A GYROTRON is a radio-frequency device which, when used to power a weapon, efficiently produces extremely short, very high-energy pulses of microwave and millimeter wave radiation. {See illustration above.}

It relies on a fundamental principle of physics: Namely, that the direction and speed of a charged particle are influenced by a magnetic field. Control the nature of the field, and you can control the velocity of the electrons, whose motion in turn produces electromagnetic waves. It works like this:

1) An electron gun (not unlike that in a TV picture tube) releases a number of electrons into a chamber surrounded by a cylindrical "gun magnet coil." The combined effect of the gun's focusing properties and the coil's magnetic field bends the direction of the electrons, forcing them into a swirling "cyclonic" motion.

2) The swirl then enters the next cavity, which is permeated by a second and much stronger magnetic field generated by the main magnet coils. This further alters the motion of the electrons. The electron flow and the microwave signal in the cavity reinforce each other, causing the beam to bunch together in a compressed stream and to vibrate at a given resonance. The magnitude of the magnetic field and the dimensions of the cavity together determine the frequency.

3) The pulsating cloud of electrons and its attendant radio-frequency wave then move to an intermediate section called the "interaction cavity." Thereafter, the beam emerges into the "beam collector area." Here the stream of electrons is dissipated onto the water-cooled walls of the chamber, leaving the RF signal to exit through the "output waveguide."

The gyrotron effects can be achieved using room-temperature components. But to boost the strength of the pulse and generate wave frequencies in the neighborhood of 60 gigahertz (60 billion cycles per second), manufacturers such as Varian Associates in Palo Alto, Calif., use supercooled coils. Because electrical conductivity increases as temperature decreases, liquid helium is employed to make the main magnet superconducting.

From a military point of view, gyrotron-based pulse weapons are attractive because they can penetrate the atmosphere at certain frequencies, without interference from smoke, dust or similar particles. And compared to other devices, they can engage and "soft kill" a greater number of targets at longer ranges with the same amount of energy.