Mode identification of surface waves excited in a planar microwave discharge

A planar high-density (similar to 10(12) cm(-3)) plasma, 22 cm in diameter and 9 cm in length, is produced by a 2.45 GHz microwave radiation of 500 W through small slot antennas in argon at 20-350 Pa without a magnetic field. Several types of azimuthal and radial standing wave mode pattern are observed in the optical emission from the plasma depending on the discharge conditions. The microwave field in the plasma measured by a movable antenna decreases exponentially in the axial direction from the quartz wall adjacent to the slot antennas, thus suggesting the propagation of surface waves in the r, theta directions. The measured azimuthal microwave field distributions and the optical emission pattern clearly show a mode transition of the standing surface wave from a TM33 mode to a TM62 mode when the pressure is decreased from 140 to 44 Pa at the constant power of 400 W. Here TMmn denotes the transverse magnetic mode of azimuthal mode number m and radial mode number n. A wave dispersion analysis based on a one-interface uniform-density model predicts these modes in a range of electron densities corresponding to those measured by a Langmuir probe in the experiment.