A scaling law of the neutral opacity and Balmer-α wing shape in high-temperature plasmas

Hydrogen atoms penetrate deep inside high-temperature plasmas through repetition of charge exchange collisions, providing the particle source to the plasma. Many works have been conducted to simulate their dynamics, however, their systematic understandings are still sparse. Recently, the authors have reported that the neutral atom emission from the core region can be detected in the far wings of Balmer-α line. To understand neutral-plasma interaction and to interpret the experimental observations, an analytical representation of their dynamics is important.

In this talk, we present a scaling law of the neutral opacity as a function of the electron density and ion temperature at the pedestal as well as their decay lengths. We establish this scaling law by approximating a plasma as a cylindrical shape with a flat-top profile at the core and exponentially decaying parameters at the edge. This scaling law leads to a power-law decay in the Balmer-α line wings, the power-law index of which is represented again by these parameters. We compared this scaling law with a simple Monte-Carlo simulation as well as spectroscopic observations of LHD plasmas, both of which show a reasonable agreement with the scaling law. This scaling law can be used not only as diagnostics but also to establish a zero-dimensional particle transport analysis including recycling and penetration of the neutrals.