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In neutral beam injection (NBI), which is a primary auxiliary heating method for tokamak plasmas, the negative hydrogen ion source (NHIS) functions as a critical front-end component governing neutral beam quality. The performance of NHIS remains a key challenge. This work presents a three-dimensional (3D) fluid model, which is developed for a double-driver NHIS to simulate and optimize surface-generated negative hydrogen ion density. A comparison of plasma parameters between the NHIS with Cs and without Cs shows that surface generation yields negative ion density one order of magnitude higher than volume generation. However, the presence of the magnetic filter field induces asymmetry in negative ion density within the extraction region. To improve this asymmetry, two approaches are proposed: (1) increasing the power of one of the drivers and (2) adding a spacer plate to the expansion region. After increasing the power of Driver I from 50 to 56 kW, the H– density asymmetry at the y = 25 cm intercept on the xy-plane (z = –22 cm) decreases from 0.04 to 0.01, and the value of H– density increases. Following the addition of a spacer plate, the H– density asymmetry further decreases to 0.004, but the value of H- density also shows a significant reduction. Finally, adding a magnetic shield to the back plate of the expansion region further optimizes H– density from 1.48×1017 m–3 to 2.50×1017 m–3, yielding a 69% increase downstream. This is because increased plasma transport into the expansion region enhances the dissociation rate of H2 molecules, thereby yielding more H atoms. The attenuation of the magnetic filter field in the driver region after adding a magnetic shield also enhances the symmetry of the H– density.
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Keywords:
- neutral beam injection system /
- negative hydrogen ion source /
- 3D fluid modeling /
- negative ion
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图 1 双驱动负氢离子源的三维结构示意图
Figure 1. Schematic diagram of the 3D geometric structure of a double-driver negative hydrogen ion source.
图 2 无Cs源(第一列)与有Cs源(第二列)中xz平面(y = 25 cm)的等离子体参数分布. 压强为0.6 Pa, 每个源功率为50 kW
Figure 2. Distribution of plasma parameters in the xz-plane (y = 25 cm) in a double-driver ion source without Cs and with Cs. The pressure is 0.6 Pa and the power of every driver is 50 kW.
图 3 无Cs源与有Cs源中xy平面(z = –22 cm)的等离子体参数分布. 压强为0.6 Pa, 每个源功率为50 kW
Figure 3. Distribution of plasma parameters in the xy-plane (z = –22 cm) in a double-driver ion source without Cs and with Cs. The pressure is 0.6 Pa and the power of every driver is 50 kW.
图 4 有Cs源中增加源I功率和添加隔板后xz平面(y = 25 cm)的等离子体参数分布, 其中压强固定为0.6 Pa
Figure 4. Distribution of plasma parameters in the xz-plane (y = 25 cm) after increasing the power of driver I and adding a spacer plate in ion source with Cs, where the pressure is fixed at 0.6 Pa.
图 5 有Cs源中增加源I功率和添加隔板后xy平面(z = –22 cm)的等离子体参数分布, 其中压强固定为0.6 Pa
Figure 5. Distribution of plasma parameters in the xy-plane (z = –22 cm) after increasing the power of driver I and adding a spacer plate in ion source with Cs, where the pressure is fixed at 0.6 Pa.
图 6 截线y = 25 cm上的H–密度分布
Figure 6. H– density at the intercept y = 25 cm.
图 7 (a)无磁屏蔽与(b)添加磁屏蔽情况下xz平面(y = 25 cm)的过滤磁场分布; (c) 轴线上的磁场分布
Figure 7. Magnetic filter field distribution in the xz-plane (y = 25 cm) (a) without and (b) with magnetic shield; (c) profile of the magnetic field distribution on the driver axis.
图 8 有Cs源中添加磁屏蔽后xz平面(y = 25 cm)的等离子体参数分布. 压强为0.6 Pa, 每源功率为50 kW
Figure 8. Distribution of plasma parameters in the xz-plane (y = 25 cm) without and with magnetic shield in ion source with Cs. The pressure is 0.6 Pa and the power of every driver is 50 kW.
图 9 有无磁屏蔽条件下等离子体参数沿轴向的分布
Figure 9. Distribution of plasma parameters along the axial direction with and without magnetic shield.
图 10 有Cs源中添加磁屏蔽后xy平面(z = –22 cm)的等离子体参数分布. 压强为0.6 Pa, 每源功率为50 kW
Figure 10. Distribution of plasma parameters in the xy-plane (z = –22 cm) without and with magnetic shield in ion source with Cs. The pressure is 0.6 Pa and the power of every driver is 50 kW.
图 11 有无磁屏蔽条件下截线y = 25 cm上的等离子体参数分布
Figure 11. Distribution of plasma parameters at the intercept y = 25 cm with and without magnetic shield.
