When RF waves are applied in tokamaks with metal walls, sheath rectification effects associated with the fields induced in the scrape-off layer (SOL) may lead to enhanced plasma-wall interactions (i.e. heat-loads in the limiters, RF-induced impurity sources) which can endanger the integrity of the machine and limit the RF power. Although many codes are available to describe the wave-particle physics in the plasma core, the modelling of the RF wave interactions in the presence of a low-density plasma is much less explored since the RF physics describing the involved mechanisms is not yet fully understood and the solution of the problem is numerically demanding due to the excitation of millimetric waves in the SOL and the close interaction of these waves with the complex antenna and wall geometries.
Currently, some numerical tools are being used to simulate the RF antenna near fields in the presence of magnetized plasmas, but they have their limitations. For instance, the well-known in-house code TOPICA, which is typically used to couple realistic antenna geometries with the hot plasma inside the reactor, needs a vacuum buffer area of separation between the antenna and the plasma. Therefore, it neglects all the physical phenomena related to the interaction of the RF waves with the low-density plasma close to the antenna (as the Lower Hybrid Resonance (LHR)). Other software such as the commercial packages CST, HFSS and COMSOL or in-house codes such as MFEM, have been customized to take into account the close interaction of the near-fields with the low-density plasma, but they fail to find a solution around the LHR due to numerical instabilities associated with the finite element formulation implemented inside them. Simplifications can be used to reach convergence (neglect gyrotropy, increase electron density to avoid LHR), but the fields obtained can be very different to the real ones (even if the input impedance of the antennas are similar to the ones measured) and this difference can affect the accuracy of derived magnitudes as the sheath rectification effects, which use these fields as an input.
In this work we try to overcome all the limitations mentioned above by customizing the open-source finite element code ERMES to read measured plasma density profiles from files, incorporate these measurements in a 3D CAD representation of the RF antennas and calculate the near-fields and other relevant magnitudes in the presence of cold magnetized plasma. ERMES implements a stabilized finite element formulation which allows it to simulate near fields of the antenna in a continuous gyrotropic non-homogeneous media without limits in the minimum value of the plasma density. Benchmarking of this approach is underway and comparison against measurements, semi-analytical approaches and other codes will be presented.