Speaker
Description
One of the main challenges in magnetically confined fusion research is the development of plasma-facing materials able to withstand the harsh environment of long-term plasma exposure. Linear plasma devices are widely used to address this issue. GyM [1] is one such device, capable of generating steady-state plasmas with electron temperatures up to 15 eV, densities in the range of 10¹⁵–10¹⁷ m⁻³, and ion fluxes up to 10²¹ m⁻²s⁻¹. GyM is currently being upgraded to BiGyM within the framework of the NEFERTARI project, funded by NextGenerationEU. The upgrade introduces a pair of helicon wave-generating birdcage radiofrequency (RF) antennas [2] to extend the operational range toward divertor-relevant plasmas with densities up to 10¹⁹ m⁻³ and ion fluxes of 10²³ m⁻²s⁻¹.
The work carried out in this project focused on validating the initial design choices taken for the device by investigating antennas performance and wave propagation. The figure of merit for this scope was the RF power density deposited by the wave, since maximizing such quantity represents the optimal energy transfer to the plasma.
A full 3D finite element model was developed using COMSOL [3], which reproduces the electromagnetic behaviour of the device and allows to study the propagation of the helicon waves generated by the antennas in a plasma. The model sensitivity to key physical parameters (neutral pressure and temperature, and plasma temperature) was tested and the effect of different antenna phasing on the helicon wave patterns was investigated in a hydrogen plasma. A set of different vessel geometries was investigated, each with a specific magnetic field profile, reaching plateau values of 18 mT. The results of this analysis allowed to identify the configuration that maximises the power deposition at the sample position and the best antennas phasing. The maximum power density value of about 105 W/m³ can be reached at the sample, up to two orders of magnitude higher than in the least favourable configurations.
The modelling activity supported the validation of the optimal design for BiGyM and offers a valuable tool for assessing antennas performances and serves as a reference on plasma operating conditions.