly can then be lowered into the corresponding side of the instrument body. An alignment jig is used to make sure the boards are centered and that the inner plate is centered on the board, to insure the 1.2mm plate spacing is precisely maintained (Figure 2-7). On the ion side, the outer four screws that hold in the board assembly and also secure the lid are countersunk. The lid has a smaller opening with its own cover to allow access to the test connector which can be used to input test pulses to the amplifiers. On the electron side, a connector cover backing plate must be installed behind the board before the side connector cover can be installed (Figure 2-8). At this point, the sensor head is complete, with the 37 pin MDM connectors from the amp boards hanging out of the bottom. These connectors are mated to boards in the DPU in Sweden before the sensor head is attached to the DPU body. Figure 2-7 MEDUSA Alignment Jig (Ion Side) 27 Figure 2-8 MEDUSA Electron Side Connector Cover and Backing Plate 28 3. Instrument Development Three MEDUSA instruments have been fabricated over the last two years: an engineering model and two flight models. The second flight model reused many of the engineering model parts. The development of this instrument, which I conducted at SwRI and IRF, includes not only the fabrication and assembly of parts, but also all the testing and modifications that were necessary before calibrations could be made. 3.1. Engineering Model The engineering model (EM) of the MEDUSA instrument was designed to be identical to the flight models with the exception of the amplifier boards. In the place of amplifiers, pulsers were installed to simulate pulses to the Swedish DPU. In the case of the EM, the pulser boards for each side were identical. The voltage was not run to the MCP's or deflection plates, although the high voltage boards were installed and built close to the flight configuration (Figures 3-1 and 3-2). Most of the difficulties faced during the assembly of this and the prototype flight model were due to tight spacing. The extreme miniaturization meant that many of the gaps between parts were below the manufacturing tolerances of the parts, causing the "tight fit" of the parts to often be too tight. 29 Figure 3-1 MEDUSA Engineering Model Figure 3-2 MEDUSA Engineering Model 30 One such occurrence was with the outer MCP mounting ring, the heart of the most complex area of the mechanical design. First, the outer edge of the ring had to fit inside the cut out on the HV board, but since the radii of each were made exactly the same, it would not pass through the hole. The inner radius of the HV board was shaved slightly in the places where it touched the mounting ring in order to obtain a proper fit. The next hurdle came when it was found that there had been no provision made for the heads of the 12 screws that held various MCP stack components to the ring and the ring to the amplifier board. The worst case of this was with the lowest level screws that held the lower finger springs of the MCP's in place. The heads bowed out the outer edge of the ring and also interfered with the MCP's ability to rest on the springs. My first solution was to use Ryton screws which would not short to the upper MCP and whose heads could be cut down to fit, but we needed to carry the MCP bias voltage to the finger springs via one of these screws, so the threads were coated with copper. 3.2. Prototype/Flight Model The prototype, which was also the first flight model of MEDUSA, was started concurrently with the engineering model since the machine shop preferred to make all parts at the same time. However, the main body pieces were not fabricated until after the engineering model was successfully assembled and tested. The only aluminum parts to undergo any changes were the door and backing plate for the electron side test connector. It was decided that more clearance was needed for the connector. 31 After the parts were made they were sent to Los Alamos National Laboratory, NM, for Ebanol-C coating. This is a hot dip process, so the entire main body parts and inner plates were coated. When the parts returned, completely blackened, the exterior was buffed smooth and painted with a special conductive white paint so that only the collimator and outer deflection plate retained the coating. Unlike the EM, the prototype lid was covered with aluminum tape instead of paint, a change that the spacecraft engineers suggested after thermal tests with the EM in Sweden (Figures 3-3 – 3-4). The amplifier boards for each side of the instrument differed in several ways from each other and from the EM boards. Most importantly, each flight board contained the 16 Amptek A111F preamplifiers, which were connected to the anodes and test inputs. The amps stood on edge on the back side of the electron board, but laid down on the top side of the ion board (Figure 3-5). Since the electron side anodes must be biased to high voltage in order for the charge cloud emitted from the MCP to reach them, each amp input on that board also contained a high voltage capacitor to isolate the bias voltage from the amplifiers. The Nanonics connector on the ion board was the same as the EM, however it was moved to the back side of the board on the electron side. The large number of amplifiers and related components made the spacing very critical, but everything on both boards was made to fit properly within the instrument body. The high voltage board, supplied by IRF, consisted of the transformer circuitry that produced the high voltage for the deflection plates and MCP's from the supplied spacecraft power. It also contained all the necessary control circuits for varying and 32 Figure 3-3 MEDUSA Prototype Flight Model Figure 3-4 MEDUSA Prototype Flight Model 33 Figure 3-5 MEDUSA Electron Amplifier Boards 34 monitoring the high voltages. Linear relationships between the high voltages and monitor voltages (Figures 3-6 – 3-9) allowed the MCP and deflection voltages to be monitored remotely. In a classic miscommunication error with our partners in Sweden, the pads for the 9-pin connector between our amplifier/MCP mount board and their high voltage board were laid out differently on each board. To overcome this, we improvised by running jumper wires from one board to the other, eliminating the connector all together. Much care was taken at this point to make sure the boards were connected correctly. Another change from the EM was the way in which power was transferred to the MCP's. After the metal coated Ryton screws failed to live up to expectations, the screws were changed to uncoated Ryton and the holes in the outer mounting ring were enlarged to allow a special copper washer to go between the metal PEM nut carrying the current and the finger spring that contacted the lower plate. This allowed for a more positive connection. The four upper finger springs attached to the inner MCP mount were deleted after it was decided that they were unnecessary. This allowed the inner mount to be attached by those four screws before the MCP was installed. 