Gyrotron tube windows

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. 

Values of the dielectric loss of CVD diamond have been measured over the past 3 years as a suitable material grade for dielectric window applications was being developed [5]. For open resonant cavity measurements, samples are usually required to be of at least 30 mm in diameter and of thickness in excess of 0.87 mm depending on the measurement frequency and the accuracy required. For recent CVD diamond, values of tan 5 below 10 have been achieved. A specific example is a window 100 mm in diameter and 1.6 mm thick which exhibited a tan 8 value of 0.6 ( 0.2) 10 . This is the lowest value so far reported for CVD diamond and would enable the material to be used as output windows in Gyrotron tubes of powers in excess of 2 MW as discussed in 2.4. 

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

Figure 31. Examples of CVD diamond windows mounted onto vacuum flanges. The two at either end show 100 mm diameter windows mounted to conventional CONFLAT vacuum flanges. The one in the middle illustrates a double flange configuration designed specifically for Gyrotron tube assemblies. All these vacuum assemblies have been tested to be vacuum tight to better than 10-9 mbar I s" after thermal cycling up to 450°C.

A metal-to-diamond bonding technique has been developed to attach CVD diamond windows to vacuum flanges or other assemblies. This is required to mount the windows to the Gyrotron tubes or reactor ports via suitable waveguide sections. Figure 31 shows examples of three 100 mm diameter CVD diamond windows mounted to two different types of flanges. The two at either end of the picture are... 

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