Development of CVD diamond r.f. windows for ECRH

The recent development of CVD diamond windows for high power gyrotrons is summarized. Highlights of this work include the manufacture of a window 100 mm in diameter and 1.6 mm thick having a uniform loss tangent of <1×10−5 at 145 GHz, and a window 120 mm in diameter and 2.25 mm thick with losses below 3×10−5 at 145 GHz. Moreover, the dielectric loss and permittivity have been found to be relatively stable over a wide temperature range from 100 to 700 K compared to other materials such as sapphire or gold doped silicon. More recently, development work has concentrated on achieving a robust and cost effective manufacturing process able to produce windows matching the requirements of 1 MW CW gyrotrons for ECRH applications. Typical dielectric properties for this new grade of material are presented using a 110 GHz gyrotron window 114 mm in diameter and 1.715 mm thick as an example. Data show a loss tangent in the region of 2×10−5 at 145 GHz across the 80 mm clear aperture region of the disc. A numerical model is used to calculate the temperature and stress profile of the window when subjected to high power beam conditions. It is shown that these dielectric properties, in conjunction with a thermal conductivity in the region of 2000 W m−1K−1 at room temperature and a fracture strength in excess of 250 MPa, make this grade of diamond suitable for use as an output window on 1 MW CW gyrotrons. A new method of mounting CVD diamond windows to metal flanges has been developed to allow vacuum bakeout to be performed at 550°C compared to the previous limit of 450°C. This increase in bakeout temperature promises dramatic reductions in the time taken to achieve optimum vacuum conditions in the gyrotron. The new mounting utilizes a novel graded seal arrangement to achieve low thermal mismatch stresses and good dimensional stability during thermal cycling. A 75 mm diameter window mounted using this method has been shown to survive five thermal cycles up to 550°C for 1 h without degradation of its sealing performance.