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Fostering Visions for the Future: A Review of the NASA Institute of Advanced Concepts F Three NIAC Phase II Projects Infused into NASA’s Long-Term Plans MINI-MAGNETOSPHERIC PLASMA PROPULSION Project of Robert M. Winglee, University of Washington The Mini-Magnetospheric Plasma Propulsion (M2P2) system, proposed by Robert Winglee and John Slough of the University of Washington, was funded in 1998 as a Phase I effort followed by a Phase II effort in 1999 (Figure F-1). M2P2 is a revolutionary means for spacecraft propulsion that efficiently utilized the energy from space plasmas to accelerate payloads to much higher speeds than can be attained by present chemical oxidizing propulsion systems.1,2 The system utilized an innovative configuration of existing technology based on well-established principles of plasma physics. It offered the potential of feasibly providing cheap, fast propulsion that could power an Interstellar Probe, as well as powering large payloads that may be required for a crewed mission to Mars. The M2P2 system utilized low-energy plasma to transport or inflate a magnetic field beyond the typical scale lengths that can be supported by a standard solenoid magnetic field coil. In space, the inflated magnetic field would be used to reflect high-speed (400 to 1000 km/s) solar wind particles, thereby attaining an unprecedented acceleration for a power input of only a few kilowatts. Initial estimates were made for a minimum system that would provide a thrust of about 3 Newton continuous (0.6 MW continuous) power at a specific impulse of 104 to 105s, producing an increase in speed of about 30 km/s in a period of 3 months. As part of the NASA Institute for Advanced Concepts (NIAC) Phase I effort, several laboratory-scale models were developed and tested to FIGURE F-1 Mini-Magnetospheric Plasma Propulsion concept. SOURCE : Courtesy of Robert M. Winglee, University of Washington. 1 R. Winglee, J. Slough, T. Ziemba, and A. Goodson, Mini-magnetospheric plasma propulsion: Tapping the energy of the solar wind for spacecraft propulsion, Journal of Geophysical Research 105(20):833, 2000. 2 R.M. Winglee, J. Slough, T. Ziemba, and A. Goodson, Mini-magnetospheric plasma propulsion: High speed propulsion sailing the solar wind, p. 962 in 2000 Space Technology and Applications International Forum, M.S. El-Genk, ed. CP504. American Institute of Physics, College Park, Md., 2000.
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Fostering Visions for the Future: A Review of the NASA Institute of Advanced Concepts FIGURE F-2 Optical emissions from an injected neutral puff into the plasma. SOURCE: Courtesy of Robert M. Winglee, University of Washington. improve understanding of the proposed magnetic inflation process and to confirm models of the effect.3,4 These tests included that measurements of the plasma parameters at the helicon source and at the magnetic equator and perturbations in the magnetic field caused by plasma injection along dipole field lines. The tests demonstrated plasma confinement by the M2P2 followed classical linear scaling up to the point where wall effects became important, and the tests demonstrated plasma inflation. This finding was instrumental in leading to NASA evaluation and testing in a much larger chamber. The Phase I effort developed extensive models for the effect. This modeling was based on the fluid equations for plasmas, but the equations for conservation of mass and energy were combined in a multifluid treatment. This is more complex than traditional MHD modeling, which combines the equations into a single-fluid treatment. The multifluid approach required that the dynamics of the electrons and the different ions species be kept separate. The modeling was detailed and led to the amount of solar wind deflection with dipole tilt and the total force imparted onto the M2P2. On the basis of these detailed calculations and the development of a laboratory prototype, a Phase II award was made. As part of the NIAC Phase II project, a simulation model5,6 was developed where the magnetic field was represented by either a point dipole or a finite width solenoid and studies were performed to resolve processes occurring in close proximity to the magnet. The modeling was complicated by the physics of wall interactions, observed in the test program, that cause mirror currents, sputtering, and plasma sheaths. These effects were not incorporated into the model due to computational limitations. Despite those limitations, both the modeling and the tests in a 1-m-diameter chamber gave evidence that the M2P2 prototype had proven transport of magnetic flux. Figure F-2 shows quenching of the plasma initially followed by expansion of the closed field lines. The emission extends both downward and further into the chamber as the models predict. These initial NIAC Phase II tests led to further testing at MSFC in an 18 ft × 32 ft vertical vacuum chamber and used a plasma source from the SEPAC program for comparisons with the M2P2 3 R.M. Winglee, T. Ziemba, J. Slough, P. Euripides, and D. Gallagher, Laboratory testing of Mini-Magnetospheric Plasma Propulsion prototype, p. 407 in 2001 Space Technology and Applications International Forum, M.S. El-Genk, ed., CP552, American Institute of Physics, College Park, Md., 2001. 4 T. Ziemba, R.M. Winglee, and P. Euripides, Parameterization of the laboratory performance of the Mini-Magnetospheric Plasma Propulsion (M2P2) prototype, 27th International Electric Propulsion Conference, October 15-19, 2001. 5 R. Winglee, T. Ziemba, P. Euripides, and J. Slough, Computer modeling of the laboratory testing of Mini-Magnetospheric Plasma Propulsion (M2P2), International Electric Propulsion Conference Proceedings, October 14-19, 2001. 6 R. Winglee, T. Ziemba, P. Euripides, and J. Slough, Computer modeling of the laboratory testing of Mini-Magnetospheric Plasma Propulsion (M2P2), International Electric Propulsion Conference Proceedings, October 14-19, 2001.