35 36 37 38 39 3.3. Bench Testing Once assembly was completed it was necessary to devise a method to perform functional tests on the MEDUSA FM to be certain all of the components were operating normally before shipping it to IRF. Before beginning this process, I contacted our partners in Sweden in order to learn what was needed to operate the instrument. According to the IRF engineers, whose DPU contained the spacecraft interface, our sensor received six DC power inputs all using a common ground. These included four power inputs (28 V, 12 V, -12 V for the Swedish high voltage converter board, and 5 V for the HV board and for the amplifier power and grid) and two control voltages. The first control voltage was the MCP bias control voltage which regulated the high voltage to the MCP's and the second was the deflection control voltage which controlled the high voltage charging the inner deflection plate. The outputs from the sensor consisted of an MCP bias monitor voltage, a deflection monitor voltage, and the output from the 16 anodes. Each of the monitor voltages was linearly related to the corresponding high voltage (Figures 3-6 – 3-9). All these connections were repeated for each side since they are wired and are operated completely independently of each other. The 32-pin MDM connector from one sensor side was connected to a breakout box into which was patched the six DC power supplies, two DC volt meters, and an oscilloscope. The oscilloscope could be connected to any of the 16 digital outputs one at a time (Figure 3-10). Using this crude setup (Figure 3-11), I was able to test and troubleshoot the electronics on the bench, while taking precautions not to ramp up any high voltage while at atmospheric pressure. The MCP's were left off for most of the bench testing. 40 Figure 3-10 MEDUSA Bench Setup Diagram 41 Figure 3-11 MEDUSA Bench Testing Setup 42 3.4. Chamber Testing Once I had the instrument working on the bench in a low-voltage environment, I moved the setup to the electron gun vacuum chamber facility. This facility contains remotely controlled motors for movement in the plane orthogonal to the electron beam, as well as for rotations in and about the instrument center (Figures 3-12 – 3-13). A special fixture was fabricated to attach the instrument to the motors. The two sets of wires at the back of the sensor passed through a slot in the circular platform of the fixture where one or the other could be connected to an internal chamber harness. A feedthrough connection through the chamber wall allowed power and signal to pass to our test setup. The ion side was tested using a lithium coated filament connected to a two amp DC power supply. The ground of this supply was floated at 1000 V above the chamber ground to accelerate the boiled-off lithium ions (Figure 3-14). The ions would then feel an acceleration towards ground (-1000 V relative to the filament). The instrument was positioned such that the collimator was the closest ground, which accelerated the particles into the aperture. This method, while not quantitatively accurate, was sufficient to test the basic function of the instrument. 43 Figure 3-12 Chamber Diagram 44 Figure 3-13 Chamber Vacuum Diagram 45 Figure 3-14 Ion Source Diagram 46 The initial tests showed noise on most channels to the extent that the signal from the lithium source could not be seen at all. This prompted a cycle of noise reducing procedures on the bench separated by chamber tests to check the noise levels. One of the main sources of noise on the inputs turned out to be due to the close proximity of the high voltage boards, which were only a few millimeters from the amplifiers. Several insulation methods were explored, but the solution turned out to be a special conductive cloth; other insulating materials proved to be thicker and stiffer than the cloth, causing clearance problems. The conductive cloth was sandwiched between thin pieces of fiberglass insulation and then mounted between the closely spaced power and amplifier boards. The test equipment itself was also a major source of noise; the grounding of the various power supplies was adjusted to minimize noise from that source, and the test leads to the instrument were insulated with aluminum foil. Eventually the noise was brought down to an acceptable level, and I could see counts related to the ion source. Once I started measuring counts, however, I found that the K factor differed substantially from its expected value. As mentioned in section 1.5, the K factor E K is simply the ratio of the energy of a particle (E) relative to the voltage (V) V required to pass the particle through the deflection plates. Investigation into this discrepancy led us to discover that the spacing between the plates was incorrect. The plate spacing was set by the distance of the amp board to the housing, which was in turn set by the standoffs and the HV board thickness. The board thickness was found to be 47 incorrect. The instrument had been designed assuming the board would be of a standard American thickness, but since the board was fabricated in Sweden, the board thickness was a European standard. Thin shims were fabricated to supplement the standoff height to create adjustment for this spacing. Since there was no way to directly measure the separation distance, the thickness of each element in the board stack had to be precisely measured and compared to the design so that the proper height could be added to the standoffs. Vacuum chamber measurements of the K factor were used to fine-tune the shim thicknesses so that the calculations gave the proper K factor. Another source of the plate spacing difficulties was discovered much later. The inner deflection plate, which was originally designed as a two-piece structure, had been changed into a single part. This change, however, introduced an error in the plate curvature that was not found until the second flight model, MEDUSA-2, was being fabricated from the engineering model. The curvature of the inner plates turned out to be almost the same as the outer plates, so that when placed in their mounted position, they were too far away at the top and too close together at the sides. After completing the troubleshooting and testing procedures at SwRI, the FM was carefully packaged in double nitrogen-filled bags inside an aluminum case and shipped to Kiruna, Sweden, for integration with the DPU and calibration. 