Harmonic and higher order mode mm-wave klystrons

速调管是一种众所周知的高效率放大器，具有简单的结构和可扩展的尺寸。它的设计通常带有基模的圆柱形可再入腔。然而，随着器件频率的增加，结构的尺寸减小。在毫米波频率下，这导致了两个问题：1)制造复杂的小尺寸结构。2)间隙电压随着间隙变短而减小，导致增益减小。文献中报道的大多数毫米波速调管概念只是微波速调管的更小版本。即使原则上尺寸可以根据频率的增加来调整，但当频率高于50 GHz时，制造挑战和束流特性是实现工作器件的巨大障碍。因此，在95 GHz左右的频率范围内也是如此，这对高带宽通信、雷达和成像应用具有极大的兴趣。本方案旨在通过两种创新的设计方案来实现95 GHz速调管来克服上述障碍。第一种解决方案是在高阶模式下运行腔，在禁带截面上选择与基模相似的Ez场分布。该设计将采用方形或矩形的凹腔，以兼容光刻制造技术。高模运行允许设计尺寸比基模运行情况下更大(至少3-5倍)的腔。这简化了技术工作，并使通过机械微加工或光刻工艺进行高质量制造成为可能。此外，束隧道可以比基模中的大，以支持更高的束电流。为了增加相互作用，将使用沿漂移管分布的若干个中间聚束器腔来增加束流调制。另一种方法使用较低频率的输入腔来调制束流。当光束沿漂移管束向下传播时，开始形成谐波，因此可以激励输入腔的整数次谐波频率处的高次模出射腔，从而起到高功率倍频的作用。由于输入可以很容易地从更低的频率获得，因此更具成本效益的高功率微波源，我们能够克服设备的任何中等增益。

The Klystron is a well-known, high efficiency amplifier, with a simple structure and scalable dimensions. It is typically designed with cylindrical reentrant cavities in the fundamental mode. However as the frequency of the device increases the size of the structure decreases. At mm-wave frequencies this leads to two problems: 1) Manufacturing the complex small scale structures. 2) The gap voltage decreases as the gap gets shorter leading to less gain.Most mm-wave klystron concepts reported in the literature are simply smaller versions of microwave klystrons. Even if, in principle the dimensions can be scaled according to the frequency increase, the fabrication challenges and the beam characteristic represent a huge obstacle to the realization of a working device when the frequency is higher than 50 GHz. This is consequently true for the frequency range around 95 GHz, which is of great interest for high bandwidth communication, radar and imaging applications.This proposal is aimed to overcome of the above-mentioned obstacle by the realization of a 95 GHz klystron by two innovative design solutions. The first solution is to operate the cavity at a higher order mode, chosen with similar Ez field distribution in the gap cross-section as the fundamental mode. The design will adopt reentrant cavities with square or rectangular shape, to be compatible with a photolithographic fabrication technique. The higher mode operation permits the design of the cavities with dimensions larger (at least 3-5 times) than in the case of fundamental mode operation. This eases the technological effort and makes possible a high quality fabrication by mechanical micromachining or by photolithographic processes. Further, the beam tunnel can be larger than in fundamental mode, to support higher beam current. In order to increase the interaction a number of intermediate buncher cavities, spaced all along the drift tube, will be used to increase the beam current modulation.A separate approach uses a lower frequency input cavity to modulate the beam current. As the beam travels down the drift tube beam harmonics start to form hence a higher order mode output cavity at an integer harmonic frequency of the input cavity can be excited hence acting as a high power frequency multiplier. As the input can be readily available from a lower frequency and hence more cost effective high power microwave source we are able to overcome any moderate gain of the device.