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Microwave Windows

Dielectric Properties and Applications

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 to 170 GHz frequency region with output powers in excess of 1 MW.

A specific requirement in this application is that the window needs to be approx. 100 mm (4") in diameter, 1.6 mm to 2.3 mm (0.063" to 0.091") thick, depending on microwave frequency, and that the loss should be relatively uniform over the area of the window to avoid hot spots. Results of


a finite element calculation of the temperature rise at the centre of a CVD diamond traversed by a 1.2 MW beam for different values of dielectric loss show that thermal runaway is oberserved for losses above tan d  = 2 x 10-4. This is due to the decrease of the thermal conductivity of diamond with temperature. For values of loss below tan d = 10-4, the temperature rise is less than 240 °C for a simply water cooled edge, which is acceptable for diamond.

At microwave frequencies in the range 72 to 145 GHz, the absorption of radiation in low loss samples is usually measured as the decrease in the Q factor of a resonant cavity and expressed in terms of the tangent of a loss angle d.

During recent years much attention has been focused on improving the dielectric properties of CVD diamond in the mm wave band. Measurement of these properties is typically made at 145 GHz on samples of material at least 30 mm (1.18") in diameter and in excess of 0.85 mm (0.034") thick, depending on frequency. While samples prior to 1997 had a dielectric loss tangent (tan d) in the range 100 -1000 x 10-6 recent material has shown that tan d in the range < 10 - 1000 x 10-6 can routinely be achieved. The following table summarizes the properties of a dielectric grade of Diafilm supplied commercially in sizes up to 120 mm diameter x 2.25 mm thick.




Loss tangent,
d (x 10-6 )


20 - 50

Equivalent absorption
-1 )


0.0014 - 0.0035

Relative permittivity, er



Data measured at f=145 GHz (l = 2.1 mm)

The right graph shows the dielectric loss profile of a 100 mm (4") window, 1.6 mm (0.063") thick measured with a scanning open cavity resonant system with a spatial resolution of 6 mm (0.24"). It can be seen from this graphic that values of tan d below 10-5 (close to or below the sensitivity of the equipment used for these measurements) are possible and that the material is uniform over the active aperture of the window.

The predicted temperature excursion in a window such as that shown for a 1.2 MW beam is less than 15 °C, and therefore this material is potentially suitable for Gyrotron tubes of powers well in excess of 1 MW. This places CVD diamond as the only material capable of running continuous wave at powers in excess of 500 kW without resorting to exotic cryo-cooling systems as is the case with sapphire.

*Courtesy of R. Heidiger, R. Spoerl, Forschungszentrum Karlsruhe

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