The ion polytropic coefficient in a collisionless sheath containing hot ions

Abstract

The fluid approach has been widely used to study plasma sheath dynamics. For a sheath containing hot ions whose temperature is greater than the electron's, how to truncate the fluid hierarchy chain equations while retaining to the fullest extent of the kinetic effects is always a difficult problem. In this paper, a one-dimensional, collisionless sheath containing hot ions is studied via particle-in-cell simulations. By analyzing the ion energy equation and taking the kinetic effects into account, we have shown that the ion polytropic coefficient in the vicinity of the sheath edge is approximately constant so that the state equation with the modified polytropic coefficient can be used to close the hierarchy chain of the ion fluid equations. The value of the polytropic coefficient strongly depends on the hot ion temperature and its concentration in the plasma. The semi-analytical model is given to interpret the simulation results. As an application, the kinetic effects on the ion saturation current density in the probe theory are discussed.

Effect of High Ion Temperature on the Polytropic Coefficient in the End Region of GAMMA 10/PDX

“…Together with the result in Ref. [9], this result suggests that γ was lowered by a hightemperature ion component resulting from the ICRF heating. Without the RF2, the evaluated γ exceeded 3.0, the upper limit in the plasma fluid model.…”

“…In the RF2 operation, the ion energy spectra obtained by the IEA had a high T i|| component. The value of γ evaluated in this experiment might have been lowered under the effect of a high-temperature ion component resulting from ICRF heating, as also mentioned in [9].…”

“…Next, we consider the effect of the high-temperature component of the ions on γ, mentioned in Ref. [9]. The authors of [9] suggested that a high T i component in collisionless plasma would reduce γ below its value in the widely used model.…”

“…[9]. The authors of [9] suggested that a high T i component in collisionless plasma would reduce γ below its value in the widely used model. Figures 6 (a) and (b) show the energy spectra obtained by the IEA in the RF2 and no-RF2 operations, respectively.…”

“…Theoretical and numerical studies using plasma fluid models have shown that the sheath characteristics are affected by ion temperature T i [4][5][6][7] and γ [8]. In a onedimensional (1D) particle model, the authors of [9] also showed that the hot T i|| component, which is higher than T e , imparts strong kinetic effects and reduces the γ in collisionless plasma below the widely used value in 1D adiabatic flow, where the temperature is anisotropic and γ = 3 in the flow direction. However, as experimental investigation of this phenomenon is lacking, investigating the effect of high T i|| on γ and C s is important.…”