The role of sheath and acceptance angle in front of a retarding field energy analyzer for plasma flow analysis

Measurements of plasma flow are of key interest in a number of plasma environments and applications. In laboratory magnetized plasmas, the directional Langmuir or Mach probe is a well-proven `in-situ' diagnostic tool to obtain the flow velocity. However, in non-magnetized or weakly magnetized plasmas, this method does not readily yield reliable velocity measurements, as it has been shown by numerical and experimental studies that the collection of upstream ions to the rearward probe surface can be significant. In this study, we utilize the analysis of data from 3D PIC simulations of ion velocity distributions in the vicinity of a negatively biased object embedded in a collision-less, source-free plasma with and without flow. The simulations allow us to study how the grounded probe housing of a retarding field ion energy analyzer (RFEA) affects the distribution of ions and their collection at different angles with a flowing, electropositive plasma. Comparisons are carried out with RFEA measurements in an inductively coupled helicon plasma.