表 1 模型中考虑的反应
Table 1. Reactions included in this model.
反应 描述 参考文献 1. $ {\text{e}} + {{\text{H}}_2} \to {\text{e}} + {{\text{H}}_2} $$ \mathrm{e}+{\mathrm{H}}_{2}\to \mathrm{e}+{\mathrm{H}}_{2} $ 弹性散射 [25] 2. $ e + H \to e + H $ 弹性散射 [25] 3. $ {\text{e}} + {{\text{H}}_2} \to 2{\text{e}} + {\text{H}} + {{\text{H}}^ + } $$ \mathrm{e}+{\mathrm{H}}_{2}\to 2\mathrm{e}+\mathrm{H}+{\mathrm{H}}^{+} $ Dissociative ionization [26] 4. $ {\text{e}} + {{\text{H}}_2} \to 2{\text{e}} + {\text{H}}_2^ + $ Molecular ionization [26] 5. $ {\text{e}} + {{\text{H}}_2} \to {\text{e}} + {\text{H}} + {\text{H}} $$ \mathrm{e}+{\mathrm{H}}_{2}\to \mathrm{e}+\mathrm{H}+\mathrm{H} $ Dissociation [27] 6. $ {\text{e}} + {{\text{H}}_2} \to {\text{e}} + {\text{H}} + {\text{H}}(n = 2) $$ \mathrm{e}+{\mathrm{H}}_{2}\to \mathrm{e}+\mathrm{H}+\mathrm{H}(\mathrm{n}=2) $ Dissociation [28] 7. $ {\text{e}} + {\text{H}} \to 2{\text{e}} + {{\text{H}}^ + } $$ \mathrm{e}+\mathrm{H}\to 2\mathrm{e}+{\mathrm{H}}^{+} $ Ionization [26] 8. $ {\text{e}} + {\text{H}} \to {\text{e}} + {\text{H}}(n = 2, 3) $$ \mathrm{e}+\mathrm{H}\to \mathrm{e}+\mathrm{H}(\mathrm{n}=\mathrm{2, 3}) $ Excitation [26] 9. $ {\text{e}} + {\text{H}}({\text{n}} = 2, 3) \to 2{\text{e}} + {{\text{H}}^ + } $$ \mathrm{e}+\mathrm{H}(\mathrm{n}=\mathrm{2, 3})\to 2\mathrm{e}+{\mathrm{H}}^{+} $ Ionization [26] 10.$ {\text{e}} + {\text{H}}_2^ + \to {\text{e}} + {{\text{H}}^ + } + {\text{H}} $$ \mathrm{e}+{\mathrm{H}}_{2}^{+}\to \mathrm{e}+{\mathrm{H}}^{+}+\mathrm{H} $ Dissociative excitation [26] 11.$ {\text{e}} + {\text{H}}_2^ + \to {\text{e}} + {{\text{H}}^ + } + {\text{H}}(n = 2) $$ \mathrm{e}+{\mathrm{H}}_{2}^{+}\to \mathrm{e}+{\mathrm{H}}^{+}+\mathrm{H}(\mathrm{n}=2) $ Dissociative excitation [28] 12.$ {\text{e}} + {\text{H}}_2^ + \to {\text{H}} + {\text{H}} $$ \mathrm{e}+{\mathrm{H}}_{2}^{+}\to \mathrm{H}+\mathrm{H} $ Dissociative recombination [29] 13.$ {\text{e}} + {\text{H}}_3^ + \to {\text{e}} + 2{\text{H}} + {{\text{H}}^ + } $$ \mathrm{e}+{\mathrm{H}}_{3}^{+}\to \mathrm{e}+2\mathrm{H}+{\mathrm{H}}^{+} $ Dissociative excitation [28] 14.$ {\text{e}} + {\text{H}}_3^ + \to 3{\text{H}} $$ \mathrm{e}+{\mathrm{H}}_{3}^{+}\to 3\mathrm{H} $ Recombination [29] 15.$ {\text{e}} + {\text{H}}_2^ + \to 2{\text{e}} + 2{{\text{H}}^ + } $$ \mathrm{e}+{\mathrm{H}}_{2}^{+}\to 2\mathrm{e}+{2\mathrm{H}}^{+} $ Dissociative [26] 16.$ {\text{e}} + {{\text{H}}_2} \to {\text{e}} + {{\text{H}}_2}(v = 1 - 14) $$ \mathrm{e}+{\mathrm{H}}_{2}\to \mathrm{e}+{\mathrm{H}}_{2}(\mathrm{w}=\mathrm{1, 2}, 3) $ Radiative decay and excitation: EV [30] 17.$ {\text{e}} + {{\text{H}}_2}(v = 1 - 14) \to {\text{e}} + 2{\text{H}} $$ \mathrm{e}+{\mathrm{H}}_{2}(\mathrm{w}=\mathrm{1, 2}, 3)\to \mathrm{e}+2\mathrm{H} $ Dissociation [31] 18.