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This Defense Intelligence Reference Document from the Defense Intelligence Agency, dated 21 November 2010, was produced in FY 2010 under the Advanced Aerospace Weapons System Applications (AAWSA) Program. It reviews propulsion concepts that use plasmas and magnetohydrodynamics. It finds the Ajax MHD energy bypass concept meaningless below Mach 12 but calls a reverse energy bypass with a Virtual Cowl potentially practical. It also predicts that electric propulsion will become standard for spacecraft.
UNCLASSIFIED/ ,'F&II. &Ffilii,td, Wlii &Hl!o>t Similarly, if a voltage source (e.g., a battery) is connected to the electrodes placed on either side of the flow in such a way that the applied e.m.f. acts against the induced Faraday e.m.f., then the current will flow in the direction opposite to the Faraday current, and the jxB force will be in the direction of the flow. This will be an MHD accelerator that converts the battery-supplied electrical energy partially into enthalpy of the flow and partially into Joule dissipation in the circuit. The corresponding load factor, k, defined as the ratio of the applied electric field E and the product uB, k=E/uB, is greater than 1 in this accelerator configuration. In generator and accelerator devices, the interaction between the induced motion of electric charges across the B field and that field results in an e.m.f. induced along the flow. This secondary e.m.f. is called the "Hall e.m.f.," and the magnitude of Hall effect increases with the ratio of electron-cyclotron frequency, c,)B=eB/m, and the electron collision frequency, v. This ratio is called the electron Hall parameter, Qe. As the Hall parameter approaches 1, the Hall current directed along the flow increases at the expense of the transverse Faraday current, resulting in reduction of the jxB force. To reduce or eliminate the Hall current, the electrodes placed on either side of the flow are normally segmented and thus form multiple pairs. Each electrode pair has a proper resistor and/or battery in its circuit. Theoretically, this segmented-electrode Faraday configuration enables the performance equal to that without the parasitic Hall effect. However, as the Hall parameter increases, so does the voltage fall between the adjacent electrode segments, so that eventually arcing between the segments starts, effectively negating the advantage of segmenting. A better (and more "natural") MHD configuration at high values of the Hall parameter is the one where each electrode pair (with the electrodes on either sides of the flow and right across each other) is shorted, and the voltage is either extracted (in the generator case) or applied (in the accelerator case) along the flow, between the first and the last electrode pair. This is called the "Hall configuration." A useful dimensionless parameter reflecting the strength of MHD interaction is called the MHD interaction parameter, or the Stuart number, and it represents the ampere body force effect relative to the flow momentum flux: S ~ ,:;BL pu In this equation, a is the electrical conductivity of the fluid, B is the magnetic field strength, Lis the characteristic linear dimension, pis the fluid density, and u is the velocity. Therefore, for a significant MHD effect in high-speed, high dynamic pressure flow (e.g., in hypersonics), the conductivity and the B field strength must be high. Herein lies the principal problem for aeronautical MHD applications. Indeed, normal air is not an electrical conductor. Air does become ionized and thus electrically conducting as it is heated to very high temperatures (3,000-10,000K or higher), such as those achieved in shock and boundary layers around reentry vehicles, at Mach numbers M=12-25 or so. The ionization fraction then reaches 10·5-10· 2, and the conductivity from 100 to about 3,000 mho/m ensues. Seeding 12 UNCLASSIFIED/I OR 01 PICll!IL ~91! 8HLV
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Report, from the dia collection. The PDF is mirrored here; the original link is above. 32 pages are in the text index: search them above, or from the library's search.