48 3.5. Troubleshooting at IRF The complete unit was tested after its arrival in Sweden. During integration with the DPU, the team at IRF could not get rid of enough noise to fulfill the science objectives. This was attributed to the highly complex amplifier boards (16 amps on a round 93 cm2 board) in conjunction with the interference generated by the close proximity of the electronics within the DPU. Their solution was to fabricate new amplifier boards that would use the eight-channel Mocad amplifiers, thus greatly simplifying the layout. Each of the Mocad amplifiers could do the job of eight of the Ampteks, and were not very different in size. Other simplifications included the deletion of the test inputs and the 37-pin Nanonics connectors. In place of the Nanonics connector, the leads were soldered directly onto holes in the board. This worked well for the electron side; however, it created complications for the ion side. The wires, which were connected to the board, had to run through the hollow standoffs before terminating at the MDM connector. Once this was done the ion amplifier board could not be removed from the sensor body. Fortunately, there was enough slack in the wires for the board to be oriented so that it could be worked on while still attached. The new boards used smaller metric screws for the MCP mounting hardware, so the head clearance of the metal screws was no longer a problem. Since none of the IRF personnel working on the project had experience with the assembly of the sensor, I went to Kiruna to aid with the board swap. 49 After completing the amplifier board modifications and installing the new amplifier board, the electron side of the instrument functioned very well and had acceptably low levels of noise. The ion side, however, had four channels that were still noisy. Upon closer inspection, I discovered that the four lower finger spring locations corresponded with the four noisy ion channels. After discussing options with the Swedish engineers, I decided to develop a mounting method that did not require these springs (Figure 3-15). In order to get rid of the springs, I first had to devise a new way of getting power to the lower MCP. Since the used area of the MCP's was towards the outside, a spring on the inside area would work well for transferring voltage. Unfortunately, the inner MCP mount used solid insulated feet for lower support, not springs. A spare finger spring was cut in half, to make it thinner, and soldered to the hole in the board for the assembly screw that threads into two of the inner mount feet. Part of the mount foot had to be cut away to allow the spring to attach to the board, but the stability of the MCP on the mount was not adversely affected. The second change I had to make was to come up with an alternate method of supporting the bottom MCP at the outer edge. Without the finger springs, I needed something to back up the MCP's around the outside. What I came up with was a ring made of Delrin that would fit between the outer mounting ring and the anode array with a thickness equal to the feet of the inner mount (Figure 3-15). This new arrangement got rid of the noise from the finger springs. 50 Figure 3-15 Modified Lower Spring and Outer Support Ring 51 3.6. Second Flight Model The second flight MEDUSA, or MEDUSA-2, was constructed for the Swedish Institute of Space Physics' Munin satellite project. The instrument body and various other parts were reused from the original engineering model. Its design was almost identical to the final version of the prototype, with a few exceptions. Most importantly, the inner deflection plate curvature error was caught early on in this project, and new corrected plates were made. Also, the modifications made to the MCP mounting system on the ion side of the prototype were incorporated into an improved set of mounting hardware. Finally, many of the parts whose fit was too tight during the previous assemblies were modified slightly to allow adequate clearance. The components of MEDUSA-2 were hand-carried to IRF in Sweden prior to the second calibration of the original MEDUSA. Following the calibration, work began to assemble and test the second model. The new MCP mounting hardware included a simplified inner mount, since the inner finger springs had been omitted. The feet of the inner mount were replaced with a continuous circular shelf, on which a thin copper washer was placed. The copper washer was connected to a wire though a hole in the inner mount and supplied power and inner support to the lower MCP (figure 3-16). Outer support was again supplied by a Delrin ring located just inside the outer MCP mount. This system proved to be much more reliable for MCP support and power, except that the inner mount was not firmly attached to the board until the 5 volt grid was installed, making assembly more difficult. 52 Figure 3-16 Modified Inner Support Ring 53 4. Calibration Scientific quality is assured through calibration and characterization of the detector. Calibration of the instrument was performed at IRF in their ion/electron calibration facility during November of 1997. These calibrations were repeated in July 1998 when launch delays allowed a more thorough treatment of the amplifier characteristics. 