Near-Field Radiated from an Overmoded Corrugated Waveguide

Modern Electron Cyclotron Resonance Heating (ECRH) systems in Nuclear Fusion Devices employ high-power gyrotrons operating in the millimeter wave frequency range. Typical output powers are at or above 1 MW with pulse lengths of several seconds up to continuous wave (CW). Low-loss transmission can be achieved by optical transmission via reflecting metallic mirrors or with corrugated waveguides employing the low-loss HE11 mode. In case of waveguide transmission lines, the Gaussian-like beam ejected from the gyrotron is coupled to the waveguide via a set of beam conditioning mirrors. Similarly, at the output of the transmission line, the beam is radiated from the open-ended waveguide via focusing mirrors to the launching antenna. This is possible due to the excellent coupling of the HE11 mode to the Gaussian free-space TEM00 mode. However, near field focusing effects cannot be neglected, since the applied quasi-optical mirrors are all placed within the near field region. This holds especially for CW systems, where cooling of mirror surfaces can become critical. Near-field focusing effects are investigated both numerically and experimentally.