A Cutoff Waveguide ICRF Coupler for Large Tokamaks

This paper proposes a cutoff waveguide Ion-Cyclotron Range of Frequencies (ICRF) coupler in heatings of fusion plasma for large tokamaks. Structures of tokamaks do not permit enough aperture areas for using cylindrical waveguides in the propagative region. However, if the power source to excite the coupler is located near its aperture, electromagnetic power is expected to pass through the cutoff region as a tunnel effect and radiate into the fusion plasma. We calculate characteristics of the normalized radiation power and the input impedance for this cutoff waveguide coupler, regarding its aperture as a slot. Numerical results show that the cutoff waveguide coupler has a few million watts handling power when the effective dielectric constant of the plasma is between 60 and 100. Experiments for a one-tenth model of actual size are performed using air and water instead of the fusion plasma. Measured input impedance showed a fine agreement with the theory for typical ones. Furthermore, experiments verify that its input impedance can be controlled by the location of a short-circuiting plunger. All these results suggest that the cutoff waveguide coupler is very attractive for ICRF heating.