Measurement of ablation cloud parameters and plasma potential with on/off-axis pellet injection in the JIPP T-IIU tokamak

A pellet injection system with controllability of the injection angle has been developed and installed in the JIPP T-IIU tokamak. A long helical ablation cloud (‘tail’) has been observed when an ice pellet is injected into the tokamak plasma using the controllable injection angle system. The ‘tail’ does not flow along the direction of the total magnetic field lines of the torus. In order to understand the details of the ice pellet ablation structure in the ‘tail mode’, the local parameters of electron density and temperature within the ablation cloud are obtained by a spectroscopic method. The electron density determined from Stark broadening of the Balmer β line is in the range 1.0 × 1016−1.0 × 1017 cm−3 while the temperature obtained from the line-to-continuum intensity ratios is in the range 1–4 eV. It has also been observed by a heavy ion beam probe (HIBP) measurement that the potential at the plasma centre becomes more negative and the potential profile becomes deeper negatively after pellet injection.