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An electromagnetic pulse is a burst of electromagnetic energy. It may occur in the form of a radiated, electric or magnetic pulse depending on the source. Electromagnetic pulse is commonly abbreviated EMP and pronounced by saying the letters separately (E-M-P). EMP is generally damaging to electronic equipment, and its management is an important branch of electromagnetic compatibility (EMC) engineering.
An electromagnetic pulse is a relatively short burst of electromagnetic energy over a spread of frequencies. Pulses are typically characterised by:
An electromagnetic pulse, EMP, also sometimes called transient disturbance, arises where the source emits a short-duration pulse of energy. The energy is usually broadband by nature, although it often excites a relatively narrow-band damped sine wave response in the victim. Some types are generated as repetitive and regular pulse trains.
Types of EMP divide broadly into natural, man-made and weapons effects.
Types of natural EMP event include:
Types of military EMP include:
Types of (civilian) man-made EMP event also include:
An EMP typically contains energy at frequencies from DC (zero Hz) to some upper limit depending on the source. The whole range of concern is sometimes referred to as "DC to daylight", with optical (infrared, light, ultraviolet) and ionizing (X and gamma rays) ranges being excluded.
The highest frequencies are generated by NEMP bursts and continue up into the optical and ionizing ranges. Other types can leave a visible trail, such as lightning and sparks, but these are side effects of the current flow through the air and are not part of the EMP itself.
The waveform of a pulse describes how its amplitude changes over time. Real pulses tend to be quite complicated, so simplified models are often used. Such a model is typically shown either as a diagram or as a mathematical equation.
Double exponential pulse
Damped sinewave pulse
Most pulses have a very sharp leading edge, building up quickly to their maximum level. The classic model is a double-exponential curve which climbs steeply, quickly reaches a peak and then decays more slowly. However pulses from a controlled switching circuit often take the form of a rectangular or "square" pulse.
Lightning is unusual in that it typically has a preliminary "leader" discharge of low energy building up to the main pulse, which in turn may be followed at intervals by several successively smaller bursts.
EMP events usually induce a corresponding signal in the victim equipment, due to coupling between the source and victim. Coupling usually occurs most strongly over a relatively narrow frequency band, leading to a characteristic damped sine wave signal in the victim. Visually it is shown as a high frequency sine wave growing and decaying within the longer-lived envelope of the double-exponential curve. A damped sinewave typically has much lower energy and a narrower frequency spread than the original pulse, due to the transfer characteristic of the coupling mode. In practice, EMP test equipment often injects these damped sinewaves directly rather than attempting to recreate the high-energy threat pulses.
The direct effect of a large EMP is to induce high currents and voltages in the victim, damaging electrical equipment or disrupting its function. A very large EMP event such as a lightning strike is also capable of damaging objects such as trees, buildings and aircraft directly, either through heating effects or the disruptive effects of the very large magnetic field generated by the current. An indirect effect can be electrical fires caused by the heating. Most engineered structures and systems require some form of protection against lightning to be designed in.
NEMP weapons are designed to maximise such effects, especially on electronic systems, and are capable of destroying susceptible electronic equipment over a wide area. The popular media often depict such EMP effects incorrectly, causing misunderstandings among the public and even professionals, and official efforts have been made in the USA to set the record straight. 
A smaller ESD event can damage electronic circuitry by injecting a high-voltage pulse, besides giving people an unpleasant shock. Such an ESD event can also create sparks, which may in turn ignite fires or fuel-vapour explosions. For this reason, before refuelling an aircraft or exposing any fuel vapour to the air, the fuel nozzle is first connected to the aircraft to safely discharge any static.
Minor EMP events, and especially pulse trains, cause lower levels of electrical noise or interference which can affect the operation of susceptible devices. For example a common problem in the mid-twentieth century was the interference emitted by the ignition systems of gasoline engines, which caused radio sets to crackle and TV sets to show stripes on the screen. Laws had to be introduced to make vehicle manufacturers fit interference suppressors.
