## 2 Overview of Nuclear EMP  
A high-power EMP refers to an EMP with powerful energy, such as a nuclear EMP generated by a nuclear explosion at a high altitude or a non-nuclear EMP produced by an artificial generator. For nuclear EMPs, up to 50 kV/m is generated over a broadband of several Hz to several GHz. Here, 50 kV/m is defined by U.S. military standards (MIL-STD) and International Electrotechnical Commission (IEC) standards. When the distance between two points is 1 m, the mutually induced voltage is 50,000 V [22; 23]. The generated EMP causes a flow of strong electric currents in electronic circuits in electric/electronic devices and communication systems, thus destroying them in a form of flashover or melting.  
As shown in Figure 1, EMPs are classified into nuclear EMPs (high-altitude nuclear EMPs) and non-nuclear EMPs. However, non-nuclear EMPs include lightning EMPs (LEMPs) and geomagnetic storms, which are naturally occurring phenomena and distinguished from artificially generated nuclear EMPs and HPEMs.  
For nuclear EMPs, gamma rays produced by a nuclear explosion at a high altitude (usually higher than 30 km in the sky) collide with the atmosphere and radiate high-energy EMPs, which cause damage to a broad area. Specifically, high-power EMPs are produced due to the enormous energy generated by the powerful flash that persists from between several seconds to several minutes after a nuclear explosion. Although it has no fatal effect on human bodies, it causes a flow of overcurrent, momentarily, in various electric/electronic devices, communication systems, power control systems, aviation control systems, etc., thereby paralyzing their functions. A nuclear explosion produces a very short and intensive nuclear EMP. As instantaneous EMP currents are inducted on all kinds of metal conductors, the currents may penetrate the equipment or damage electric/electronic devices, resulting in their failure.  
The EMPs generated at a low altitude can be considered to be similar to HEMPs, but the characteristics of the radiation are different. To produce nuclear EMPs, a nuclear weapon must be detonated above the atmosphere. Although there is no distinct boundary to the atmosphere, the typical properties of nuclear EMPs are shown when detonation occurs at an altitude higher than 30 km. In other words, in the case of a nuclear explosion occurring at a low altitude (approximately below 20 km), the generation of a high-power EMP is less because the density of air molecules is high. However, it can be said that the physical impact of the radioactive storm and fallout is larger than that of high-power EMPs. The generated high-power EMP spreads as if a circular shape centered on the explosion point is expanding continuously. The thickness of the circular shape reaches tens of meters. If the circular shape reaches the Earth's surface, a short EMP can be felt, and after several nano-seconds, the high-power EMP increases. After several hundred nano-seconds, all electric and electronic devices stop working. High-power EMPs exist in a high-frequency band between tens of MHz and tens of GHz. The high-power EMP caused by a nuclear explosion is distributed in a wide range from a low-frequency band to hundreds of MHz.