Compression of frequency modulated pulses using a high order helically corrugated waveguide

A method was studied to generate high-power short-microwave pulses by using a principle of compression (reduction in pulse duration accompanied by an increase in pulse amplitude) of a frequency-swept wave packet propagating through a novel dispersive medium. A new helically-corrugated waveguide was designed and constructed to act as the frequency dispersive medium. It was constructed from copper and had 5-fold right-handed corrugations (m=5), which coupled a circularly polarized TE3,1 mode having large group velocity with a near-cutoff counter-rotating TE2,2 mode. The use of these relatively high-order modes ensured sufficient RF breakdown strength at GW powers. Simulations and experiments were carried out using an X-band relativistic backward wave oscillator (RBWO) designed to produce a chirped output signal, with power of hundreds of MWs, being compressed to a multi-GW pulse using the 5-fold helically corrugated waveguide. When the top of the voltage pulse driving the RBWO decreased from 550kV to 300 kV and the beam current increased from 8 to 10 kA the RBWO generated radiation with power of 0.6-0.8 GW whilst frequency sweeping from 10 to 9.6 GHz over 15 ns. Compression of pulses down to a half-width of 2.2 ns accompanied by a 4.5-fold power increase up to a value of 3.2 GW was achieved. This microwave compression method has the potential to compress 750MW, 80ns duration chirped pulses to produce world record pulses of power 18GW.

Demonstrating the potential of helical compressor operation means that these passive microwave components can be applied relatively easily at the output of commercially available frequency agile amplifiers or tuneable oscillators to directly increase the peak power and hence increase the range of RaDAR for applications based on these generators. Techniques for increasing the peak power of vacuum tubes can be problematic as it requires operation of the microwave source at either higher voltages or currents. The use of a passive microwave compressor at the output of an amplifier to increase the peak power can be achieved without the need for a further increase in beam voltage or current, hence removing the need for more complicated power supplies and increased X-ray shielding The generation of short pulse high power microwave radiation has important applications in Radar.

Experiments performed at 5.8kW, demonstrated that X-band microwave pulses of 80-ns duration with a 5% frequency sweep (9.6GHz to 9.0GHz) can be compressed into 145kW, 1.5-ns pulses having 25 times higher peak power.

The research has contributed to a better understanding of the physics of this novel pulse compressor and consequently possibly its use in radar applications. British industry, for example TMD Ltd and MBDA have interest in the generation and application of high power microwave radiation and through them, this research may have commercial impact.