Density enhancement near lower hybrid resonance layer in m=0 helicon wave plasmas

Radial density profiles are measured with various frequencies, powers, pressures, and gas masses in $\text{m=0}$ helicon wave plasmas. The radial density profiles show clear mode changes for the E mode (capacitive coupling), the H mode (inductive coupling), and the W mode (wave coupling) as the rf (radio frequency) power increases. The radial density profile hardly changes as the frequency varies in the E and the H modes but changes substantially in the W mode; it changes from a central peak profile to a hollow density profile near the lower hybrid resonance frequency. The change of radial density profile near the lower hybrid resonance frequency occurs only at pressures below 20 mTorr, and the resonance effects become very weak at higher pressures. The change of the radial density profile may be explained by the density enhancement near the lower hybrid resonance layer. The experiments with various magnetic fields and gas masses show a close correlation between the lower hybrid frequencies and the density profile changes.