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 10$^{15}$–10$^{17}$ m$^{-3}$, and ion fluxes up to 10$^{21}$ m$^{-2}$ s$^{-1}$. 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 10$^{19}$m$^{-3}$ and ion fluxes of 10$^{23}$ m$^{-2}$ s$^{-1}$.
A full 3D finite element model was produced using COMSOL [3], in support of the design of BiGyM. The model reproduces the electromagnetic behaviour of the device and allows to study the propagation of the helicon waves generated by the antennas in a hydrogen plasma. This contribution focuses on presenting the model, its sensitivity to key physical parameters and the effect of different antenna phasing on the helicon wave patterns. The model was also employed to investigate the RF power density distributions in a set of different vessel geometries, each with a specific magnetic field profile, reaching plateau values of 18 mT. The results of this analysis allowed to identify the configuration that maximizes the power deposition at the sample position, reaching a maximum power density of about 10$^{5}$ W/m$^{3}$ at the sample, up to two orders of magnitude higher than in the least favorable configurations.
The modeling 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.
Acknowledgements
This work has been carried out within the framework of Italian National Recovery and Resilience Plan (PNRR), funded by the European Union – NextGenerationEU (Mission 4, Component 2, Investment 3.1 – Area ESFRI Energy – Call for tender No. 3264 of 28-12-2021 of Italian University and Research Ministry (MUR), Project ID IR0000007, MUR Concession Decree No. 243 del 04/08/2022, CUP B53C22003070006, "NEFERTARI – New Equipment for Fusion Experimental Research and Technological Advancements with Rfx Infrastructure").
References
[1] A. Uccello et al. (2023), Front. Phys., 11:1108175
[2] Ph. Guittienne et al. (2024), DOI: 10.1088/978-0-7503-5296-3.
[3] COMSOL Inc Multiphysics software from COMSOL – http://comsol.com
