Spatial Characterization of Wave Modes in Argon Helicon Plasma Source

ABSTRACT We report spatial characterization of multiple wave‐coupled modes in an argon helicon plasma. Under 0.8 Pa discharge and 0.05 T axial magnetic field, increasing RF power from 800 to 2000 W produces four distinct wave modes (W1–W4) at the antenna center. Observable modes decrease axially and radially with decaying plasma density. Critical densities for mode transitions are 1.3 × 10 12 , 3.2 × 10 12 , 5.0 × 10 12 and 8.0 × 10 12 cm −3 . Low‐order W1 mode shows flat density and temperature radial profiles, while high‐order W3, W4 modes form a prominent central density peak and “blue core”. Axial wave measurements capture standing‐traveling wave patterns with phase jumps for axial wavenumber extraction. Based on helicon wave dispersion theory, we identify two transition mechanisms: axial transitions from rising axial eigenmode numbers, and radial transitions corresponding to varied radial wave structures. Multiple evidence including profile evolution and scale length variation supports our interpretation. Higher‐order mode extinction originates from insufficient local plasma density. This work builds a quantitative framework linking density thresholds to eigenmode spatial persistence to support selective mode control. Full‐vector wave field mapping is required for precise radial eigenmode verification in future studies.