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

A waveguide can be used to compress frequency-modulated pulses. Consider compression of a quasi-monochromatic pulse with frequency monatonically varying in time from one frequency (w1) to another frequency (w2). This pulse is then propagated down a dispersive medium in which the group velocity of the wave is a function of frequency only. If the wave group velocity in the dispersive medium is an increasing function of frequency, vgr(w2) > vgr(w1), then the tail of the pulse will overtake its leading edge, resulting in pulse shortening and a corresponding growth in the amplitude if the losses are sufficiently low. Compression experiments were first carried out using a smooth metal waveguide and were found attractive because of its capability of handling high power. However a serious drawback of a smooth waveguide as a powerful compressor is its operation very close to the cut-off. In optimum cases, the frequency at the beginning of an input pulse should be only 0.5-1% above the cut-off frequency. If one uses such a compressor at the output of a powerful amplifier, then the low-frequency part of the amplification band will be reflected back to the amplifier resulting in its possible parasitic self-oscillation (RF isolation using unidirectional elements is impossible at very high power). We propose to design and construct a waveguide with a helical corrugation of its inner surface as the frequency dispersive medium, to couple a pair of circularly polarized partial modes of the smooth waveguide having significantly different group velocities. Pulse compression using a 3-fold helically corrugated waveguide using an optimised frequency swept pulse generated by a state-of-the-art programmable Arbitary waveform generator and microwave sweeper will be studied, with the predictions of theory compared to experiment. We propose also to design, build and investigate the use of a larger diameter 5-fold helically corrugated waveguide as the frequency dispersive medium to compress MW frequency swept pulses generated by a gyrotron travelling wave amplifier. The advantage of using a 5-fold helical structure as compared to a 3-fold helical waveguide is that it requires the use of a higher order mode and hence the diameter of the compressor is increased which means that higher peak power radiation can be propagated down the compressor before the electric fields becomes excessively large resulting in RF breakdown. A favourable wave dispersion can be synthesized for the higher-order modes (near cut-off TE2,2 mode which couples to a counter-rotating TE3,1 mode) excited in a 5-fold helical compressor resulting in an increase of the helical waveguide diameter by a factor of 1.5-2 (without significant overlapping of the coupling bands) and correspondingly in an enhancement of its RF breakdown strength. Unique high power (multi-MW), short pulse (~1ns) radiation required in a number of applications will be generated.