Download Development of an Ion/Electron Plasma Spectrometer

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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
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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.
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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.
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Figure 1-1 The Astrid-2 Satellite
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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
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Figure 1-2 The Munin Satellite
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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.
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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
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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
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Figure 1-3 Top Hat Analyzer
Figure 1-4 Top Hat Analyzer
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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
2rR
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.
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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
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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.
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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
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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).
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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,
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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.
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Figure 2-2 MEDUSA Diagram
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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
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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
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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
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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
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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
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Figure 2-4 MEDUSA Amplifier Board
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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
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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
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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)
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Figure 2-8 MEDUSA Electron Side Connector Cover and Backing Plate
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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.
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Figure 3-1 MEDUSA Engineering Model
Figure 3-2 MEDUSA Engineering Model
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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.
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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
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Figure 3-3 MEDUSA Prototype Flight Model
Figure 3-4 MEDUSA Prototype Flight Model
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Figure 3-5 MEDUSA Electron Amplifier Boards
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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
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Figure 5-12 Distribution of Successful Input Locations
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Figure 5-15 Dual Mask Model
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Figure 5-16 Distribution of Successful Input Locations
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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
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Figure 5-19 Diagram of Output Mask Model
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Figure 5-22 Flat Collimator Model
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Figure 5-23 Distribution of Successful Input Locations
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Figure 5-26 Flat Collimator Entrance Mask Model
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Figure 5-27 Distribution of Successful Input Locations
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Figure 5-30 Distribution of Successful Input Locations
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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.
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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.
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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.
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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
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Figure 6-1 Data Production Flowchart
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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.
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Figure 6-2 Select Mode Data in IDFS format
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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
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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.
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References
Blomberg, L.G., Marklund, G.T., Lindqvist, P.A., and Bylander, L., "Astrid-2: An
Advanced Auroral Microprobe", December 1997.
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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
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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
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January, 1988.
Sablik, M.J., Scherrer, J.R., Winningham, J.D., Frahm, R.A., Schrader, T., "TFAS (A
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Sanderson, T.R., and Henrion, J., "Measurement of the Geometrical Factor of an
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Theodoridis, G.C., and Paolini, F.R., "The Angular Response of Spherical Plate
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World Wide Web References
Astrid-2 Poster
http://plafyd.plasma.kth.se/alp/space/astrid2/astrid2.html
Munin Satellite Homepage
http://munin.irf.se