Phase resolved two-dimensional emission pattern of a low-frequency helium discharge

Time resolved two-dimensional (2-D) plasma-induced emission patterns of the 3/sup 3/D to 2/sup 3/P transition in a three-torr low-frequency helium discharge have been measured. The emission patterns are found to be asymmetric for complementary phases of the current at 30-KHz excitation frequency, however, they are symmetric for 200-kHz excitation frequency. The excitation of the 30-KHz discharge is similar to that of a dc discharge with increasing and decreasing current.