(CAPS Links)
The ion mass spectrometer (IMS) provides
species-resolved measurements of the flux of positively charged
atomic and molecular ions as a function of energy/charge vs aperture
entry direction. The IMS uses a toroidal top-hat electrostatic
analyzer (for energy/charge data and to create a narrow field
of view) combined with a linear electric field time-of-flight
mass spectrometer (for mass/charge and other species-resolving
data). The IMS consists of an aperture cover/actuator,
a toroidal analyzer, carbon
foils, a time-of-flight spectrometer,
microchannel plates, amplifier/discriminators,
a time-to-digital converter, a spectrum
analyzer module, and high-voltage power
converters contained in high-voltage units 1 and 2. For information
on these components, click on their names.
(IMS Links)
The IMS aperture cover protects
the IMS during ground handling and during launch and early flight.
The cover is a strip of flexible material that fits over the
IMS annular aperture without creating a hermetic seal. To open
the cover the strip is reeled into a container by a spring after
the strip is released by a wax thermal actuator (WTA).
Closing the cover requires ground handling to unreel the cover
and re-attach its end to the cover-release mechanism. Thus, in
flight, the aperture cover is a one-time use device.
The toroidal analyzer (toroidal
refers to the configuration of the target reflector) consists
of a baffled collimator (a device used to create a parallel beam
of particles) mounted on a toroidal "top hat" electrostatic
analyzer. The geometry of the collimator determines the narrow,
annular (i.e., ring-shaped) IMS field of view of 12 degrees by
160 degrees, which is divided into eight angular "pixels"
of 12 degrees by 20 degrees each. An electric potential between
the inner and outer conductors of the electrostatic analyzer
allows through this device only ions having energies within a
range selected by the analyzer potential (i.e., only ions with
certain energies will have trajectories, at a given analyzer
potential, that navigate through the analyzer without being stopped
by hitting a wall). Energy spectra are taken by stepping the
analyzer potential through a set of 64 spaced values.
Eight thin carbon foils are arranged
in an arc along the exit of the toroidal analyzer. The foils
are the entrance to the time-of-flight spectrometer chamber formed
by the linear electric field (LEF) rings. A -15 kV potential
accelerates the positive ions exiting the toroidal analyzer into
the foils, and this permits ions entering the IMS with as little
as 1 eV of energy to pass through the foils. Molecular ions traveling
through the foils are usually broken into their constituent atoms
and/or molecular fragments. Molecular and atomic ions and molecular
fragments exit the foils as neutrals or ions, and upon exiting
the neutrals and ions eject secondary electrons into the time-of-flight
chamber.
The time-of-flight spectrometer
is a cylindrical chamber in the IMS, bounded by linear electric
field (LEF) rings, where particles exiting the carbon foils encounter
an electric field with a strength that increases linearly with
distance parallel to the LEF axis of symmetry. The linear electric
field is generated by a stack of thirty equally spaced aluminum
rings along which a network of gigaohmn resistors establishes
a quadratic electric potential with a total potential drop across
the stack of rings of 30 kV.
The LEF focuses positively charged particles
with energies up to approximately 15 KeV, independently of the
energy and angle with which they exit the carbon foils. Microchannel
plate (MCP) multipliers are located at the ends of the stack
of LEF rings. Detection by the "start" (ST) MCP of
rapidly travelling secondary electrons from the carbon foils
is used as a time-of-flight "start" signal and for
determining an ion's elevation angle with respect to the IMS
aperture. Simple harmonic motion of ions in the LEF, together
with knowledge of their energy gained from the toroidal analyzer
setting, relates their time-of-flight, from carbon foils to the
LEF MCP (at the other end of the chamber from the ST MCP), to
mass per charge.
The two microchannel plates (MCPs)
each consist of three circular plates of lead oxide glass with
a multitude of microscopic channels running at an incline through
the thickness of each plate. Electrons, ions, and neutrals striking
the outer plate cause secondary electrons to cascade down the
semiconductive walls of the channels. With a potential drop of
about 1 kV across the thickness of a plate, there is a yield
of about 300 electrons per incident particle, or a total gain
of 106 electrons across the three-plate stack.
Nine anodes under the ST MCP and one anode
under the LEF MCP collect the electrons emitted from the MCPs
and pass the signals to two high-speed current amplifiers
(one "start" amplifier for the eight 20-degree-wide
anodes, and one "stop" amplifier for the LEF anode
and the center ST anode). Two constant fraction discriminators
accept the amplifiers' signals and send digital timing pulses
(independent of the amplifiers' signal amplitude) to the time-to-digital
converter (TDC). Also sent to the TDC is the identification of
which of the eight annular anodes was the source of the "start"
pulse and whether a "stop" pulse came from the LEF
anode or the center ST anode.
