An Electronic Bomb or High Power Microwave (HPM) device is a weapon designed to attack electronic systems while being harmless to people.

HPM devices are designed to destroy electronic equipment in command, control, communications and computer targets. They produce an electromagnetic field of such intensity that their effect can be far more devastating than a lighting strike. The high powered energy pulse is called an Electromagnetic Pulse (EMP)

One (unclassified) approach to producing the required pulse is a device called an Explosive Pumped Flux Generator. In this a charged bank of capacitors energises a coil wrapped around a copper tube, which itself contains high explosives. On detonation, the explosives expand the tube from the back and moves rapidly forward, forcing the tube to make progressive contact with the coil and causing a short circuit. This has the effect of crushing the magnetic field at the same time as reducing the coil's inductance. The resultant spike lasts tens to hundreds of microseconds and can produce peak currents of tens of millions of Amps and peak energies of tens of millions of Joules. By comparison, a typical lighting strike produces around 30,000 Amps.

Many essential systems need a semiconductor in some way. Failure of semi-conductive chips could destroy industrial processes, railway networks, power and phone systems, car ignitions (especially gasoline driven cars) and access to water supplies. Semiconductor devices fail when they encounter an EMP because of the local heating that occurs. When a semi-conductive device absorbs the EMP energy, it displaces the resulting heat that is produced relatively slowly when compared to the time scale of the EMP. Because the heat is not dissipated quickly, the semiconductor can quickly heat up to temperatures near the melting point of the material. Soon the device will experience short-circuiting and fail.

It would effectively turn back civilisation about 200 years in the area of impact.

Hardly a 'Mostly Harmless' device, heh ?

There are (expensive) protective measures that can be taken to 'harden' critical systems, however. The first method is metallic shielding. The alternative is tailored hardening.

Metallic shielding basically uses the cage of Faraday effect and "Excludes energy propagated through fields in space." Shields are made of a continuous piece of steel or copper. A metal enclosure generally does not fully shield the interior because of the small holes that are likely to exist. Therefore, this type of shielding often contains additional elements to create the barrier. Commonly, only a fraction of a millimetre of a metal is needed to supply adequate protection.

The alternative method, tailored hardening, is a more cost-effective way of hardening. In this method, only the most vulnerable elements and circuits are redesigned to be more rugged, withstanding much higher currents.

In both cases, interfaces with the outside world are especially vulnerable: communication links or electrical connections with the outside world might exist which would still provide an impact path for the Electromagnetic Pulse

Source: New Scientist