4.1. Facility The calibration of both sides of MEDUSA was carried out in Kiruna, Sweden, at the Swedish Institute for Space Physics (IRF). The calibration facility (see Figure 4-1) consists of a single vacuum tank and pumps that can be rolled along a track between the ion and electron sources. The ion source uses a high voltage element that emits energetic electrons. These electrons pass through Argon gas, which results in Ar+ ions being emitted from the other side. These ions are accelerated to the desired energy, run through a 90º cylindrical deflector to filter out ions not at the energy selected, and then focused through a series of electromagnetic lenses into a homogeneous beam. The beam flux is measured with a Faraday cup, the center of which is open to allow part of the beam to pass through to the experiment. The electron source is a modified electron microscope where the microscope lenses are used to focus the beam, then a Faraday cup measures the 54 Figure 4-1 Calibration Facility Diagram 55 intensity. A phosphorus screen can be rotated into place in order to observe the beam and insure homogeneity. (Figures 4-2 – 4-5). The sensor and DPU were mounted on a platform that could be remotely manipulated to vary the elevation and azimuth of the incident particle beam. The output of the DPU was routed to a computer, which was running a satellite simulator. Thus, the computer could communicate with the DPU just as the satellite would. This software also included housekeeping displays and a graphical display of the energy steps as 32 concentric rings partitioned by 16 radial lines to divide each step into the proper sectors (Figure 4-6). The software also includes an accumulation counter, which recorded the totals for each step and for an entire calibration sweep. 4.2. Procedure and Results Unless otherwise noted, the procedures and results described below are from the second calibration performed in July of 1998. I began the calibration by determining the voltage at which to run the MCP's. The gain of the MCP's increases with increasing voltage; however, there is a point at which the gain levels off and only noise is increased due to a phenomenon called ion feedback. On the electron side, a graph of particle counts per second versus MCP bias voltage (Figure 4-7) did not show a clear partition between the leveling off of gain and the onset of ion feedback. There was, however, an area that appeared to be the transition between the gain leveling off and the steeper slope 56 Figure 4-2 MEDUSA Mounted in Vacuum Tank Figure 4-3 Argon+ Ion Source and Support Equipment 57 Figure 4-4 Electron Source and Pumps Figure 4-5 Electron Source and Support Equipment 58 Figure 4-6 Calibration Software Screen Shot 59 of ion feedback. Therefore, it was decided to use a bias voltage near the bottom of this transition of 1801 V. The ion side showed a clear plateau before the onset of ion feedback, so the bias was set at the upper end of the plateau, i.e. 1776 V (Figure 4-8). These values roughly correspond with the peak found previously by Swedish graduate student Ulrik Eklund on his MCP tests at IRF. Eklund conducted dark count and radioactive source testing of MCP's and MSP's during the spring of 1998. The MCP source tests showed the best peak at a bias voltage of approximately 1700 to 1800 V. The second exercise was to identify some general characteristics of the sensor. Except for the first test, none of the following were repeated in the second calibration. The first graph generated showed the relation between detector counts and beam flux (Figure 4-9). This proved to be an approximately linear relationship, as expected, but fluctuations in beam diameter limited the Faraday cup accuracy at low flux levels. Next, the characteristics of the automated elevation scans had to be characterized. Elevation scanning determines the elevation acceptance under different conditions, and aids in removing error associated with fluctuations in elevation as the azimuth is changed from one sector to another. In order to correlate the scan data with the actual angle in degrees, a manual elevation scan was performed (Figure 4-10) and repeated in the automated mode (Figure 4-11). This allowed the two curves to be correlated so that the actual angles of the auto scans could be constructed from the manual scan. The manual scan also showed that the maximum acceptance angle for the ion side was at 3º above horizontal (horizontal =14.5º in Figure 4-10) and that the full-width half-maximum (FWHM) of the acceptance curve was approximately 2º. This was different from 60 61 62 63 64 65 expected from the models and will be discussed later. An azimuth scan was then performed indicating an overlap of a couple of degrees between sectors, although the FWHM was only 18.5º, compared with the 22.5º physical width of the anodes (Figure 412). I also determined that the support vanes occluded about 4º in between the two sectors on either side of the supports. This was also expected. The energy dependent geometric factor is a measure of the relative sensitivity of a particle instrument as a function of the input particle energy. The development of the equation used with the calibration data is as follows, beginning with the general statement: G (E) = Counting rate/Incident flux The units are (counts/s)/(particles/cm2 s sr) so that the units of geometric factor are cm2 sr (Sanderson 1975). However, this must be written in terms of those quantities that can be easily measured in the calibration setup. First of all, we change flux to intensity (particles/cm2 s), which can be found by multiplying the Faraday cup current by a constant factor. The solid angle of acceptance is approximated by the product of the elevation and azimuth angles of acceptance, which is acceptable for small enough angles. The azimuth angle is determined on a top hat sensor by the width of the sectors, so it is independent. The elevation angle, however, affects our counting rate so each incremental counting rate elevation must be summed. This can be written as 66 67 G( E ) Azimuth elevation counting rate Intensity or G ( E ) A e cps I The sample rate for MEDUSA is 4 samples per second, so the total number of terms in the sum will be 4 sweep time (for an automated sweep through elevation). Since the differential angle e is the same for each term, we can pull it out. This is the angle swept out in one sample time, or 1 second. We know the sweep velocity de/dt in rad/s from 4 timing the elevation sweep, so we multiply it by 0.25 to get e. The summation term is now simply the sum of the counts per second of each