Cassini

The Cassini Plasma Spectrometer (CAPS) will measure the flux 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 CAPS.

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:

Ion mass spectrometer
Ion beam spectrometer
Electron spectrometer
Data processing unit
High-voltage power supply
Actuator

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:

Aperture cover/actuator
Toroidal analyzer
Carbon foils
Time-of-flight spectrometer
Microchannel plates
Amplifier/discriminators
Time-to-digital converter
Spectrum analyzer module
High-voltage power converters contained in high-voltage units 1 and 2.

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:

Hemispherical electrostatic analyzer
Channel electron multipliers
Amplifiers/discriminators
High-voltage power converters

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:

Spherical analyzer
Microchannel plates
Amplifiers/discriminators
Sensor management unit
High-voltage power converters

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:

Sensor and actuator data control interfaces
Housekeeping analog-to-digital converter
Safe/arm control
Wax thermal actuator (WTA) driver
CPUs with memory
Bus interface unit
Low-voltage power converters
Supplemental and replacement heaters
Radiator
Principal structure of CAPS

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.

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