The effect of radiation trapping on the ambient gas pressure in a stationary high-density He arcjet plasma

The purpose of this study is to investigate the effect of radiation trapping on the ambient He gas pressure by measuring He I resonance (1¹S-2¹P, 58.4 nm) and forbidden line (1¹S-2³P, 59.1 nm) emissions in a stationary high-density He arcjet plasma. Here, by varying the gas pressure in the expansion chamber of arcjet, the resonance line intensity drastically changed due to a self-absorption, while the forbidden one did not suffer from it owing to less absorption coefficient.

It is found that the behavior of resonance intensity is divided into four regions. In the region I, the intensity is considerably increased, and subsequently it is decreased with gas pressure in the region II. In the region III and IV, the intensity is decreased more gently.

The reason for these is explained by the interaction between plasma and ambient He gas, that is, plasma cooling due to ambient gas, three-body collisional recombination and optical decay into ground state.

Furthermore, by comparing the allowed/forbidden intensity ratio with that by a collisional model taking into account a radiation trapping, we derive an atomic temperature and a plasma radius.