Confirmation of a new concept of ICRF antenna by modelling and experiments

The new ICRF antenna concept proposed in Milanesio and Maggiora (2014) has been confirmed. In the ANTITER II code a layer of low loss dielectric replaces the artificial dielectric covering the back-plate of the antenna box. This allows producing any surface impedance Z_S at the dielectric boundary. The W7-X antenna is taken as a reference example. High Q resonances are obtained for discrete values of Z_S at a given frequency. They correspond to TE_n,0 cavity modes of the antenna box, partially filled with dielectric. These resonances can occur when the wave of the considered mode is propagating into the dielectric and evanescent in the vacuum part. The conclusions of the modelling have been experimentally verified on an antenna box partially filled with a dielectric; excited either by a strap or a side loop and loaded by a salted water dummy load. The high Q resonances are damped by the dummy load at the aperture of the antenna box but also by the losses in the dielectric. The loading also shifts their resonance frequencies. Therefore: (i) resonance tracking is mandatory either by acting on the dielectric (or equivalent one) or on the generator frequency, (ii) very low loss dielectric or equivalent artificial one is needed to avoid the dissipation of a significant part of the power in it. The strap is not needed to excite the resonance: appropriate side loops can be used and designed to be close to matching for the average loading condition.