April 23, 2009 High-power microwave (HPM) bombs that use an enormous electromagnetic radio pulse to disable computers, electronics, vehicles, guided missiles and communications while leaving people and structures unharmed have been under investigation in research labs for a number of years. Until recently these weapons have been impractically large at over 3.5 meters long, but researchers at Texas Tech University have now built a self powered device with U.S. Army funding that measures 15 cm in diameter and only 1.5 meters long, making it small enough to be considered portable.

The device being tested at an arsenal in Huntsville, Alabama should produce a peak power of 35 MW with a pulse length of 100 to 150 nanoseconds, emitting a microwave beam in the 2- to 6-GHz range.

You may be wondering how so much energy can be generated with such a small portable device. Firstly it is due to the apparent power generated by compressing a lot of energy into a very short period of time. Where electrical power is normally measured in kilowatt hours on your utility bill, the peak pulse of this E-bomb lasts just 36 billionths of an hour. Secondly, an E-Bomb is a device that can convert the output of high explosives into radio waves. The 1.5 meter long Texas Tech HPM contains three main components: a power generator in the form of a flux compression generator (FCG), a microwave source called a vircator (for virtual cathode oscillator), and an antenna that radiates the resultant high-power microwave radiation.

The source of all this power is the Flux Compression Generator (FCG). In an FCG, the energy is primarily stored as chemical energy in an explosive like plastic C4. It consists of a metal pipe with a helical stator coil wound inside it like a solenoid and a second smaller diameter armature tube which contains the C4 explosive, with an insulating layer between the two in a coaxial arrangement. The process is started with a 12-volt lead acid battery one end of the coil which provides a field current. Once detonated the explosive front propagates through the explosive in the armature pressing the inner pipe against the outer, rapidly compressing the magnetic field and generating a pulse of electromagnetic energy. An FCG is a one use device as it is destroyed by the explosive and although they are relatively cheap, they become less efficient as they get smaller.

In the second part of the process, the FCG’s energy pulse is fed through an inductor producing a voltage of about 100 kilovolts. This voltage powers the vircator, which converts the energy into microwaves. The same vircator can also be driven by power sources other than a FCG such as explosive or propellant driven Magneto-Hydrodynamic (MHD) generators or by a nonexplosive power generator that don't self-destruct such as a Marx generator, although these tend to be much larger than a simple FCG.

Texas Tech is working on a Marx generator in the hopes of making a portable directed energy weapon called a microwave cannon. The first application may be to stop vehicles by using a HPM to destroy the electronics and shut the engine down.

The amount of damage an E-bomb can do depends on its ability to couple the energy into the target. There are several ways for the microwave power to enter a system. Front door coupling happens when the pulse weapon couples with an antenna associated with radar or communications equipment. Back door coupling occurs through fixed electrical wiring and cables that connect equipment like mains power or telephone wires. Lower frequencies work better on standing wiring while higher frequencies can work better through antennas. In both cases a high voltage standing wave enters the equipment and these spikes cause damage to electric power supplies and electronic components. For example a typical semiconductor such as a microprocessor is designed to operate at 3.3 – 5 volts. A large voltage spike can do extensive damage often requiring the replacement of most semiconductors in the equipment.

Lab testing on the portable HPM has begun, but the technology is not expected to reach the field any time soon.

Paul Evans

Via: IEEE Spectrum (Image: http://blog.puppetgov.com/).