$ {\text{e}} + {{\text{H}}_2}(v = 1 - 14) \to {\text{H}} + {{\text{H}}^ - } $$ \mathrm{e}+{\mathrm{H}}_{2}(\mathrm{w}=\mathrm{1, 2}, 3)\to \mathrm{H}+{\mathrm{H}}^{-} $ Dissociative electron attachment: DA [26] 19.$ {\text{H}}_2^ + + {{\text{H}}_2} \to {\text{H}}_3^ + + {\text{H}} $$ {\mathrm{H}}_{2}^{+}+{\mathrm{H}}_{2}\to {\mathrm{H}}_{3}^{+}+\mathrm{H} $ Ion formation [32] 20.$ {\text{e}} + {{\text{H}}^ - } \to 2{\text{e}} + {\text{H}} $$ \mathrm{e}+{\mathrm{H}}^{-}\to 2\mathrm{e}+\mathrm{H} $ Electron detachment: ED [28] 21.$ {\text{H}}_2^ + + {{\text{H}}^ - } \to {\text{H}} + {{\text{H}}_2} $$ {\mathrm{H}}_{2}^{+}+{\mathrm{H}}^{-}\to \mathrm{H}+{\mathrm{H}}_{2} $ Mutual neutralization: MN [33] 22.$ {\text{H}}_2^ + + {{\text{H}}^ - } \to 3{\text{H}} $ Mutual neutralization: MN [39] 23.$ {\text{H}}_3^ + + {{\text{H}}^ - } \to 2{{\text{H}}_2} $$ {\mathrm{H}}_{3}^{+}+{\mathrm{H}}^{-}\to 2{\mathrm{H}}_{2} $ Mutual neutralization: MN [33] 24.$ {\text{H}}_3^ + + {{\text{H}}^ - } \to 4{\text{H}} $ Mutual neutralization: MN [39] 25.$ {\text{H}}_{}^ + + {{\text{H}}^ - } \to {\text{H + H}} $ Mutual neutralization: MN [39] 26.$ {\text{H}}_{}^ + + {{\text{H}}^ - } \to {\text{H + H}}(n = 2, {\text{ }}3) $ Mutual neutralization: MN [33] 27.$ {\text{H}} + {{\text{H}}^ - } \to {\text{e}} + {{\text{H}}_2} $$ \mathrm{H}+{\mathrm{H}}^{-}\to \mathrm{e}+{\mathrm{H}}_{2} $ Associative detachment: AD [33] 28.$ {\text{wall \& PG: H}}_3^ + \to {{\text{H}}_2} + {\text{H}} $$ {\mathrm{H}}_{3}^{+}+\mathrm{w}\mathrm{a}\mathrm{l}\mathrm{l}\to {\mathrm{H}}_{2}+\mathrm{H} $ Ion wall recombination [34] 29.$ {\text{wall \& PG: H}}_3^ + \to 3{\text{H}} $ Ion wall recombination [34] 30.$ {\text{wall \& PG: H}}_2^ + \to {{\text{H}}_2} $$ \to {\mathrm{H}}_{2} $ Ion wall recombination [34] 31.$ {\text{wall \& PG: H}}_2^ + \to 2{\text{H}} $ Ion wall recombination [34] 32.$ {\text{wall \& PG: }}{{\text{H}}^ + } \to {\text{H}} $ Ion wall recombination [34] 33.$ {\text{wall \& PG: H}} + H \to {{\text{H}}_2} $$ \mathrm{H}+\mathrm{H}+\mathrm{w}\mathrm{a}\mathrm{l}\mathrm{l}\to {\mathrm{H}}_{2} $ H wall recombination [35,36] 34.$ {\text{wall \& PG: H}}(n = 2, {\text{ }}3) \to {\text{H}} $ H(n) wall recombination [35,37] 35.$ {\text{wall \& PG: }}{{\text{H}}_2}(v = 1 - 14) \to {{\text{H}}_2} $$ {\mathrm{H}}_{2}(\mathrm{w}=\mathrm{1, 2}, 3)+ $ Vibrational de-excitation: WD [35,38] 36.$ {\text{PG: H}} \to {{\text{H}}^ - } $ Surface generation [40] 37.$ {\text{PG: }}{{\text{H}}^ + } \to {{\text{H}}^ - } $ Surface generation [40] 38.$ {\text{PG: H}}_2^ + \to 2{{\text{H}}^ - } $ Surface generation [40] 39.$ {\text{PG: H}}_3^ + \to {{\text{H}}_2} + {{\text{H}}^ - } $ Surface generation [40] 40.$ {\text{PG: H}}_3^ + \to 3{{\text{H}}^ - } $ Surface generation [40] 
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