Ion-sound velocity at the plasma edge in fusion-relevant plasmas

Highlights: ► We determine plasma-sheath boundary via ion-sound velocity for warm plasmas. ► We compare the ion sound velocity with the ion directional velocity. ► Necessity of local polytropic coefficient function at the plasma sheath. -- Abstract: The plasma-sheath boundary is confirmed to be a surface at which the ion directional velocity toward the wall attains the ion-sound velocity (i.e., the "sonic point" or "Mach-1 point"). Expressing this statement quantitatively is a complex task which has been solved to a satisfactory degree for low temperature plasmas only, where the ion temperature is negligible with respect to the electron population temperature (i.e., "cold-ion" plasmas). In this paper we tackle this problem for "warm-ion" plasmas, in which the ion temperature is arbitrary. The proposed method is perfectly suited for fusion-relevant plasmas