氩气压力对螺旋波放电影响的发射光谱诊断及仿真研究

螺旋波等离子体源以其高电离效率与高密度优势受到多个领域的青睐。螺旋波放电高电离效率的机理或者功率耦合模式，一直是困扰该领域学者的难点之一，对于放电过程与特性的诊断则是揭示其物理机制的重要途径。光谱诊断能够克服介入式诊断手段对等离子体的干扰同时受等离子体烧蚀等弊端，且响应速度快、操作灵活。为研究螺旋波等离子体的放电特性以及气体压力的影响，开展了以氩气为工质气体的光谱实验研究，并针对实验开展了Helic程序数值模拟。通过改变光纤探头焦距调整径向诊断位置，得到谱线强度的径向分布。由氩原子4p-4s能级跃迁产生的谱线主要集中在740～920 nm区间，谱线相对强度较离子激发谱线较强。实验研究发现，在较低氩气压力范围（0.2 Pa<P_Ar<1.0 Pa），随着压力增加，放电光强迅速增加，但是当压力增加到大于1.0 Pa之后，光强增长的趋势变缓，甚至部分谱线的相对强度不再增长，达到类饱和状态，朗缪尔探针测量得到离子密度变化趋势与其相似。光强分布在靠近径向边界处（r≈4 cm）存在凸起，且随压力增加，该凸起分布更为明显。通过对电子温度的计算发现，压力增加到一定程度将影响放电均匀性。仿真结果显示，增大压力，功率沉积密度的径向分布逐渐向径向边界处积累，与实验观察到的谱线强度径向凸起相一致，螺旋波与TG波的耦合效率增加。随着气体压力的增加，E_r的径向边界峰值降低，原因是波所受阻尼增强，TG波被有效地局限于径向较窄的边界处。电流密度轴向分量J_z在等离子体内部和边界处的峰值呈显著的减小趋势，可见，虽然压力增加一定程度上提高了等离子体密度，但却相应的减小了电离率，导致轴向电流密度受限。但是径向电流密度J_r却呈现先减小后增大的趋势，且增长幅度明显，综合来看，放电效率有所提高。可见适当增加气体压力，有助于提高放电的功率耦合效率和强度，增加等离子体密度。光强比值法是针对线性谱线参数计算的典型方法，Helic程序亦是专业领域内认可度很高的计算工具，结果可靠，分析方法具有可借鉴性。实验及仿真结果对于提高氩气工质下的螺旋波放电强度提供了一定的参考价值。

Emission Spectroscopy Diagnosis and Simulation Study of Argon Pressure Effect on Helicon Wave Discharge

Helicon plasma sources have gradually been widely adopted in various research fields due to their high ionization efficiency and high density. Lacking in understanding of the mechanism for the high ionization efficiency and the power coupling mode of helicon discharge has been a great challenge for scholars to deal with in this field. To diagnose the discharge process and the characteristics is an important way to reveal its physical mechanism. Spectral diagnosis can avoid the interference of contact measurements on plasma and is free from plasma ablation. It responds quickly and is flexible to operate. In order to study the discharge characteristics of helicon plasma and the influence of gas pressure, researches on emission spectral diagnosis of argon discharge and numerical simulation of the Helic code for these experiments were conducted. The radial profiles of the line intensity were obtained by changing the focal length of the fiber optic probe to adjust the radial diagnostic position. Atomic emission lines of argon are mainly concentrated in the 740～920 nm region, which are generated by the transition of argon atoms between the 4p-4s energy levels and stronger than the relative intensity of ion lines. It can be found that at lower pressure ranges (0.2 Pa<P_Ar<1.0 Pa), the discharge intensity increases rapidly with pressure, and tend to be nearly saturated when the pressure reaches 1.0 Pa or larger. Langmuir probe measurement shows a similar trend in ion density. A “bump-on-boundary” of the line intensity profile was observed near the radial boundary (r≈4 cm), which was more obvious as the pressure increased. By calculating the electron temperature, it was found that the discharge uniformity will be influenced when the pressure is increased to a certain extent. The simulation results show that the radial profile of power absorption gradually increases toward the radial boundary, which is consistent with the experimentally observed “bump-on-boundary” of the line intensity. And the coupling efficiency of the helicon-TG waves increases. As the gas pressure increases, the radial boundary peak of E_r decreases because the TG wave is more damped and effectively confined to the narrower radial boundary. The current density J_z shows a significant decrease in the peaks inside and at the boundary of the plasma. It can be seen that although the pressure increase improves the plasma density to some extent, the ionization rate is correspondingly reduced, resulting in limited axial current density. However, the radial current density J_r firstly decreases and then increases, and the growth rate is obvious. Overall, the discharge efficiency has improved. Appropriately raising the gas pressure helps to improve power coupling efficiency and strength of the discharge, as well as the plasma density. The light intensity ratio method is a typical method for the calculation of linear spectral line parameters. The Helic code is also a highly recognized tool in the professional field. Therefore, the results are reliable and the analytical methods have the value of reference. The experimental and simulation results provide a certain reference value for improving the helicon discharge intensity under argon working fluid.