The time-to-digital converter (TDC)
measures the time interval between the start and stop pulses
from the amplifier/discriminators. The TDC outputs an interval
length to the spectrum analyzer module, along with information
identifying the "start" anode and the "stop"
anode. The "start" anode identity determines which
of the eight 20-degree-wide IMS elevation resolution elements
an ion entered through. A device in the TDC (a pulser) can inject
simulated MCP signals just downstream of the MCP anodes, via
high-voltage isolating capacitors, in order to test the signal
handling functions of the IMS and related hardware and software
in the CAPS data processing unit.
The spectrum analyzer module (SAM)
provides the data gathering, sorting, and transfer functions
between the TDC and the data processing unit (DPU). Time interval
data from the TDC are "binned" or grouped into a pre-selected
(by the DPU) set of time channels associated with certain selected
ion species, whether atomic or molecular. The SAM processor performs
a deconvolution of the time-of-flight spectra to obtain ion identifications,
which it passes on to the DPU.
The IMS has four programmable pulse width-modulated
high-voltage power converters . These converters supply
high voltage to the toroidal analyzer, the LEF rings, and the
LEF MCP.
The second major CAPS subassembly is the
ion beam spectrometer (IBS). The IBS measures the flux
of positively charged ions of all species as a function of energy/charge
and aperture entry direction. The IBS consists of a hemispherical
electrostatic analyzer, channel electron
multipliers, amplifiers/discriminators,
and high-voltage power converters. For
information on these components, click on their names.
(IBS Links)
The hemispherical electrostatic
analyzer consists of two conductive hemispheres of slightly
different radius, mounted concentrically one within the other
in such a way that there is a small gap between the two conductors.
The electric field in the gap selects the range of energy per
charge and angular direction that ions are allowed to have in
order to pass through the analyzer. Energy spectra are taken
by stepping the analyzer potential through a set of spaced values.
Three apertures, each defining a field of view of 1.5 degrees
by 150 degrees, spaced 30 degrees apart, allow particles to enter
the analyzer. Particles are "focused" into three channel
electron multipliers 180 degrees from each of the entrance apertures.
Ions entering the channel electron multipliers
(CEMs) strike the inner semiconducting surfaces of the devices
and spawn secondary electrons, which bounce down the curved channel
in the CEMs, spawning more electrons with each bounce. In this
way, the entry of an ion into a CEM results in a pulse of approximately
108 electrons being collected by an anode at the exit
of the CEM.
The amplifiers/discriminators amplify
the electron pulses collected by the CEM anodes and send the
signals to the DPU's IBS interface. A device in the IBS (a pulser)
can inject simulated CEM signals just downstream of the CEM anodes
in order to test the signal handling functions of the IBS and
related hardware and software in the DPU.
The IBS has two programmable pulse width-modulated
high-voltage power converters, which supply high voltage
to the hemispherical analyzer and the CEMs.
The third major CAPS subassembly is the
electron spectrometer (ELS), which measures the flux of
electrons as a function of energy/charge and aperture entry direction.
The ELS consists of a spherical analyzer,
microchannel plates, amplifiers/discriminators,
a sensor management unit, and high-voltage
power converters. For information on these components, click
on their names.
(ELS Links)
The spherical analyzer consists
of a baffled collimator mounted on a spherical "top hat"
electrostatic analyzer. Collimator geometry determines the narrow,
annular ELS field of view of 5 degrees by 160 degrees, which
is divided into eight 20-degree "pixels." An electric
potential between the inner and outer conductors of the electrostatic
analyzer allows through this device only electrons having energies
and angles within a range selected by the analyzer potential
and top-hat collimation. Energy spectra are taken by stepping
the analyzer potential through a set of 96 spaced values.
Two 90-degree "long" annular
(i.e., curved) microchannel plates (MCPs) are arranged
in a 180-degree arc along the exit of the spherical analyzer.
electrons striking the surface of the MCPs are multiplied into
signals collected by an arc of eight 20-degree anodes.
The anode signals are passed into
eight amplifiers/discriminators via high-voltage isolating
capacitors and are then accumulated by the sensor management
unit. The anode identity determines which of the eight 20-degree-wide
ELS elevation resolution elements an electron entered through.
The sensor management unit (SMU)
controls the voltage level of the ELS high-voltage power converters
in accordance with requests from the CAPS data processing unit
(DPU) and passes counts of electrons (from each of the eight
angular resolution elements) detected by the ELS to the DPU.