sample. Since each sample time is 0.25 seconds long, we multiply each by 4 to get counts per second. After we do the sum, we multiply everything together. Combining these terms, we are left with the formula G( E ) A de total counts I dt sweep Where de/dt is the sweep velocity, which is measured in the beginning, and I and counts are recorded for each sweep. For A, I used the FWHM mentioned above of 18.5º. However, for the purposes of comparison to our simulated data (see raytracing section below), we omit the azimuth, writing the geometric factor in terms of cm2 rad, and adding the efficiency term ef. The MCP efficiency terms are described in the particle sensor section and the grid transparency is the geometrical open area of the 5V grid (a 100x100 lines/inch mesh of 0.001 inch wire): ef = MCP efficiency grid transparency MCP open area = 0.6 0.8 0.45 = 0.216 68 so now, G ( E ) 1 de total counts I ef dt sweep Using the above equation, we can create geometric factor versus energy curves for both the electron and ion sides. On the electron side, the deflection voltage was set to 753 V, corresponding to an energy passband centered at about 4.3 keV. Each elevation scan was from 2º below to 5º above horizontal, while the beam energy for each scan was varied between 3.5 to 5.1 KeV. This resulted in the graph of Figure 4-13. The average peak geometric factor is 5.84x10-4 cm2 rad, while the theoretical value is about 6.6 times this value. On the ion side, an elevation scan of 5º above to 2º below was used over the energy range of 5.5 to 8.1 KeV. The center energy was approximately 6.5 KeV, which corresponds to the deflection voltage of 1648 V. Figure 4-14 shows the experimental and simulated curves for these data. The experimental maximum geometric factor is 1.31x10-3 cm2 rad, 2.8 times smaller than the theoretical curve. Due to the incorrect inner deflection plate radius, and spacing, the instrument was very sensitive to azimuthal variations in the plate spacing. A K factor per sector analysis was performed (Figures 4-15 and 4-16) so that later, this variation could be subtracted from the data. As the graph shows, the variation is basically sinusoidal with a variation of about 15% of the K factor on the ion side and 11% on the electron side. The actual variation of spacing of non-concentric S h cos R2 R12 h(cos 2 1) circles can be shown to be 69 70 71 (see Appendix A) which in this case is almost identical to the cosine wave S ' h cos R2 R1 . This is the function that was fit to the data. The variation in gain per sector, also shown in Figures 4-15 and 4-16, is anticorrelated with the K factor. This illustrates how the plates are "pinching off" part of the throughput when the plates are closer together than nominal. 72 73 74 5. Raytracing Model Calibration data are compared with results from a raytracing model simulation. In order to better understand the characteristics of the instrument, it was modeled in detail with a modified 2.5 dimensional raytracing code. The output of the raytracing model is compared with the calibrations performed at IRF. Additional model runs were undertaken with different collimator geometries. This study was intended to provide greater insight into the impact of such changes on the geometric factor and energy resolution of the instrument. It was found that the greatest decrease in E/E without significant loss of geometric factor is achieved by changing the opening angle of the collimator to 0. These findings are very useful in our understanding of the instrument. 5.1. Model Setup and Parameters The raytracing was performed using code developed by Ronald Holsinger of Los Alamos National Laboratory, Martin Sablik of SwRI, and others. The model grid and equipotential surfaces were calculated using the Los Alamos developed Poisson software. This model was then used as input to the Fortran raytracing code written by Dr. Sablik. It was necessary to make several modifications to the raytracing code in order to accommodate the more complicated geometry required for this instrument. In particular, the grid positions of the aperture edges and length of the aperture could no longer be calculated, and had to be explicitly input due to the unconventional dual aperture. Also, there was no provision for particles passing through above the top-hat plate and possibly 75 exiting out the other side of the instrument. Code was added to filter out these particles without throwing out potentially successful particles. These additions allowed a very realistic model of the instrument to be used in the raytracing of over 13 million particles. The input parameters used for the first run were the same as previous "simple top-hat" model runs and were as follows: Z (height) full width of collimator, 50 steps (azimuthal position) 15 to each side of center line 0.5 separation V (elevation velocity angle) Inner plate voltage 11.5º to each side of horizontal (90), 0.5º separation 200 V Energy range Number of time steps 550 eV to 2500 eV, 10 eV steps 500 Table 5.1 These parameters are designed to correlate with various physically significant parameters on the real instrument. Z is the vertical distance over which the incoming particles can enter. Since in our case the particles are "emitted" from just beyond the edge of the collimator, there is no reason for this distance to be any larger than the collimator opening, which is what was chosen. is the azimuthal spread of particles emitted from the vertical Z line. Since the instrument has cylindrical symmetry, this is the same as setting the width of an uni-directional beam (figure 5-1). V sets the elevation angle at which each particle enters the system. Since the opening angle of the collimator is 5º, the only reason to go beyond that is for particles that start at the very top or bottom and come in towards the center at a steeper angle than the collimator walls. The plate voltage and energy range are self-evident, although until the K factor is established 76 Figure 5-1 Beam Equivalence Diagram 77 the energy range must be fairly large so that enough particles will get through for good statistics so the K factor can be found and the range adjusted. The other input parameters were also maximized for the model to give better resolution for the same number of particles by decreasing range and step distance. The final