Spectroscopic characterization of a- and .-mode in a capacitively-coupled plasma in the high pressure range up to atmospheric pressure

Summary form only given. Optical characteristics of different capacitive discharge modes, namely alpha- and gamma-mode, were investigated in the high gas pressure (10 torr) up to the atmospheric pressure. The plasma was produced between parallel copper electrodes using a 13.56 MHz RF source. An alpha-mode was excited as igniting the plasma at a low current region. At a certain discharge voltage and current, however, the alpha-mode was abruptly transformed into a gamma-mode with a voltage drop and a discharge volume contraction. Using optical spectroscopic methods, different discharge parameters were investigated. Because of the helium discharge gas, both discharge modes emitted intense excited atomic helium spectral lines, such as 587.6 nm and 667.8 nm. From the experimental results, the gamma-mode showed more intense emission and higher excitation temperature than that of the alpha-mode. On the other hand, the rotational temperature, corresponding to the gas temperature at the atmospheric pressure, were 350 K and 473 K for alpha-and gamma-mode, respectively