# Comment on "Ion velocity analysis of rotating structures in a magnetic linear plasma device" [Phys. Plasmas 25, 061203 (2018)]

## TL;DR

This paper critiques a previous study on ion drift velocities in plasma, highlighting that the original analysis used inappropriate assumptions about geometry and ion behavior, leading to invalid conclusions.

## Contribution

It clarifies the importance of considering cylindrical geometry and ion Larmor radius effects, providing a more accurate interpretation of ion drift in plasma devices.

## Key findings

- Classical ExB drift formula is invalid in cylindrical geometry.
- Ion Larmor radius can exceed plasma column diameter.
- Ion movement is primarily parallel to electric field after creation.

## Abstract

In a recent paper (Phys. Plasmas 25, 061203, 2018), the authors have presented the analysis of the electric ion drift velocity experienced by heavy ions created in a plasma submitted to a low magnetic field. Unfortunately, they have used the classical ExB drift formula that is valid only in slab geometry. The authors have not taken into account that the cylindrical geometry induces a slow electric drift of the ions around the axis of the column. Moreover, the low magnetization of the ions induces a Larmor radius that is larger than the diameter of the plasma column. The movement of the ions immediately after their creation is parallel to the local electric field, not perpendicular as indicated by the authors. Most often the ions are neutralized before experiencing the electric drift calculated along the classical guiding center theory. This has not been taken into account carefully by the authors so that the theoretical analysis of the Laser Induced Fluorescence measurements presented in this paper is clearly invalid.