Like any electromagnetic interference, the threat from EMP needs to be controlled. This is true whether the threat is natural or man-made.
Therefore, most control measures focus on the susceptibility of equipment to EMP effects, and hardening or protecting it from harm. Man-made sources, other than weapons, are also subject to control measures in order to limit the amount of pulse energy emitted.
The discipline of ensuring correct equipment operation in the presence of EMP and other RF threats is known as electromagnetic compatibility (EMC).
NEMP is the abrupt pulse of electromagnetic radiation resulting from a nuclear explosion. The resulting rapidly changing electric fields and magnetic fields may couple with electrical/electronic systems to produce damaging current and voltage surges.
In military terminology, a nuclear warhead detonated hundreds of kilometres above the Earth's surface is known as a high-altitude electromagnetic pulse (HEMP) device. Typically the HEMP device produces the EMP as its primary damage mechanism. The nuclear device does this by producing gamma rays, which in turn are converted into EMP in the mid-stratosphere over a wide area within line of sight to the detonation.
Non-nuclear electromagnetic pulse (NNEMP) is an electromagnetic pulse generated without use of nuclear weapons. Devices that can achieve this objective include a large low-inductance capacitor bank discharged into a single-loop antenna, a microwave generator and an explosively pumped flux compression generator. To achieve the frequency characteristics of the pulse needed for optimal coupling into the target, wave-shaping circuits and/or microwave generators are added between the pulse source and the antenna. Vircators are vacuum tubes that are particularly suitable for microwave conversion of high-energy pulses.
NNEMP generators can be carried as a payload of bombs, cruise missiles (such as the CHAMP missile) and drones, with diminished mechanical, thermal and ionizing radiation effects, but without the political consequences of deploying nuclear weapons.
The range of NNEMP weapons (non-nuclear electromagnetic pulse bombs) is much less than nuclear EMP. Nearly all NNEMP devices used as weapons require chemical explosives as their initial energy source, producing only 10−6 (one millionth) the energy of nuclear explosives of similar weight. The electromagnetic pulse from NNEMP weapons must come from within the weapon, while nuclear weapons generate EMP as a secondary effect. These facts limit the range of NNEMP weapons, but allow finer target discrimination. The effect of small e-bombs has proven to be sufficient for certain terrorist or military operations. Examples of such operations include the destruction of electronic control systems critical to the operation of many ground vehicles and aircraft.
The concept of the explosively pumped flux compression generator for generating a non-nuclear electromagnetic pulse was conceived as early as 1951 by Andrei Sakharov in the Soviet Union, but nations keep work on non-nuclear EMP classified until similar ideas emerge in other nations.
Large EMP simulators were built in the United States, the Soviet Union, the United Kingdom, France, Germany, the Netherlands, Switzerland and Italy. The purpose of these large EMP simulators was to test equipment and vehicles (including ships and aircraft) for their resistance to high-altitude nuclear EMP.
Information about the EMP simulators used by the United States during the latter part of the Cold War, along with more general information about electromagnetic pulse, is now in papers under the care of the SUMMA Foundation, which is hosted at the University of New Mexico.
The SUMMA Foundation web site documents the huge wooden ATLAS-I simulator (better known as TRESTLE, or "The Sandia Trestle") at Sandia National Labs, New Mexico, which was the world's largest EMP simulator. Nearly all of these large EMP simulators used a specialized version of a Marx generator. The SUMMA Foundation offers a short documentary on its web site called TRESTLE: Landmark of the Cold War.
The US Navy also has a facility called the Electro Magnetic Pulse Radiation Environmental Simulator for Ships I (EMPRESS I).
The large forces generated by electromagnetic pulses can be used to shape or form objects as part of their manufacturing process.
References to EMP weapons in popular fiction go back at least to 1965, however EMP did not gain a significant presence until the mid 1980s.
The popular media often depict EMP effects incorrectly, causing misunderstandings among the public and even professionals, and official efforts have been made in the USA to set the record straight.