Observations of spatially constant peak Ar II emission phase threshold at higher helicon densities

Summary form only given. This paper presents results of a curious observation of changes in helicon plasma source Ar II optical emission characteristics related to wave character at higher densities when the coupled power or magnetic field is increased. Experiments on both the WOMBAT experiment at Canberra and the University of Wisconsin helicon sources measured similar excited Ar II optical emission characteristics, yet the magnetic fields, coupled rf powers and plasma densities are quite different. The wave and peak optical emission characteristics measured during the 70 ns rf period change from a spatially traveling wave character at lower powers and magnetic fields in the "transition mode" to an apparent spatially constant phase at higher coupled powers and magnetic fields where the plasma is in the "blue mode". The "blue mode" is achieved on the WOMBAT experiment by increasing the coupled power from 2.3 kW to 3.4 kW at a magnetic field of 100 G with a rise in density from 1.2 to 1.8/spl times/10/sup 12//cc. The "blue mode" regime is achieved in the UW experiment by increasing the magnetic field from 200 G to 1000 G at a constant coupled power of 800 W with the plasma density rising from 0.4 to 2.5/spl times/10/sup 13//cc. Contour plots of the space-time emission modulation characteristics and the wave magnetic field plots illustrate the transition from a traveling wave to an increased standing wave character at higher densities where the optical peak emission phase is constant relative to the driving rf phase as the optical probe is moved. The emission characteristics are shown to correspond to the dominant m=+l mode in both the experiments although the probe was centered on the axis in the WOMBAT experiment and located just outside the Pyrex vacuum chamber for the UW experiment. The optical emission and wave characteristics and plasma deviation from a Maxwellian are discussed for the different regimes. Possible reasons for the emission observations including - n increased standing wave character of the wave at higher densities and a threshold for stimulated emission are discussed for the different regimes.