[P-EM15] Space Plasma Physics: Theory and Simulation

[PEM15-07] Neutral depletion and the density limit of the helicon plasma

*Shogo Isayama¹, Shunjiro Shinohara², Tohru Hada¹, Shih Hung Chen³ (1.Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 2.Institute of Engineering, Tokyo University of Agriculture and Technology, 3.Department of Physics, National Central University)

Keywords:Helicon plasma, Neutral depletion, Density limit

Neutral depletion is a process in which neutral density decreases due to processes of ionization, neutral heating, and the drag (collision) by ions. In laboratory plasma sources, some experimental [1] and theoretical studies [2] suggest that the neutral depletion plays an important role in determining the plasma transport and the maximum plasma density. The neutral depletion caused by the collision with ions, is known to occur when the plasma pressure is comparable to the neutral gas pressure. If this is the case, the neutrals will naturally be cleared away from the region of high plasma pressure so that the net force balance is maintained. The plasma density consequently saturates to some density limit as the input power is increased. However, the relationship between the neutral depletion and the density limit is not well understood. Physical understanding of this saturation process is important to overcome the density limit of plasma sources.
In our study, we have constructed the self-consistent fluid model to investigate the time evolution of the helicon discharge with the neutral dynamics [3, 4]. In our simulation, the plasma density of ~10¹⁹ m^-3 is generated, and the neutrals are depleted to ~10²⁰ m^-3 under a typical set of experimental parameters. When the input power is increased, the plasma density temporarily grows to ~10²⁰ m^-3, but the neutrals are seriously depleted to ~10¹⁸ m^-3. In this situation, the electron temperature considerably increases since the power absorption (Joule heating) via Coulomb collision still occurs while the loss of energy by the ionization of neutrals becomes very small. As a result, the plasma density decreases to ~10¹⁹ m^-3 due to the increase of outflow flux which is created by the strong pressure gradient. In this presentation, we discuss the effect of the neutral depletion and the density limit of the helicon plasma.

[1] S. Shinohara, D. Kuwahara, K. Yano and A. Fruchtman, Phys. Plasmas, 23 122108 (2016).
[2] A. Fruchtman, J. Phys. D: Appl. Phys. 50, 473002 (2017) (review paper).
[3] S. Isayama, S. Shinohara, T. Hada and S. H. Chen, Phys. Plasmas, 26 023517 (2019).
[4] S. Isayama, S. Shinohara, T. Hada and S. H. Chen, Phys. Plasmas, 26 053504 (2019).