1. Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, Xi’an 710038, China
2. Aviation Maintenance NCO School, Air Force Engineering University, Xingyang 464000, China
3. School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China
Abstract：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<PAr<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 Er decreases because the TG wave is more damped and effectively confined to the narrower radial boundary. The current density Jz 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 Jr 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.
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