Overview of the Canadian Electric Field Instrument (CEFI) for Swarm Brian Moffat (a), John Hackett (a), David Knudsen (b), Jan-Erik Wahlund (c), Lennart Åhlén (c), Nico Stricker (d) (a) COM DEV Ltd., Cambridge Ontario, (b) University of Calgary (c) Swedish Institute of Space Physics (d) European Space Agency (ESA) Abstract This paper describes the Swarm CEFI instrument which has been selected to fly aboard the Swarm constellation of satellites. Swarm is one of the Earth Explorer missions in ESA s Living Planet Programme. The Swarm mission will provide the best ever survey of the geomagnetic field and its temporal evolution, in order to gain new insights into the Earth System by improving our understanding of the Earth s interior and physical climate. The mission will utilize a constellation of three spacecraft each with identical instrument payloads. The CEFI instrument is designed to measure various features of the local plasma environment in orbit. The CEFI instrument incorporates an ion imager that makes use of an electrostatic analyzer similar to those used on the Cold Plasma Analyzer (CPA) [1] instrument on Freja and the Thermal Plasma Analyzer (TPA) instrument on Nozomi as well as the GEODESIC [2,3], CUSP and JOULE sounding rocket missions. A pair of Langmuir probes is also included as part of the instrument to allow characterization of the electron environment and to measure the Swarm spacecraft potential. The instrument is composed of three main parts: the SII sensor head assemblies, the Langmuir Probe sensors and the electronics assembly. The Swarm Mission [4] The objective of the Swarm mission is to provide the best ever survey of the geomagnetic field and its temporal evolution, and gain new insights into improving our knowledge of the Earth s interior and climate. The Swarm concept consists of a constellation of three satellites in polar orbits. Two satellites will fly in formation below 450 km altitude while a third satellite will fly below 530 km altitude but always above the other two. High-precision and high-resolution measurements of the strength and direction of the magnetic field will be provided by each satellite. In combination, they will provide the necessary observations that are required to model various sources of the geomagnetic field. CEFI Overview 1
The goal of the CEFI instrument is to characterize the electric field about the earth by measuring the plasma density, drift, and acceleration at high resolution. CEFI derives its heritage from the Freja CPA instrument, the Nozomi TPA instrument and the CUSP, JOULE and GEODESIC sounding rocket missions. The CEFI Instrument is comprised of three main parts: the Suprathermal Ion Imager (SII) sensors, the Langmuir Probe (LP) sensors and the Electronics Assembly. The electronics assembly contains all of the electronics necessary to support power supply, sensor data acquisition, instrument control and communications with the spacecraft bus. The Electronics Assembly and SII sensors will be positioned on the ram face of each Swarm spacecraft along with the Langmuir probes positioned preferably on the ram and nadir faces of each spacecraft and connected to the electronics assembly with wire harnesses. SII Sensors Radiator Electronics Assembly SII Sensors Figure 1: CEFI Solid Model Electrical Interface Connectors The SII sensors are based on a unique particle focusing scheme first developed for and flown on the Freja Cold Plasma Analyzer. This design was developed at the University of Calgary. Ions enter a narrow aperture slit and are then deflected by a pair of hemispherical grids that create a region having electric fields directed radially inward. Incoming low-energy positive ions are accelerated toward the center of the spherical system, whereas ions with larger kinetic energies travel farther toward the edge of the detector, creating an energy spectrum as a function of detector radius as depicted in Figure 2. 2
Figure 2: CEFI particle focusing system (Figure courtesy of J. Burchill) Particles arriving from out of the plane of Figure 2 land at different azimuths on the image plane. The resulting image from each SII sensor is a 2-D cut through the ion distribution function, from which one can calculate ion density, drift velocity (2-D), temperature, and higher-order moments. The two SII sensor head assemblies are oriented such that the aperture slits are oriented perpendicularly to each other, enabling 3-D characterization of the ion distribution. The detector shown in Figure 2 consists of a Micro Channel Plate (MCP), a phosphor-coated fiber taper and a frame transfer CCD. Ions entering the instrument s aperture slit are deflected by the electrostatic analyzer towards the MCP. When the charged particles strike the MCP, the signal is amplified through secondary emission processes. The voltage applied across the MCP controls the gain of the device. In