Harmonic and higher order mode mm-wave klystrons

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.