A double disk window for the JET EP ECRH system

Detailed calculations of the microwave transmission characteristic of a double disk window for the Joint European Torus (JET) EP ECRH project are presented in this paper. In order to obtain sufficiently low power reflection (<1%), the pair of chemical vapor deposition (CVD)-diamond disks have to be within the following geometrical specifications: mean thickness of both disks used in one window unit ddisk0=(1.111±0.010) mm; mechanical tolerance of each disk ±0.005 mm. Calculations show that depending on ddisk0, the optimum gap distance is between dgap≅3.4 mm and 3.2 mm for 113.3 GHz. The optimum gap distance dgap is slightly different for 170.0 GHz, namely between dgap≅3.2 and 3.0 mm. With the assumed thickness tolerance of ±5 μm the reflectivity of the double disk window unit is in the worst case −25 dB at 113.3 GHz and about −22 dB at 170 GHz. In both cases the bandwidth is as large as about 2 GHz.