Iowa Space Grant Consortium
Project Title: Toroidal Plasma Analyzer
Project Manager: Dr. William Paterson
Complete Mailing Address: Dr. William Paterson
University of Iowa
Department of Physics and Astronomy
Iowa City, IA 52240
Phone: 319-335-1864 Fax: 319-335-1753 E-mail: firstname.lastname@example.org
ISGC Affiliate: The University of Iowa – Department of Physics & Astronomy
Student Participant: Shawn Conklin
The objectives of this proposal are the assembly and testing of components for an innovative plasma analyzer designed for future spaceflight missions. Plasmas comprised of electrically charged ions and electrons pervade most of space. During four decades of spaceflight, instruments designed to measure these particles have been an effective tool for exploration. New sensor technology offers the opportunity to redesign the basic analyzer configuration with the goal of reducing the mass and volume of the analyzer plates that guide the particles to the sensors. A new plate design that fulfills this goal has been modeled to determine the particle trajectories that will be accepted by this analyzer. This proposal seeks funding to support testing of a laboratory calibration unit to validate the model computations and verify the functionality of the design. An undergraduate mechanical engineering student has provided support for the design work that has already been completed, and this student will participate directly and importantly during the proposed testing of the model analyzer. The project can be completed within the scope of the ISGC Seed Grant program by utilizing a test facility previously developed for calibration of plasma analyzers that are now successfully in service on the Galileo and Geotail spacecraft. It is anticipated that successful testing of the design will strengthen the standing of future proposals based on this concept.
Electrostatic plasma analyzers measure the directions and energies of electrically charged ions and electrons with energies typically in the range 1 eV - 50 keV. Parameters determined from the measurements include the densities, temperatures, and bulk flow speeds of the plasmas, and the data also provide evidence of the physical processes that drive motions of the plasmas and evidence of their source. The measurements are relevant for the analysis of geophysical processes that include the occurrence of geomagnetic storms and substorms that are responsible for the northern and southern lights, and also for interference with radio communications on Earth, for disruption of electrical power distribution grids, and for damage to Earth orbiting spacecraft. Plasmas also play important roles in altering both natural and human-engineered surfaces in space. The Space Plasma Physics Particles and Imaging research group at the University of Iowa has successfully designed and built plasma instrumentation for NASA missions that include the PLS for the Galileo spacecraft now surveying Jupiter's magnetosphere, and the CPI for the Geotail spacecraft that has been continuously monitoring plasmas in the vicinity of our own planet since 1992.
We are currently researching the feasibility of new instrument designs with the goal of reducing both the mass and the volume of future plasma analyzers while maintaining their capabilities for resolving particle directions and energies. Smaller and lighter instruments would reduce costs associated with space launch, and significant reductions in mass and size are required for instruments to be flown on fleets of small scientific spacecraft that are envisioned as a means of making simultaneous multi-point observations within the vast and complex plasma-physical systems characteristic of space.
Advances in sensor technology provide an opportunity for redesign of the electrostatic plates that guide charged particles to the sensors. In the past, multiple sensors (channeltron detectors) were used to divide the viewing space of the instrument and thus provide the needed resolution of particle directions. For example, the Galileo PLS employs seven electron sensors and a separate set of seven ion sensors to divide the instrument field-of-view into seven contiguous segments. These sensors are arrayed along one edge of concentric electrostatic guide plates. To accommodate the placement of multiple sensors the plates are relatively large and massive, having a shape that is approximately a 1/4 section of a sphere.
It has been demonstrated that a single microchannel plate (MCP) sensor can be operated in a mode that allows it to replace the multiple sensor assemblies used in the past. A set of plates that would guide the charged particles to this MCP is depicted in Figures 1 and 2. This plate assembly has a shape that is a quarter section of a torus. In comparison with the quarter sphere plates used in earlier instruments the volume and mass of the plates are reduced by approximately 2/3. The configuration shown can be used to simultaneously measure hot plasma ions, hot plasma electrons, and solar wind ions. The plate gaps depicted in Figure 2 are the same as those used for the CPI and PLS Analyzers. Plates 1 and 4 are always at ground, providing the protection required from the high voltages on plates 2 and 3. Hot plasma electrons (1 eV to 50 KeV) are funneled to the sensor by the electrostatic field between grounded plate 1 and the positive voltage (0.1 V to 4.9 kV) on plate 2. Hot plasma ions, of similar energy, are funneled to the sensor by the electrostatic field between the positive voltage on plate 2 and the potential of zero volts applied on plate 3. Solar wind ions (150 eV to 7 KeV) are funneled to the sensor by the electrostatic field between a positive potential applied to plate 3 (5.5 V to 258 V) and a grounded plate 4.