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Fostering Visions for the Future: A Review of the NASA Institute of Advanced Concepts helicon source.7 Figure F-3 shows density contours when the SEPAC is operated by itself and when the M2P2 operates in conjunction with SEPAC. The plasma plume is substantially thickened both horizontally as well as vertically. Most surprisingly, the plasma plume is affected all the way in to close proximity of the plasma source. Modeling confirmed that this deflection of the external plasma is associated with the inflation of the mini-magnetosphere. FIGURE F-3 Density contours showing inflation with the M2P2. SOURCE: Courtesy of Robert M. Winglee, University of Washington. Additional tests and modeling confirmed that M2P2 led to expansion of the magnetic field to several tens of magnetic radii. The tests also showed the existence of a plasma depletion layer between the SEPAC and M2P2 plasmas. This gap is analogous to the magnetopause of Earth where there is deflection of solar wind by the terrestrial magnetosphere. Its persistence in the experiment indicates that the mini-magnetosphere is stable over long periods. Other data confirmed that the plasma within the mini-magnetosphere was well confined and that continued plasma production leads to an increasing buildup of the mini-magnetosphere. These experiments were able to quantify the performance of the prototype through comparative studies of the laboratory test results with the simulation results. The results showed that the transport of flux within the mini-magnetosphere had a very distinctive signature, where the flux inside the magnetosphere declined and the flux outside the initial closed region of the vacuum dipole increased. As flux was transported outward, both the simulations and the observations showed a pileup of the terrestrial magnetic field. The perturbations observed were small at only ~1 G, but this change in magnetic field was sufficient to drive the field lines into the walls of the laboratory chambers that are available. In addition, both the simulations and the experimental results showed that this same type of magnetic field perturbation was able to deflect plasma at large distances and produce observable effects all the way into the throat of an external plasma source. These results were all strong indicators that the inflation of a mini-magnetosphere was achieved and that the closed magnetic field geometry of M2P2 provides an efficient means for deflecting external plasma winds at much greater distances than could be accomplished by a magnet alone. Inflation and deflection are the key tenants of the M2P2 system, and the experimental confirmation of the simulation results in the laboratory provided strong evidence that the high thrust levels (1-3 N) reported in the original description should be achievable for low energy input (~500 kW) and low propellant consumption (<1 kg/day). Further testing to measure the thrust levels attainable by the prototype, however, did not confirm measurable thrust. In the 2001 to 2002 time frame, the M2P2 concept was considered as a viable, emerging technology by the NASA Decadal Planning Team and the NASA Exploration Team. Within NASA, these teams were created to generate and assess innovative concepts for NASA senior leadership that allowed new approaches to human and robotic space exploration. Specifically, these teams were chartered to develop options that could achieve major scientific goals over the subsequent 20 years using advanced technologies and could take advantage of the capabilities that astronauts made available on site. External to NIAC, the M2P2 was funded by various NASA organizations to continue experiments confirming computer models as noted above. Continued development of a high-powered helicon component and collaboration between the JSC VASIMR program and the M2P2 program was 7 R.M. Winglee, T. Ziemba, J. Slough, P. Euripides, D. Gallagher, P. Craven, W. Tomlinson, J. Cravens, and J. Burch, Large scale laboratory testing of Mini-Magnetospheric Plasma Propulsion (M2P2)—Enabling technology for planetary exploration, 12th Annual Advanced Space Propulsion Workshop Proceedings, April 3-5, 2001.