parameters were: Z (height) (azimuthal position) V (elevation velocity angle) Inner plate voltage Energy range Number of time steps full width of collimator, 100 steps 5º to each side of center line 0.25º separation 7.5º to each side of horizontal (90º), 0.25º separation 200 V 950 eV to 1650 eV, 10 eV steps 500 Table 5.2 5.2. Comparison with Calibration Data The detailed nature of the raytracing model allowed for more accurate results than previous models, which were simple top-hat models scaled to the dimensions of the MEDUSA radii. The main difference is that the dual collimator created an asymmetry for each side of the instrument, allowing particles at steeper downward angles to be accepted. My model, shown in Figure 5-2, contains all of the design elements that made MEDUSA's dual collimator unique. The runs were performed with 200 volts on the electron side inner deflection plate, which was the one being used, and -200 volts on the ion side, to account for any fringing effects the upper sensor might have on the lower. 78 Figure 5-2 Realistic MEDUSA Model 79 The following comparisons are between the original "scaled" model (Figure 5-3), the new realistic model (Figure 5-2), and calibration data taken in Sweden in November 1997 and in August 1998. The notation for each will be SM, RM, NC, and AC respectively. The i and e refers to the ion and electron sides of the instrument which are denoted only for the calibration data. The models were performed with "electrons", although the theoretical data should be the same for ions. The major statistics are summarized in Table 5.3. The most noticeable difference between the SM and the RM is in the geometric factor. The peak fell from a value of 4.31 x10-3 cm2 rad (Figure 5-4) to 3.82 x10-3 cm2 rad (Figure 5-5), a decrease of about 11%. The E/E increased slightly, however, from 21% up to 22.4%. The longer collimator on the RM more than made up for it being wider due to it being for both the ion and electron sides. The single collimator FWHM elevation angle was 8.5º (Figure 5-6), while the full length double collimator had a FWHM of 8.1º (Figure 5-7). Label SM RM NC e NC i AC e AC i G factor (x10-3) 4.31 3.82 0.011 corrected 0.020 corrected 0.58 corrected 1.31 corrected E/E 21 22.4 28.0 25.0 14.9 24.6 K factor 6.10 6.15 4.6 – 5.8 5.8 7.45 – 8.45 4.9 – 5.7 FWHM elev. 8.5 8.1 2 Table 5.3 The poor correlation between the model (RM) and calibration data (AC) can be accounted for by the inner defection plate radius problem. Figures 5-8 and 5-9 illustrate 80 Figure 5-3 Simple MEDUSA Model 81 82 83 84 85 Figure 5-8 Initial Spacing – Incorrect Inner Radius 86 Figure 5-9 Spacing after Addition of Shims 87 what the plate spacing looked like before and after the 0.8 mm shims were added to correct the K factor. As can be seen, the final position leaves the plate spacing too large at the top and too narrow at the bottom. This results in a "squeezing off" effect severely limiting the elevation acceptance of the instrument and correspondingly decreasing the geometric factor. In addition, the electron side circuit boards were not seated the same as in earlier tests (the shims were designed to bring the K factor to back around 6), so that the K factor was much larger than predicted. This leads to an even more pronounced "squeezing off", decreasing the geometric factor and E/E further from the model. This change is not entirely bad, though, since there is expected to be a large enough flux of electrons at most times for good statistics, and a smaller E/E increases the energy resolution. Also, a larger K factor means a higher peak energy can be accepted, although the large variation of K factor in azimuth will complicate data reduction. It is hoped that a future version of the modeling software will be able to handle the geometry of non-concentric plates so that a more complete understanding of the current instrument can be formed. There are, however, sufficient calibration data to properly interpret the data once science operations began in January, 1999. 5.3. Collimator Study Results In addition to the model representing the prototype, alternate models were examined in order to see how various "masks" within the collimator would affect the 88 geometric factor and E/E. It was hoped that the E/E could be lowered from 23% down to around 8% on a modified MEDUSA for application on the European Space Agency's Mars Express mission. Initially, masks were used to block certain portions of the entrance aperture in the hopes of narrowing the energy acceptance and lowering the E/E. Output from the realistic MEDUSA model was reexamined, and a plot of successfully counted particles was plotted by their initial azimuth and elevation values (Figure 5-10). This diagram was used to select the original masks used in the study. The first mask, shown in Figure 5-11, is a 1 mm thick ring located directly under the edge of the top hat, sticking up halfway into the collimator. This geometry reflected the distribution of successful input locations, Figure 5-12, which shows that the peak is just above the halfway point in Z. The results of the test are given in Figures 5-13 and 5-14. The geometric factor was decreased by 26% to 2.82 x10-3 cm2 rad, while the E/E was decreased only 11% to 20.6%. A second mask was then included. This mask was also a ring coming down a quarter of the way into the collimator from the tip of the top hat. It was added to further mask the range of accepted particles (Figure 5-15). This also did not have the desired effect, as can be seen in Figures 5-16 through 5-17. The geometric factor was reduced an additional 42% to 1.64 x10-3 (57% less than the original run) and the E/E only decreased an additional 10% to 18.0% (20% less than run 1). At this time the top-hatmask idea was abandoned because it was realized that input masking can not produce the desired characteristics and the following variations were examined. 