Helium volumetric recombining plasma formation for energetic ion injection in radio-frequency plasma device DT-ALPHA

The spatial distribution of helium volumetric recombination in a radio-frequency (RF) plasma device was investigated in two different plasma production cases. It was revealed that the radial distribution of volumetric recombination was strongly localized in the peripheral region of a cylindrical plasma at a higher RF heating power and lower neutral pressure case. In contrast, volumetric recombination was widely distributed around the plasma column at a lower RF heating power and higher neutral pressure. To understand helium recombining plasma formation, the electron-ion temperature relaxation time was evaluated for each plasma production case. The electron-ion temperature relaxation time in the gas puffing region becomes much smaller than the plasma confinement time in the latter plasma production case, whereas it is much larger than the plasma confinement time in the former plasma production case. This result indicates that energy transfer from electrons to bulk ions plays an important role in helium recombining plasma formation in an RF plasma device.