The field-of-view for the plate configuration shown in Figure 1 has been investigated using a computer model to trace the trajectories of particles between the plates. The MCP detector (not shown) could be placed directly beneath the circular exit aperture. However, it is found that the resolution of particle directions would then be degraded as compared with earlier quarter-sphere guide plates. To recover the resolution of directions it is necessary to add cylindrical drift tubes as a stage between the exit aperture of the plates and the MCP detector.
The goal to be pursued in the proposed work is the development of a simplified model analyzer sufficient for testing of the basic plate configuration and for verification of the computer modeling of particle trajectories. This model analyzer will consist of plate 1 and plate 2. Initial testing would investigate the field-of-view for the basic plates without the use of drift tubes. The drift tube stage would then be inserted between the plates and the sensor for testing of the complete assembly. Both electrons and ions could be tested by reversing the voltage applied to the plates. An MCP detector is not required for this testing, and the cost of such a detector for this purpose is not justified. A single channeltron detector would be used instead. Testing would be carried out using the laboratory facility developed for testing of the Galileo and Geotail plasma instrumentation. This facility is located in Van Allen Hall on the Campus of the University of Iowa. The facilities include vacuum chambers equipped with motor driven calibration fixtures used to change the orientation of the instrument model. An electron gun and an ion gun used to calibrate the Galileo and Geotail instruments would provide the sources of electrically charged particles, and the vacuum chambers are equipped with Helmholtz coils to nullify Earth's magnetic field which otherwise deflects the particle beams. The field-of-view of the instrument for particles of various energies would then be mapped as the orientation of the instrument was changed while holding the particle source in fixed position. The quarter-toroid plates required for this project are currently being CNC machined as part of a student engineering project in collaboration with the Particles and Imaging group. The channeltron detector would be chosen from a set of spare detectors that are available at no cost. Drift tubes can be constructed from stock materials. The funding requested for this project would support the efforts of an engineering student engaged in the work required to configure the model instrument and the test facility. Funding also would support the efforts of senior personnel required for supervision of the student and analysis of the test data. Successful verification of the engineering model can be expected to strengthen the standing of future proposals based on this concept that are to be submitted in response to solicitations from NASA for plasma instrumentation.
WILLIAM R. PATERSON
B.S. (1982), M.S. (1987), Ph.D. (1990), all from The University of Iowa, Iowa City, Iowa
William R. Paterson is a Research Scientist and Adjunct Professor in the Department of Physics and Astronomy at the University of Iowa. Responsibilities include teaching undergraduates about the chemistry and physics of our environment and analysis of measurements from plasma instruments on board the Galileo spacecraft at Jupiter and the Geotail spacecraft orbiting Earth. These measurements are used to investigate naturally occurring plasma phenomena within planetary magnetospheres and interplanetary space. Current research is directed towards understanding the transport and energization of plasmas within Earth's magnetosphere using measurements from Geotail and studies of the Jovian magnetosphere and the environs of the major Jovian moons using measurements from Galileo. Dr. Paterson is a member of the American Geophysical Union.
Paterson, W. R. and L. A. Frank, Hot Ion Plasmas From the Cloud of Neutral Gases Surrounding the Space Shuttle, J. Geophys. Res., 94, 3721-3727, 1989.
Paterson, W. R., L. A. Frank, S. Kokubun and T. Yamamoto, Geotail Survey of Ion Flow in the Plasma Sheet: Observations Between 10 and 50 RE, J. Geophys. Res., 103, 11,811-11,825, 1998.
Paterson, W. R., L. A. Frank, S. Kokubun and T. Yamamoto, Geotail Observations of Current Systems in the Plasma Sheet, in Geospace Mass and Energy Flow: Results From the International SolarTerrestrial Physics Program, Geophys. Monogr. Ser., vol. 104, pp. 201-211, AGU, Washington, D.C., 1998.
Paterson, W. R., L. A. Frank and K. L. Ackerson, Galileo Plasma Observations at Europa: Ion Energy Spectra and Moments, J. Geophys. Res., 104, 22,779-22,791, 1999.
Frank, L. A., K. L. Ackerson, W. R. Paterson, J. A. Lee, M. R. English, and G. L. Pickett, The Comprehensive Plasma Instrumentation (CPI) for the GEOTAIL spacecraft, J. Geomag. Geoelectr., 46, 23-37, 1994.
Frank, L. A. and W. R. Paterson, Intense Electron Beams Observed at Io With the Galileo Spacecraft, J. Geophys. Res., 104, 28,657-28,669, 1999.
Frank, L. A. and W. R. Paterson, Observations of Plasmas in the Io Torus With the Galileo Spacecraft, submitted to J. Geophys. Res., 1999.
Frank, L. A. and W. R. Paterson, Return to Io by the Galileo Spacecraft: Plasma Observations, submitted to J. Geophys. Res., 1999.
Frank, L. A., W. R. Paterson, J. B. Sigwarth and S. Kokubun, Observations of Magnetic Field Dipolarization During Auroral Substorm Onset, submitted to J. Geophys. Res., 1999.
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