Suitability of using diamond shaped aluminum seals for evacuated ECRH waveguide transmission line components

Oversized circular corrugated waveguides are generally employed in transmitting high power RF beams from mm-wave sources like Gyrotrons, as they have been proved to be most efficient (low loss) waveguides for transmitting high power in the mm-wave frequency range. ITER-India, the Indian domestic agency for ITER project, has the responsibility to supply a set of two, high power (1MW), long pulse (3600 s) Gyrotron sources at a frequency of 170 GHz to ITER for Electron Cyclotron Heating & Current Drive applications. ITER-India is establishing an ITER-India Gyrotron Test Facility (IIGTF) for testing 1MW class Gyrotrons to support ITER deliverables. An evacuated set of corrugated waveguide components of 63.5 mm diameter will be used to transmit the gyrotron output power into a dummy load in the test facility. The vacuum in the waveguide components ensures that there is no RF breakdown occurs while transmitting the RF power of the order of 1MW. As the waveguide components are generally made out of aluminum material, and as the RF joints are to be electrically conducting, the spring loaded metallic seals are employed between the waveguide joints to provide the vacuum sealing and the electrical continuity. Spring loaded seals are tubular in cross-section and have complicated design due to which they are costly and easily not available. In this context, an alternate seal concept, that is low cost and easily machined has been tested for its vacuum compatibility and verified its suitability for aluminum corrugated waveguides. In order to address the leak tightness with this alternate option, aluminum seals with diamond shaped edge on both sides are prepared and tested for vacuum leak tightness of the order of 10^-10 mbar-l/s and results are illustrated in this paper. Apart from the vacuum compatibility, the other main concern that was to be addressed by these tests was to ensure that the sharp edge (diamond shape) of the seal does not permanently damage the waveguide sealing surface as the materials of the seal and the waveguide is the same. This paper presents problem identification, approach, test schematic, the leak tightness results and suitability of using diamond shaped aluminum seals for high power evacuated corrugated waveguide transmission line components. (author)