Development of ion sensitive probe and its application to RF plasma device DT-ALPHA

An ion sensitive probe was developed and introduced into the radio-frequency (RF) plasma source DTALPHA. The collector current was investigated by changing the position of the recessed collector electrode and the offset voltage to optimize these two parameters for ion temperature evaluation. It was found that the ion temperature could be overestimated when the retardation of bulk electrons is insufficient. In addition, it was also found that secondary electron emission from the collector surface results in overestimation of ion temperature. The dependence of ion temperature on RF heating power was then investigated. The ion temperature increased, and the ratio of ion temperature and electron temperature became close to 1 as RF power increased. This trend could be interpreted as a temperature relaxation between ions and electrons. The ion temperature dependence on neutral pressure was also investigated. Ion temperature monotonically decreased with increasing neutral pressure.