Hydrogen ion-induced surface damage of copper grids in RF ion sources for fusion NBI
Xiaona Li, Jianhua Lv, Weiyuan Ni, Chao Chen, Miao Zhao, Xingquan Wang, Mengchao Li, Guangjiu Lei
Abstract
• A numerical model shows extraction voltage affects H + implantation energy and Cu grid swelling, critical for NBI optimization. • Conical surface, with ∼ 50 % lower flux, has worse swelling due to suppression-shadowing, challenging flux-damage links. • H-induced swelling alters roughness, changing work function and impairing ion beam extraction efficiency/stability in fusion systems. During the operation of extracting hydrogen ions in NBI RF ion systems, the plasma grid is exposed to prolonged irradiation with low-energy hydrogen ions, leading to surface damage. In this study, a hydrogen plasma environment is constructed to investigate hydrogen ion-induced surface damage of copper grid. During the irradiation, the ion extraction process is considered. Additionally, a numerical fluid model was developed to analyze ion implantation parameters on the flat and conical surfaces of the extraction aperture under different gas pressures and extraction voltages. The results reveal that different extraction voltages influence the electric field in the extraction region, thereby affecting the energy of hydrogen ion implantation and resulting in surface damage. Surprisingly, the copper grid exhibits more severe surface swelling on the conical surface compared to the flat surface after prolonged irradiation, even though the irradiation flux on the conical surface is approximately half that on the flat surface. The behavior of severe swelling on the conical surface can be attributed to the synergistic effect of suppression and shadowing. The surface swelling induced by hydrogen ions can alter surface roughness, subsequently impacting work function and the efficiency and stability of ion beam extraction.