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Fostering Visions for the Future: A Review of the NASA Institute of Advanced Concepts established. Through peer review, the M2P2 effort was deemed highly innovative and technically competent. This research effort created considerable interest within and external to NASA. The NIAC Phase II implementation was professional and the M2P2 team was focused on demonstrating the feasibility of this advanced concept to NASA. In 2002, a review panel that included plasma experts concluded there were additional unresolved technical issues that centered around magnet field strengths, mass, and power requirements. While partially addressed by the M2P2 team,8 this work came to a stop due to changing priorities within the agency. MICRO-ARCSECOND X-RAY IMAGING MISSION Project of Webster Cash, University of Colorado, Boulder In 1999, University of Colorado Professor Webster Cash was awarded a NIAC Phase I award for his proposal entitled “X-Ray Interferometry: Ultimate Astronomical Imaging.” The proposed concept was for an array of grazing-incidence x-ray mirrors on free-flying spacecraft, coordinated to focus the x rays on a set of beam-combining and detector spacecraft. The Phase I work validated the basic concept and suggested a method to test the predicted performance in the laboratory. Initial tests of a prototype x-ray interferometer were performed with additional NASA support at the Marshall Space Flight Center and demonstrated an angular resolution of 100 milli-arcseconds, a factor-of-5 improvement over the best previous results. In 2000 Cash’s “X-ray Interferometry” proposal was selected by NIAC for Phase II funding. Cash and his colleagues published their x-ray interferometry test results in a September 2000, issue of Nature.9 Also that year NASA incorporated this concept into its strategic plans. Dubbed MAXIM, the Micro Arcsecond X-ray Imaging Mission, this concept appeared in the National Research Council’s (NRC’s) Decadal Review of Astronomy and Astrophysics released in 2000,10 which identified x-ray interferometry for $60 million in funding over the following 10 years. The technique of interferometric imaging, combining light from a dispersed array of collector optics onto a single focal plane (see Figure F-4), has been exploited at RF wavelengths (e.g., the Very Large Array and Very Long Baseline Array for radio astronomy) and is being implemented for optical telescopes (e.g., the European Southern Observatory Very Large Telescope). Properly implemented, the technique yields angular resolution inversely proportional to the distance between the collectors, so that extremely high resolution can be obtained by placing the collectors very far apart. Cash’s NIAC Phase II x-ray interferometry proposal was an extension of this concept to x-ray wavelengths. By choosing extremely bright x-ray objects to image, he identified an ideal combination of subject and scientific motivation: to image the event horizon of a black hole. The technical credibility of the concept was clear, but the technical implementation remains extremely challenging, in part because of the difficulty of maintaining path length control to a small fraction of the very small x-ray wavelength. However, the fact that the laboratory demonstration of this capability was published in Nature testifies to the significance of this accomplishment. 8 R.M. Winglee, P. Euripides, T. Ziemba, J. Slough, and L. Giersch, Simulation of Mini-Magnetospheric Plasma Production (M2P2) interacting with an external plasma wind, AIAA Paper No. 2003-5224, July 2003. 9 W. Cash, A. Shipley, S. Osterman, and M. Joy, Laboratory detection of x-ray fringes with a grazing-incidence interferometer, Nature 407:160-162, doi:10.1038/35025009Letter. 10 National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.
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Fostering Visions for the Future: A Review of the NASA Institute of Advanced Concepts FIGURE F-4 The x-ray interferometry imaging mission concept successfully proposed in 1999 by Webster Cash for a Phase I NIAC study. SOURCE: NASA Institute for Advanced Concepts, Annual Report (2nd; 1999-2000), Atlanta, Ga., 2000, p. 23. Courtesy of Webster Cash, University of Colorado. FIGURE F-5 The x-ray interferometry approach to imaging the event horizon of a black hole is one of the methods being pursued by NASA for its Black Hole Imager mission. SOURCE: NASA Institute for Advanced Concepts, 5th Annual Report (2002-2003), Atlanta, Ga., 2003, p. 7. Courtesy of Webster Cash, University of Colorado.