89 Figure 5-10 Distribution of Successful Input Locations, Realistic Model 90 Figure 5-11 First Mask Model 91 Figure 5-12 Distribution of Successful Input Locations 92 93 94 Figure 5-15 Dual Mask Model 95 Figure 5-16 Distribution of Successful Input Locations 96 97 98 In the first of the new variations, a virtual exit mask was created using the output from the original unmasked run. Using a simple Fortran program, the ASCII output of the "normal" run was filtered by exit radial position. In the test run, only the middle third of the distance between the plates was open and particles falling within this region were written to an output file of the same format (Figure 5-19). The data were then processed as usual to create Figures 5-20 and 5-21. The geometric factor for this run was reduced to 1.34x10-3 cm2 rad (64% reduction) and the E/E was reduced to 17.1% (down 23%). The second new variation involved simply changing the opening angle of the collimator from 5º to 0º (Figure 5-22). This change produced the graphs 5-23 through 5-25, and the following results: the geometric factor was 2.86x10-3 cm2 rad, a drop of only 25% from the 5º case. The E/E also did well, dropping the most so far to 16.0%, losing 28% off of the "normal" geometry. Still unsatisfied with the E/E, I next tried adding an entrance mask to the 0º opening angle case (Figure 5-26). This geometry produced the lowest E/E of all the runs, 14%, 38.6% below normal, but also the lowest geometric factor, a dismal 0.963x10-3 cm2 rad, a decrease of almost 75%. (Figures 5-27 – 5-29). The last attempt was also a variation on the flat collimator, this time adding a thin wall down the center line to separate the two sides of the instrument. This gave a slightly higher E/E of 13.8% (34.6% reduction) and a much stronger geometric factor of 1.66x10-3 cm2 rad (56.6% below normal). (Figures 5-30 – 5-32). This convinced me that anything I did beyond flattening the collimator would tend to lower the geometric factor faster than the E/E, which was what I was trying to avoid. Therefore, the best solution also happens to be one of the simplest 99 Figure 5-19 Diagram of Output Mask Model 100 101 102 Figure 5-22 Flat Collimator Model 103 Figure 5-23 Distribution of Successful Input Locations 104 105 106 Figure 5-26 Flat Collimator Entrance Mask Model 107 Figure 5-27 Distribution of Successful Input Locations 108 109 110 Figure 5-30 Distribution of Successful Input Locations 111 112 113 mechanically, the flat collimator. It decreases the E/E by a slightly higher percentage than the geometric factor, and is about halfway to our desired 8%. Mechanically, we would add baffles to the collimator to make its geometric opening angle zero degrees. 114 6. Software Development Flight and analysis software development is the final piece of the puzzle needed to carry out MEDUSA's mission. Although the flight software for the instrument Data Processing Unit (DPU) and the DPU itself were developed at IRF, the software for processing the raw telemetry into archivable data files and viewing those data in various scientific units was developed at SwRI. The processing software, to be run on a Sun Ultra at IRF, is designed to archive the data into the standard format of the Southwest Data Display and Analysis System (SDDAS), the Instrument Data File Set (IDFS). This system can display the data with units defined by the various description files, which define the MEDUSA instrument to the software and drive the analysis software for MEDUSA data. 6.1. SDDAS Overview The Southwest Data Display and Analysis System (SDDAS) is a generalized data analysis system that allows new missions and data to be "plugged in" to existing analysis programs. This is accomplished by defining a virtual instrument with all of the necessary tables and definitions for converting the telemetry into a raw data file that can then be displayed in various science units. This standardized approach allows all spacecraft data to be handled in the same way. 115 The Instrument Data File Set (IDFS) files contain the data, timing, and meta-data for each instrument on the missions supported. The files are time indexed and consist of header files which contain slowly varying or constant parameters, data files containing the telemetry, and Virtual Instrument Description Files (VIDF's). VIDF’s contain the information needed to convert the telemetry values to the various science units including whatever decompression and calibration tables may be needed (See Appendix D). A virtual instrument need not be a one to one correlation with a single spacecraft instrument, but can be defined as any measurement or small group of measurements for which there are defined header and data files. The Plot Information and Description File (PIDF) contains limit values and labels for the available units, as well as default values (See Appendix E). These files, when generated for any spacecraft data set, allow the data to be handled and viewed with any of the graphical user interface (GUI) software available without writing custom software for each data set. This is beneficial since it greatly simplifies and speeds up the job of going from raw data to meaningful plots. Because the SDDAS handles diverse data in the same way using the same tools, multiple data sources can be viewed together under the same software. This allows data taken at the same time from any group of instruments or platforms to be displayed together, or even combined in statistical analyses. This degree of integration of many different data sets makes the analysis of data from a constellation of MEDUSA carrying satellites a straightforward procedure. 116 A web version of SDDAS is also under development, and in the future a real-time capability may be added for the MEDUSA data. This would allow anyone with an Internet browser to watch the particle spectrogram build up in near real-time as the data is relayed from the Swedish ground station and processed. This exciting new possibility will likely soon be the standard for satellite data, bringing the turnaround time from data collection to scientific study down to a mater of minutes. 