The design, analysis, and manufacturing of an oversized circular metallic corrugated waveguide with rectangular and square grooves for transmitting power from gyrotron to tokamak or dummy load have been carried out. To carry high power at millimeter wave with lower transmission loss, a corrugated waveguide is preferred. A corrugated waveguide with HE11 mode gives lower attenuation than a smooth circular waveguide with TE11 mode. The theory behind the depth and width selection of corrugations required to carry the linearly polarized (HE11) mode is explained in this paper. The proposed structures are designed and simulated in CST microwave studio software. Rectangular and square groove circular corrugated waveguides each having a length of 500\,mm were fabricated and tested using ZVA50 vector network analyzer. Based on the performance results, it is derived that the square groove corrugated waveguide gives lower insertion loss of 0.08 dB/meter than rectangular groove corrugated waveguide which gives insertion loss of 0.11 dB/meter.
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