The Vircator (VIRtual CAthode OscillatOR) is a vacuum tube microwave frequency oscillator, configured as a triode or klystron. It is classed as a High Power Microwave (HPM) device and is a pivotal component in the Electromagnetic Pulse (EMP) weapons arsenal, as well as microwave power transmission.

Early work into developing the vircator was done through a contract tasking the Tomsk Polytechnical University who have openly published their work.

The output from a vircator tube is a waveguide and possibly an antenna, and this directs the HPM beam to its target. Peak power levels of 1010 watts are possible. The wavelength of power generation from a vircator device is in the microwave frequency bands, eg 4Ghz and higher, and include useful power outputs into the X-Ray wavelengths. At these short wavelengths, the destructive power transfer into more sensitive electronics becomes very effective, with a destructive EMP type of effect.

The vircator is often powered by what is called a Marx generator (first described by Erwin Otto Marx in 1924), which produces a short duration very high voltage DC, high current pulse. In past trials, a 27 kV DC power supply was used to charge a 20-stage Marx generator, where it effectively charged 20 capacitors in parallel to the 27 kV potential. When activated, these capacitors are discharged in series with (spectacular) spark gap switching. In this configuration, the Marx generator would output a 265 kV, 3500 ampere, 21 nanosecond pulse.

Alternate power generation systems exist to achieve similar ‘DC pulse’ power sources. In electronic warfare applications, these are effectively a sized up version of the shake-a-torch where a magnet (possibly superconducting) is forced through a generator coil with the aid of conventional explosives, or indeed using a lower yield nuclear explosion. Naturally these generators produce multi gigawatt power outputs for up to 200 nanoseconds, before being destroyed by the explosive that power them.

While the Vircator was effectively invented in the 1950's it wasn't until 2009 that a PORTABLE E-BOMB test in Huntsville, Ala., researchers are testing a new high-power microwave bomb--one that creates an electromagnetic pulse capable of disabling electronics while leaving people and structures unharmed. The tests mark the first time such a device has been shrunk to dimensions that could make it portable enough to fit in a missile.
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References and further reading

IEEE Spectrum carried a detailed article in 2003 titled “Dawn of the e-Bomb” attached below.