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Electromagnetic Pulse (EMP) AccessScience from McGraw-Hill Education Page 1 of 7 www.accessscience.com Electromagnetic pulse (EMP) Contributed by: Robert A. Pfeffer Publication year: 2014 A transient electromagnetic signal produced by a nuclear explosion in or above the Earth’s atmosphere. Though not considered dangerous to people, the electromagnetic pulse (EMP) is a potential threat to many electronic systems. Discovery The existence of a nuclear-generated EMP has been known for many years. Originally predicted by scientists involved with the early development of nuclear weapons, it was not considered to be a serious threat to people or equipment. Then in the early 1960s some of the high-altitude nuclear tests conducted in the Pacific led to some strange occurrences many miles from ground zero. In Hawaii, for example, some 1300 km (800 mi) from the Johnston Island test, EMP was credited with setting off burglar alarms and turning off street lights. In later tests that were conducted in Nevada, significant EMP-induced signals were coupled to cables. These experimental results gave credibility to the potential military use of EMP and encouraged investigators to more accurately describe its origin, electromagnetic characteristics, and coupling to systems, and to develop an affordable method for system protection. Initial nuclear radiation In a typical nuclear detonation, parts of the shell casing and other materials are rapidly reduced to a very hot, compressed gas, which upon expansion gives rise to enormous amounts of mechanical and thermal energy. At the same time the nuclear reactions release tremendous amounts of energy as initial nuclear radiation (INR). This INR is in the form of rapidly moving neutrons and high-energy electromagnetic radiation, called x-rays and gamma rays. About a minute after detonation, the radioactive decay of the fission products gives rise to additional gamma rays and electrons (or beta particles), known as residual nuclear radiation (RNR). The distribution of the total explosive energy of a hypothetical fission detonation in the atmosphere below an altitude of 10 km (6 mi) is 50% blast, 35% thermal, 10% RNR, 5% INR. At higher altitudes where the air is less dense, the thermal energy increases and the blast energy decreases proportionately. See also: BETA PARTICLES ; GAMMA RAYS ; NUCLEAR FISSION ; NUCLEAR REACTION ; RADIOACTIVITY . EMP is associated with the INR output, which is a small percentage of the total explosive energy. Nevertheless, 2 2 EMP is still capable of transferring something of the order of 0.1–0.9 J ∕ m, (0.007–0.06 ft-lbf ∕ ft, ) onto a collector, more than enough to cause upset or damage to normal semiconductor devices. AccessScience from McGraw-Hill Education Page 2 of 7 www.accessscience.com WIDTH:BFig. 1 Total gamma source strength versus time for nominal 1-megaton surface burst. Both horizontal and vertical scales are logarithmic. ( After C. L. Longmire, On the electromagnetic pulse produced by nuclear explosions, IEEE Trans. Antennas Propag., AP-26(1):3–13, 1978 ) Early research first developed the physics of high-altitude EMP. Then in 1978 a consistent explanation was set forth of how EMP is related to the total gamma source strength regardless of the detonation height aboveground. Figure 1 identifies six contributions to the gamma source strength for a hypothetical 1-megaton surface burst. As the detonation height is elevated, gamma source contributions from neutrons interacting with the ground and air decrease; for a high-altitude detonation (higher than 60 km or 37 mi), the gamma source strength becomes essentially the prompt gammas from the nuclear burst, created several nanoseconds after the detonation. The gamma source strength wave shape then approaches a smooth curve, approximating a double exponential, and since EMP is generated from the gamma source strength as described below, it too approaches a double exponential wave shape. This wave shape is commonly referred to as early time EMP. It is considered the most serious problem to most electronics and electrical systems due to its energy content and wide area of coverage. EMP generation in a high-altitude burst As the prompt gammas move away from a high-altitude nuclear detonation ( Fig. 2 ), those gamma rays moving toward the Earth penetrate a denser region of the atmosphere called the source or deposition region. In this 10-km-thick (6-mi) region, approximately 25–35 km (15-21 mi) above the Earth, the highly energetic gamma rays interact with the air molecules to form Compton electrons (with energies starting at 1 MeV) and less energetic gamma rays, which then proceed in the same general direction as the original gamma rays. The fast Compton electrons eventually slow down by stripping other electrons from air molecules to form secondary electron-ion pairs. (Though these secondary electrons and ions do not contribute to the generation of the EMP, they do cause the region to become highly