Windows for High Power Gyrotron Tubes

CVD diamond windows are now being used in commercial high power CO2 lasers. An example is the Rofin Sinar Diffusion-cooled CO2 slab laser with an output power of 3500 W [48] shown in Fig. 23. In this laser, the CVD diamond window is used to seal the slab cavity, as shown schematically in Fig. 24. 

Measurements of dielectric loss in high quality CVD diamond at millimetre wave microwave frequencies, reported in 1993 [2], furnished for the first time clear evidence that the loss tangent of CVD diamond could be comparable or lower than conventional dielectrics such as sapphire or boron nitride. This initial data and subsequent first dedicated dielectric property studies [61] intensified the considerable amount of interest that had already existed in the nuclear fusion community for CVD diamond as a high power window material [62] especially for the development 

2 Applications of Diamond Synthesized by Chemical Vapor Deposition 

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At microwave frequencies in the range 72-145 GHz, the critical parameters for high-power transmission are the dielectric characteristics of the window material the dielectric loss factor tan 5 and the permittivity e[. (or the refractive index n = because they affect power absorption and reflection [42]. The dielectric loss factor tan 8 in low loss samples is usually measured as the decrease in the Q factor of a resonant cavity [43]. Low dielectric loss materials find application as the output windows of high-power microwave tubes. A specific case is that of windows for Gyrotron tubes operating in the 70-170 GHz frequency region with output powers in excess of 1 MW, as will be discussed later. 

The detailed requirements for the use of diamond as a window material in high power Gyrotron tubes can be listed as follows. 

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