parts of the orbit where ion flux is high, the voltage applied to the front surface is reduced to limit the gain of the device and preserve its life. This gain adjustment is part of an automatic gain control realized through the use of a feedback loop using the CEFI instrument faceplate current as a control input. Where sufficient gain control cannot be achieved via the MCP voltage alone, an electrostatic shutter will gate the incoming ions with duty cycles ranging from 100% to well below 1%. The charge leaving the back surface of the MCP strikes a phosphor screen which is applied as a coating to the fiber taper and causes photon emission. The role of the fiber taper is to reduce the image size to be compatible with the active area of the CCD. The CEFI sensor head assemblies will incorporate a test LED - located near the CCD - that can be used for a CCD aliveness test during all levels of testing including spacecraft integration and post-launch if required. A solid model of the CEFI sensor head assembly is shown in the following figure. 3
Inner Dome Shutter Grid MCP Fiber Taper with Phosphor Coating CCD & Preamp Board Figure 3: CEFI Sensor Head Assembly LP Sensors A Langmuir Probe assembly is included with the instrument to provide measurement of electron density, electron temperature and spacecraft potential. The Langmuir Probe design is based on hardware flown on the Cluster and Rosetta missions and was developed by the Swedish Institute of Space Physics. A bias voltage is applied to the probe and the resulting current, which is proportional to the plasma charge density, is measured. To enable simultaneous measurements of electrons and ions, dual probes are used with one probe biased at a positive potential and the other at a negative potential. A dual probe system, when used with two probes on equal potentials, also makes it possible to perform interferometric measurements. The overall height of the Langmuir Probe sensor is 10mm. A conceptual solid model of the Langmuir Probe is shown the figure below. Figure 4: Langmuir Probe Concept 4
Electronics Assembly The instrument electronics Assembly include the following electrical subsystems: Instrument Controller Detector Readout Electronics High Voltage Supply Low Voltage Supply Langmuir Probe Assemblies Each of these subsystems is connected using internal wire harnesses within the CEFI electronics assembly housing. An additional pair of wire harnesses will connect the Langmuir Probe Electronics to the Langmuir Probe Sensors. The CEFI instrument level electrical block diagram is shown in Figure 5 below. Survival Heater N-S CCD E-W CCD +28V DC Bus Feed A +28V DC Bus Feed B LVPS V, I Sensors Read Out Electronics Board Data, TLM Control CCD Clocks Langmuir Probes HVPS HV Status HV Control Instrument Controller/ Processor Board Data Command Langmuir Probe Board Ion Optics Spacecraft Bus RS422 A Spacecraft Bus RS422 B Figure 5: CEFI Electrical Block Diagram 5
Readout Electronics Board The Readout Electronics Assembly provides the signal conditioning and readout chain, programmable voltage CCD clock drivers, test LED control and housekeeping data multiplexing and A/D conversion. This subassembly includes two small pre-amp circuit boards containing the CCD and some preamplifiers. These pre-amp boards are located within the SII sensor head assemblies. All required bias and clock lines are provided to the Pre-amp Boards by the Detector Readout Electronics Circuit Board via harnessing. The Pre-amp Boards provide the amplified CCD readout information to the Detector Readout Electronics Board. The Detector Readout Electronics Circuit Board is supplied with the input supply voltages from the LVPS and clock and command lines from the Controller Board. Within this printed circuit board the voltage supply lines are regulated as required to supply the necessary biasing for the CCD. Level shifting circuits are incorporated within this assembly to provide the appropriate clock and command line logic levels required by the CCDs and readout electronics. The amplified CCD readout voltage for the two Sensor Head Assemblies is received by this subassembly and clamped and sampled. The subsequent signal is A/D converted and sent to the Controller Board for processing. The instrument housekeeping telemetry circuitry is also incorporated within the Detector Readout Electronics Board. Instrument Controller Board The instrument controller is comprised of a processor, a control FPGA, EEPROM for nonvolatile memory, SDRAM for volatile memory, an oscillator, a D/A converter for the HVPS command lines, RS422 serial command and telemetry interfaces to the spacecraft and buffering for the interfaces between the control FPGA and the Langmuir probe electronics and the CCD readout electronics. The electrostatic shutter that is contained in each SII sensor will be controlled from a signal generated by the