conductive, and therefore play an important role in determining the EMP wave shape and amplitude.) While slowing down, the very intense, short-duration flux of Compton electrons is also AccessScience from McGraw-Hill Education Page 3 of 7 www.accessscience.com WIDTH:BFig. 2 Schematic repr esentation of the EMP in a high-altitude burst. The extent of the source region varies with the altitude and the yield of the explosion. ( After S. Glasstone and P. J. Dolan, eds., The Effects of Nuclear Weapons, U.S. Department of Defense and the Energy Research and Development Administration, 3d ed., 1977 ) deflected by the Earth’s geomagnetic field, according to Eq. (1), ( 1 ) Image of Equation 1 where the deflection force vector accentF is perpendicular to the geomagnetic field vector accentB and the velocity vector accentv of a Compton electron of charge q . The Compton electrons then spiral about the geomagnetic lines, radiating electromagnetic energy in the form of EMP until they eventually recombine with local, positively charged ions. See also: COMPTON EFFECT ; SYNCHROTRON RADIATION . Characteristics for a high-altitude burst For a high-altitude nuclear detonation, the early-time radiated EMP observed at large distances from the source region can be represented in time t as an electric field E ( t ) and a magnetic field H ( t ), given by Eqs. (2) and (3), Image of Equation 2 (2) Image of Equation 3 (3) ,4 ,2 α ,6 ,−1 β ,8 ,−1 with E, 0 = 5.2 × 10 V ∕ m, H, 0 = 1.4 × 10 A ∕ m, = 4.0 × 10 s , and = 4.76 × 10 s . (In air, E and H are related by the impedance of free space, 377 ohms.) If the observer is directly below the detonation (ground zero), the polarization of both fields is predominantly horizontal; if the observer is at the horizon, the fields can have both horizontal and vertical components. The magnitude of these field components varies according to the latitude and longitude of the observer and the direction of the burst. See also: POLARIZATION OF WAVES . AccessScience from McGraw-Hill Education Page 4 of 7 www.accessscience.com WIDTH:BFig. 3 Schematic repr esentation of the EMP in a surface burst. ( After S. Glasstone and P. J. Dolan, eds., The Effects of Nuclear Weapons, U.S. Department of Defense and the Energy Research and Development Administration, 3d ed., 1977 ) The mid-time and late-time EMP, shown in Fig. 1 to be caused by ground and air inelastics, by ground and air capture, and by fission fragment interactions, could also couple sufficient energy onto electronics and electrical systems to cause unacceptable upset or even catastrophic failure. In particular, late-time EMP is similar to fields generated by major solar storms that have caused problems to electric power grids. Surface-burst EMP Should the nuclear detonation occur closer to the Earth, the EMP generation process becomes far more complex and the electric and magnetic fields become very complicated. The most dramatic change occurs with a surface burst ( Fig. 3 ). When the observer is somewhere within the source region of a near-surface burst, where the air conductivity −4 −2 varies between 10, and 10, siemens ∕ meter, the resultant electric field is predominantly vertical, and the resultant magnetic field is polarized perpendicular to the plane of the figure. These field strengths can be significantly higher than their early-time high-altitude EMP equivalents, although they cover significantly less area. The electric field polarization is due to the Compton-electron and ion-charge-separation fields, which tend to become perpendicular to the conducting Earth, leaving a resultant vertical electric field near the ground. As these Compton electrons move radially away from the detonation, they curve earthward and return to the detonation point through the conducting Earth. This Compton current loop then gives rise to a resultant magnetic field. When the observer is far from the detonation (outside the source region), the electric and magnetic fields begin to approximate the fields radiated from a vertical dipole, decaying with distance r as 1 ∕ r . These far fields are AccessScience from McGraw-Hill Education Page 5 of 7 www.accessscience.com generally not considered a problem to modern electronics and electrical systems. See also: ANTENNA (ELECTROMAGNETISM) . Internal EMP It is also possible for INR (both x-rays and gamma rays) to directly interact with systems, causing EMP signals internal to structures. This phenomenon has been called internal or system-generated EMP and is potentially a serious problem for satellites in orbit and for electronics in metallic enclosures on or near the ground. These forms of EMP are generated by x-rays interacting with satellites and gamma rays impinging on ground-based enclosures, producing currents of Compton electrons internally that then produce internal electromagnetic waves. They are very dependent upon the nuclear detonation, the system topology, and the relative position of one to the other.
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