Optimized overmoded TE01-TO-TM11 mode converters for high-power millimeter wave applications at 70 and 110 GHZ

Mode coupling in bent, oversized, smooth-walled circular waveguides was studied by means of numerical integration of coupled-mode equations in order to optimize high-power TE01-to-TM11 mode transducers at 70 and 110 GHz. Such transducers are used in the mode conversion sequence TE
on
to TE01 to TM11 to HE11 for generating the linearly polarized HE11 (Gaussian like) mode from circular electric TE
on
gyrotron modes. The balanced HE11 hybrid mode is in many respects ideal for electron cyclotron resonance heating (ECRH) of plasmas in thermonuclear fusion research, or for other technical applications. The lowest level of unwanted spurious modes is achieved with sinusoidal curvature distribution instead of constant curvature. The calculated efficiencies of 97.9 % at 70 GHz and 95.2 % at 140 GHz (I.D. = 27.8 mm, ohmic attenuation is included) are in excellent agreement with the measured ones (97.6±0.1)% and (95±1)%, respectively.