Observation of plasma density oscillation with doubled value of RF frequency in J-PARC RF ion source

Shibata, Takanori*; Shinto, Katsuhiro   ; Takagi, Akira*; Oguri, Hidetomo  ; Ikegami, Kiyoshi*; Okoshi, Kiyonori ; Nammo, Kesao*; Naito, Fujio*

Balmer alpha line intensity in J-PARC Radio Frequency (RF) negative hydrogen ion source has been measured by photometry measurement. The line intensity shows several interesting time characteristics in different phases; (1) 2 MHz (RF frequency) oscillation just after plasma ignition and (2) 4 MHz (doubled RF frequency) and 2 MHz coupled oscillation in the steady-state. From the comparison between numerical analysis, it has been explained that electron acceleration in inductively coupled electromagnetic field takes place with 4 MHz frequency which results in the 4 MHz line intensity oscillation. From the understandings of the background physics, we can conclude that this fast photometry measurement is a good diagnosis tool to understand whether RF plasma is in E-mode or H-mode in general RF ion sources.