A new approach to ICRF antennas modeling based on coupling the surface impedance matrix of the plasma to commercial antenna codes

Although modern commercial antenna codes can handle the complex 3D geometry of ion cyclotron resonance frequency (ICRF) antennas they still can not correctly describe hot fusion plasmas. In view of the impact the plasma has on the antenna-near fields and hence the need to use a sensible mock-up for the plasma behaviour, ICRF antenna modeling is currently mostly done by substituting the plasma with suitably chosen dielectric [1,2]. One of the limitations of this approach is the incorrect evaluation of the fields on the plasma surface. In this work a theoretical basis is given and a practical implementation is shown for coupling the spectral plasma surface impedance matrix [3] to modern commercial antenna codes for self-consistent correct calculation of the fields and scattering ('S') parameters of the ICRF antennas, hereby allowing to interface the antenna coupling code with a much more realistic model for capturing the subtleties of magnetized plasmas. The approach uses subsequent application of induction and uniqueness theorems of electromagnetism. In a first step the fields of the antenna in vacuum are computed. Once these incident fields are known one can use the surface impedance of the plasma to calculate the total electric and magnetic fields on the plasma surface and the power flow into the plasma. The evaluation of the S-parameters of the antenna requires a second step. We use the obtained tangential electric field on the plasma surface as a necessary boundary condition to solve the equivalent problem and find the Sparameters of the antenna and all the fields around it. This new approach is similar in physics potential to the TOPICA code [4] for its application to antenna design. Moreover, in the new approach it is possible to simulate the presence of cold low density plasma in the antenna box, which is needed for the correct evaluation of the fields and for addressing the sheath effect. The here presented, new approach is numerically more efficient and user-friendly than codes that attempt to directly incorporate the plasma response in the antenna computation. The paper also compares results obtained using the new approach with those obtained by other modeling methods. A new approach to the problem of the minimization of the toroidal electric field of the ICRH antennas is also proposed.