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Fostering Visions for the Future: A Review of the NASA Institute of Advanced Concepts NASA has continued support to further define and develop high-resolution x-ray imaging missions, and Cash’s interferometry concept has remained among the leading contenders.11 The MAXIM Pathfinder mission was the subject of a NASA-supported “Integrated Mission Design Center” study in 2002. In 2004, MAXIM was selected as one of the NASA Vision Mission studies for advanced definition. Today, the technology of x-ray interferometry that was the subject of the initial NIAC study is the first (see Figure F-5) of three competing methods that NASA is pursuing under its Black Hole Imager (BHI) mission. The BHI team presented a white paper to the NRC’s Astro2010: The Astronomy and Astrophysics Decadal Survey of the NRC and expects the BHI to be identified as one of the compelling astrophysics missions for NASA to pursue in the near future. NEW WORLDS OBSERVER Project of Webster Cash, University of Colorado, Boulder In 2004 University of Colorado Professor Webster Cash submitted a proposal for a NIAC Phase I study of a concept called the New Worlds Imager (Figure F-6); its objective was to study a variety of pinhole camera and occulting mask designs to enable imaging of planetary systems around other stars. As documented in the final Phase I study report, dated March 31, 2005, Cash and his collaborators realized that occulting masks had significant performance advantages, and they identified an occulter design that could meet the contrast requirements of the exoplanet exploration missions under active consideration by NASA as the Terrestrial Planet Finder and by ESA as Darwin. The basic concept is an occulting mask (to first order, an opaque disk) with an edge shaped like petals of a flower (see Figure F-5), but precisely designed to cancel the diffraction effects that famously result in a local intensity maximum along the axis of the occulter at the center of the expected shadow, the Arago Spot. Following the completion of the Phase I effort, in May 2005, NIAC selected a proposal for Phase II, now called the New Worlds Imager. The ultimate implementation envisioned was for a five-spacecraft constellation consisting of two sets of starshade and telescope combinations, plus a fifth spacecraft carrying a beam combiner/interferometer. This NIAC-funded work was described in an article featured on the cover of the July 6, 2006, issue of Nature.12 During Phase II, Cash and his collaborators demonstrated suppression performance <10−7 in a laboratory test of a miniature, 16-petal occulter. Both the Nature publication and the laboratory demonstration testify to the significance and technical competence of the basic concept and the research supported by NIAC. With the completion of the NIAC Phase II study, NASA has provided significant additional support for Cash’s occulter concept, and it is now one of the competitive concepts for the Terrestrial Planet Finder program. In addition, both Ball Aerospace Corporation and Northrop Grumman Corporation have made internal investments to further develop the concept in conjunction with Cash and the rest of his team. According to NIAC, “In February 2006, NASA/GSFC [Goddard Space Flight Center] announced its intent to issue a sole-source request for proposal to Northrop Grumman Corp. and Ball Aerospace Corp. for the further development of the New Worlds Imager (NWI).”13 NASA/GSFC continues to support this concept with funding. More than 40 papers have been published between 2004 and 2008 by Cash and his colleagues on this technique and its applications. In February 2008, NASA announced that a team led by Cash had been awarded $1 million for the New Worlds Observer as one of its Astrophysics Strategic Missions Concept Studies (ASMCS). The 11 See http://maxim.gsfc.nasa.gov. 12 W. Cash, Detection of Earth-like planets around nearby stars using a petal-shaped occulter, Nature 442:6, 2006. 13 NASA Institute for Advanced Concepts, Annual Report (8th; 2005-2006), Atlanta, Ga., 2006, p. 23.
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Fostering Visions for the Future: A Review of the NASA Institute of Advanced Concepts FIGURE F-6 An artist’s rendering of the New Worlds Observer concept for imaging a distant planetary system. Light from the central star is blocked by a large external occulting disk that is shaped to control diffraction of the starlight around the occulter. A telescope placed in the right location can image the surrounding planetary system without glare from the central star. SOURCE: Webster Cash, The New Worlds Imager, Final Report to the NASA Institute for Advanced Concepts for Phase I Study, NIAC Phase I study report, 2005. Courtesy of Webster Cash, University of Colorado. study now has been completed and the final report is available.14 The results of this study will be used to prepare the New Worlds Observer mission concept for the NRC’s Astronomy and Astrophysics Decadal Survey. In conjunction with the ASMCS study, NASA convened a Technical Assessment Review (TAR) panel for the New Worlds Observer mission concept. The NASA TAR report is part of the ASMCS document package. The TAR recommendations are being used to motivate further NASA investment and to prepare the technologies necessary for a successful New Worlds Observer mission. 14 Webster Cash, principal investigator, Final Report, Astrophysics Strategic Mission Concept Study, The New Worlds Observer, April 24, 2009.