controller board. The controller will poll the instrument skin current monitors, determine the appropriate shutter control parameters (pulse width and duty cycle) and then send the appropriate control waveform to the HVPS via a D/A converter on the controller board. The CEFI controller performs data processing for both the SII and LP sensors, and provides overall control of CEFI. The CEFI instrument controller will be based on a low power, SPARC architecture microprocessor. Langmuir Probe Board The Langmuir Probe Assembly measures the electron density, the electron temperature, and the spacecraft potential in the vicinity of the CEFI instrument. In addition to the Langmuir probe subsystem also measures the current from the instrument faceplate. 6
In order to reconstruct the Langmuir curve, the probe bias potential with respect to the spacecraft versus current drawn from the probe must be measured. The Langmuir Probe Board contains all of the electronics necessary to supply the Langmuir Probe and instrument faceplate bias voltages and to measure the probe and faceplate current. Low Voltage Supply The incoming 28V supply is conditioned and converted by the LVPS to provide power for all active electronics. Several types of rails are generated in the LVPS including +3.3V for the digital electronics, +/-5V for the HVPS and +/-15V for the analog electronics and LP. Other voltages required by the instrument are generated by fixed and programmable linear regulators. The LVPS has redundant interfaces to the bus power supply. Cross strapping is achieved via pairs of latching relays. High Voltage Supply (HVPS) Board The HVPS supplies all of the high voltage lines needed for the ion optics within the SII sensor head assembly. The HVPS uses +/- 5V power rails originating from the LVPS electronics. All programmable high voltages including the electrostatic grid, the MCP and the phosphor are commanded by the instrument controller using D/A converters to generate analog control lines. The signal used to control the electrostatic shutter will originate from the Instrument Controller as a waveform with the appropriate duty cycle and pulse width but at low amplitude that is mimicked by the HVPS at a higher voltage. Mechanical/Thermal Design The instrument Electronics Assembly housing consists of the two sensors, a radiator panel and electronics box as shown in Figure 1. The general construction of the electronics housing and SII sensor head structure as well as the radiator is from aluminum alloy 6061 for its low mass, high strength and good thermal conductivity. The instrument is mounted to the spacecraft using 6 attachment points. The SII sensors mount directly to the radiator so that their dissipated heat can be conducted directly to the radiator and radiated to space. The radiator consists of a thin flat aluminum panel, suitably reinforced with ribbing to meet the strength and stiffness requirements. The instrument radiator will also act as a skin current monitor so it will be coated with conductive white thermal paint. The radiator is designed to remove the heat from each SII sensor head over the range of specified instrument interface temperatures while maintaining the CCD temperature at an acceptable value that limits the dark current. The electronics box is an aluminum structure consisting of three sections or modules bolted together as shown in Figure 1. Feed through holes are provided through the center walls as necessary to allow for 7
common cabling. The center walls provide shielding between the boards and simplify the assembly. Covers are also located between the modules to provide additional shielding. Instrument Status CEFI is currently in the development phase. Some breadboard testing has been performed with the remainder planned in the next 6 months. Instrument level Preliminary Design review is planned for fall 2006. All three flight models are deliverable by fall 2008. Acknowledgements Phase A of the CEFI design was funded by the Canadian Space Agency. In subsequent phases, the instrument design and build is funded by the European Space Agency while science support is funded by the Canadian Space Agency. The instrument prime contractor is COM DEV Ltd with design support, science advisement and calibration facilities provided by the University of Calgary. The Langmuir Probe hardware along with science support is provided by the Swedish Institute of Space Physics. References 1. Whalen, B. A, et al., "The Freja F3C Cold Plasma Analyzer", Space Sci. Rev., 70, 137, 1994 2. Burchill, J. K., GEODESIC Observations of Auroral Ions, Ph.D. dissertation, University of Calgary, 2003 3. Knudsen, D. J. et al., A low-energy charged particle distribution imager with a compact sensor for space applications, Rev. Sci. Instr., 74, 202, 2003 4. ESA Living Planet Programme Swarm website: http://www.esa.int/esalp/esa3qzje43d_lpswarm_0.html 8