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

波导可用来压缩调频脉冲。考虑压缩一个频率随时间从一个频率（w1）到另一个频率（w2）呈单频变化的准单色脉冲。然后，这个脉冲沿着色散介质传播，在色散介质中，波的群速度仅是频率的函数。如果色散介质中的波群速度是频率的递增函数，vgr(w2) > vgr(w1)，则脉冲尾部将超过其前缘，如果损失足够低，则导致脉冲缩短，幅度相应增长。压缩实验首先使用光滑的金属波导进行，由于其处理高功率的能力而被发现具有吸引力。然而，作为一个强大的压缩器，光滑波导的一个严重缺点是它的工作非常接近截止点。在最佳情况下，输入脉冲开始时的频率应仅比截止频率高0.5-1%。如果在大功率放大器的输出端使用这样的压缩器，那么放大频带的低频部分将反射回放大器，导致其可能的寄生自振荡（在非常高的功率下，使用单向元件的射频隔离是不可能的）。我们提出设计和构造一个以其内表面的螺旋波纹作为频散介质的波导，以耦合一对具有明显不同群速度的光滑波导的圆偏振部分模式。将研究使用由最先进的可编程任意波形发生器和微波扫频器产生的优化扫频脉冲的3倍螺旋波纹波导的脉冲压缩，并将理论预测与实验进行比较。我们还建议设计、制造和研究使用直径更大的5倍螺旋波纹波导作为频率色散介质来压缩由回旋管行波放大器产生的毫瓦频扫脉冲。与3倍螺旋波导相比，使用5倍螺旋结构的优点是，它需要使用更高阶的模式，因此压缩机的直径增加，这意味着在电场变得过大导致射频击穿之前，更高的峰值功率辐射可以沿着压缩机传播。在5倍螺旋压缩器中激发的高阶模式（近截止的te2,2模式与反向旋转的te3,1模式耦合）可以合成有利的波色散，导致螺旋波导直径增加1.5-2倍（没有明显的耦合带重叠），相应地增强了其射频击穿强度。将产生许多应用所需的独特的高功率（多兆瓦）、短脉冲（~1ns）辐射。

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.