Spatially Resolved Ion Density Measurements in an Oxyfuel Cutting Flame

A novel variation of the Langmuir probe for spatially resolved ion density measurements in a dense flowing plasma is presented. The technique uses a spinning disc to rapidly transit a wire probe through the flame. Spatial resolution is accomplished by post-processing data collected with many wire locations. The technique resolves ion density with a resolution of 0.5 mm in a 3000 K atmospheric pressure flame. The oxyfuel cutting torch flame under study is found to have twelve hollow cones with a 1 mm base and 4 mm height with ion density 3 × 10¹⁸m^-3 along their surface. The base of the cones forms a ring, the inner diameter of which exhibits similarly large ion density, while the outer diameter appears to be weakened by entrained air. The ion densities decay over mm length scales from their peaks in the inner cones to form a semi-homogeneous annulus with ion density on the order 1 × 10¹⁷m^-3.