6.2. Production and Viewing Software It was also my responsibility on the MEDUSA instrument to write the processing code that would convert the raw telemetry received into header and data files. In addition, I was responsible for populating the VIDF and PIDF files so that the data could be displayed accurately (Figure 6-1). The data stream I had to work with contained not only the ion and electron sides of MEDUSA, but also the Photometers for Imaging the Aurora (PIA) instruments along with the timing and magnetometer data. Orbit/Attitude data had to be processed and stored for the satellite as well (See Appendix C). Six virtual instruments were defined for MEDUSA and PIA with an additional one for the OA. The production code takes the cleaned up telemetry output from the Swedish decompression software and breaks it up into its constituent pieces. MEDUSA 117 Figure 6-1 Data Production Flowchart 118 was separated into MDSE, containing the 31 electron data steps and MDSI, which contained the ion data. The first step, designated the "flyback" step for each side, is used to ramp the voltage up and is not kept in the data file. In the selected mode, only three of the sixteen sectors from each side are sent down. The three sectors are selected in real time by the magnetometer, and correspond to the data who’s velocity vectors are closest to parallel, antiparallel and perpendicular to the local magnetic field. Two separate virtual instruments, SEL_E and SEL_I, are defined for this mode. An example of select mode data is shown in Figure 6-2. PIA consists of three photometers. PIA's 1 and 2 are identical, and point 180 º apart looking out along the spin plane of the satellite. These are stored together in PIA1_2, and can be combined to form an image as they sweep over the surface of the Earth. PIA 3 always looks at the sun, since Astrid-2 is a sun-pointing satellite, and is stored in the linear data file PIA3. Along with each data structure, a Satellite Time Word or STW is included. This is converted to Universal Time (UT) and sent to each of the above files along with the appropriate data. Each UT is also sent to a subroutine that looks up the appropriate OA for that time. These data files are automatically placed in their proper locations and entered into the database for easy retrieval. 119 Figure 6-2 Select Mode Data in IDFS format 120 7. Conclusions This thesis presents the design, construction, calibration, and software development of the MEDUSA electrostatic analyzer. An engineering model and two flight models have been successfully built. The first is operating well in orbit and the second awaits launch in late 1999 aboard a second Swedish spacecraft. A next generation version has also been chosen for flight on the ESA Mars Express mission. Calibration of the prototype has also been carried out, and shows adequate agreement to simulations, taking into account a curvature error on the inner deflection plates. Data processing software has been written that allows plots to be made as soon as the data is received from the ground station via the Internet. 7.1. Scientific Value MEDUSA has so far proven itself to be a reliable, low-cost instrument capable of making the scientific measurements required for a better understanding of the behavior of the near-Earth particle environment, especially those which precipitate into the atmosphere and cause the aurora. This basic MEDUSA design, coupled with the ability to fine-tune characteristics for specific applications with the 2.5D raytracing model, should make this instrument an attractive candidate for many future satellite missions such as the ESA Mars Express, which has a very tight mass budget. The original MEDUSA instrument has been successfully built, tested, and calibrated, and launched aboard Astrid-2, and the second awaits launch aboard Munin in 1999. I am confident it 121 will prove itself as a reliable data source and an attractive alternative to the traditionally larger and more expensive energy spectrometers. The success of Astrid-2 and Munin will be a good start towards populating the near Earth environment with these dual particle detectors which will achieve the goal of the instantaneous global particle distribution needed to significantly advance near-Earth science. The international support from Sweden on this project has been instrumental in the development of MEDUSA, and we expect this partnership to continue with future missions. 7.2. Educational Value During the course of this project I have had the opportunity to work with and learn from many highly skilled people of various expertise. From this I have gained valuable insight into the process of taking an idea from drawing board to science data. I am fortunate to have had the opportunity to work hands-on on this instrument and be involved with so many aspects of its development, assembly, and testing. In addition, the theoretical insight of computer modeling and software development have prepared me to properly interpret and analyze the data that is being sent down from the first instrument. 122 References Blomberg, L.G., Marklund, G.T., Lindqvist, P.A., and Bylander, L., "Astrid-2: An Advanced Auroral Microprobe", December 1997. El-Mul Technologies Ltd., Soreq, PO Box 571, Yavne 81104, Israel, Sales Literature, 1996. Marklund, G.T., Blomberg, L.G., and Lindqvist, P.A., "Astrid-2: A Low-Budget Microsatellite Mission for Auroral Research", Poster presented at ESA/PAC symposium, May 26-29, 1997. See also WWW reference below. Marklund, G.T., Blomberg, L.G., Falthammar, C.G., and Lindqvist, P.A., "On Intense Diverging Electric Fields Associated with Black Aurora", Geophys. Res. Lett., 21, 1895 (1994) Marklund, G.T., Karlsson, T., and Clemmons, J., "On Low-Altitude Particle Acceleration and Intense Electric Fields and their Relationship to Black Aurora", J. Geophys. Res., 102, 17509 (1997) Norberg, O., Puccio, W., Olsen, J., Barbash, S., Andersson, L., Winningham, J.D., and Jonsson, U., "Munin: A Student Nanosatellite for Space Weather Information", 1998 Sablik, M.J., Golimowski, D., Sharber, J.R., Winningham, J.D., “Computer simulation of a 360° field-of-view ‘top-hat’ electrostatic analyzer”, Rev. of Sci. Inst. V59, Nu1, p146, January, 1988. Sablik, M.J., Scherrer, J.R., Winningham, J.D., Frahm, R.A., Schrader, T., "TFAS (A Tophat For All Species): Design and Computer Optimization of a New Electrostatic Analyzer", October, 1989. Sanderson, T.R., and Henrion, J., "Measurement of the Geometrical Factor of an Electrostatic Analyzer-channeltron Detector", Space Science Instrumentation 1 (1975) 351-361. Theodoridis, G.C., and Paolini, F.R., "The Angular Response of Spherical Plate Electrostatic Analyzers", Rev. of Sci. Inst. V40, Nu5, p621, May, 1969. Young, David, Experimental Space Physics class notes, 1997. 123 World Wide Web References Astrid-2 Poster http://plafyd.plasma.kth.se/alp/space/astrid2/astrid2.html Munin Satellite Homepage http://munin.irf.se