Harmonic and higher order mode mm-wave klystrons

速调管是一个著名的，高效率的放大器，具有简单的结构和可扩展的尺寸。它通常在基模中设计圆柱形可重入腔。然而，随着装置频率的增加，结构的尺寸减小。在毫米波频率下，这会导致两个问题：1)制造复杂的小尺度结构。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小版本。即使原则上尺寸可以根据频率的增加而缩放，但在频率高于50 GHz时，制造挑战和波束特性对实现工作器件构成了巨大障碍。因此，对于94 GHz左右的频率范围来说，这是正确的，这对通信和雷达应用非常感兴趣。本方案旨在通过两种创新的设计方案来克服上述94 GHz速调管设计和实现的障碍。第一种解决方案是在高阶模式下工作，选择在间隙截面上具有相似的Ez场分布作为基本模式。该设计将采用方形或矩形的可入腔，以兼容光刻制造技术。高模操作允许设计比基模操作尺寸更大（至少4 -5倍）的空腔。这减轻了技术上的努力，并使机械微加工或光刻工艺的高质量制造成为可能。此外，波束隧道可以比基模更大，以支持更高的波束电流。为了增加相互作用，将使用沿漂移管分布的若干中间聚束腔来增加光束电流调制。另一种方法是使用频率较低的输入腔来调制光束电流。当光束沿漂移管向下传播时，光束谐波开始形成，因此可以在输入腔的整数谐波频率下产生高阶模输出腔，从而充当高功率倍频器。由于输入可以是一个现成的高功率微波源，我们能够克服器件的低增益。

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 frequency is higher than 50 GHz. This is consequently true for the frequency range around 94 GHz, which is of great interest for communication and radar applications.This proposal is aimed to overcome of the above-mentioned obstacle to the design and realization of a 94 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 on 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 to design the cavities with dimensions larger (at least 4 -5 times) than in 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 exited hence acting as a high power frequency multiplier. As the input can be a readily available high power microwave source we are able to overcome the low gain of the device.