A device in the SMU (a pulser) can inject simulated MCP signals
just downstream of the isolating capacitors in order to test
the signal handling functions of the ELS and related hardware
and software in the DPU.
The ELS has two programmable pulse
width-modulated high-voltage power converters that supply
high voltage to the spherical analyzer and the MCPs.
The fourth major CAPS subassembly is the
data processing unit (DPU). The DPU manages the acquisition
and onboard data processing of all CAPS data and controls sensor
and actuator motor functions. The DPU is designed to use two
CPUs, in addition to the processor in the spectrum analyzer module.
The first CPU accumulates IMS time-of-flight spectra and compresses
all IMS data. The other CPU controls the IMS, ELS, IBS, and the
actuator and performs onboard data analysis to determine what
measurements will be taken and what data will be placed into
housekeeping and science packets.
The DPU consists of the sensor
and actuator data control interfaces, a housekeeping
analog-to-digital converter, a safe/arm control,
a wax thermal actuator (WTA) driver, CPUs
with memory, a bus interface unit, low-voltage power converters, supplemental
and replacement heaters, a radiator,
and the principal structure of CAPS. For
information on these components, click on their names.
(DPU Links)
Through its sensor and actuator
data control interfaces, the DPU performs several functions.
It accumulates IBS ion flux counts; it collects ELS and IMS data
products from the SMU, the TDC, and the SAM; and it directly
controls the IMS and IBS pulsers and high-voltage converters.
The DPU feeds ion energy/charge data to, and controls, the SAM.
In addition, the DPU controls the CAPS actuator (ACT) motor stepping
and accepts position and status data from the ACT.
The output voltage of all nine
CAPS high-voltage converters, CAPS low voltages, the actuator
position encoder, and the temperature at six different locations
in CAPS are monitored by a housekeeping analog-to-digital
converter. In addition, two temperature sensors (one in the
DPU, the other in the IMS cover release mechanism) are monitored
directly by the spacecraft. These two sensors do not require
that CAPS be powered.
When not enabled by the DPU (via CAPS software),
all high-voltage power converters have zero potential. When high-voltage
converters are enabled, a safe/arm connector on the DPU
can be used during ground handling to limit high voltages to
approximately 3 percent of what the DPU has commanded them to
be.
The wax thermal actuators (WTAs) in the
IMS cover release mechanism and the scan motor launch latch are
driven and switched by a WTA driver in the DPU.
The DPU uses two nearly identical CPU
boards, each containing its own RAM, ROM, and PACE 1750a
microprocessor.
CAPS communicates with the Command and Data
Subsystem (CDS) via a bus interface unit (BIU) that is
electrically, mechanically, and thermally accommodated within
the DPU.
All CAPS low-voltage power converters
are housed in the DPU. Power at various voltages is supplied
for use by the electronics boards in the DPU, including the BIU,
and by the amplifiers, D/A converters, and other circuitry housed
in the IMS, ELS, IBS, and ACT.
The CAPS supplemental heater
(controlled by CAPS) and replacement heater (controlled
by the spacecraft) are mounted to the inside of the DPU's top
plate.
The CAPS radiator mounts to
the back plate of the DPU.
Along with the actuator, the DPU box forms
the principal structure of CAPS. The IMS, ELS, IBS, and
the high-voltage power supplies mount to the top plate of the
DPU. CAPS mounts to the spacecraft via the actuator, which is
mounted to the bottom plate of the DPU.
The fifth major subassembly of the CAPS
instrument is the high-voltage power supply. Two redundant
power supplies, HVU1 and HVU2, mount to the top plate of the
DPU.
The last subassembly of CAPS is the actuator
(ACT). The ACT will rotate the CAPS instrument at a steady
rate over a maximum range of 184 degrees with the acceleration/deceleration
over a further 12 degrees (minimum) at either end of this range.
The steady rate and acceleration/deceleration range can be adjusted
in-flight, and the 216-degree total range of movement will be
limited by hard stops to prevent any "wraparound" effects
on the CAPS interface cables.
The Cassini Plasma Spectrometer Subsystem (CAPS) will measure
the flux (i.e., the flow rate or density) of ions as a function
of mass per charge and the flux of ions and electrons as a function
of energy per charge and angle of arrival relative to the CAPS
instrument. The CAPS Subsystem consists of six major subassemblies:
the ion mass spectrometer, the ion
beam spectrometer, the electron spectrometer,
a data processing unit, a high-voltage
power supply, and an actuator. For information
about these components, click on their names.
