Performance of fueling pellet injectors for the large helical device

Techniques of solid hydrogen pellet injection into plasmas towards steady state operation as well as further reliable hardware have been demonstrated for the large helical device (LHD). The conversion of hardware is performed to attain higher reliability and reproducibility of pellets. Major issues to improve the quality of pellets are the cooling temperature of pellet production by a cryo-cooler and the incident angle to the inner wall of the guide tube for pellets. The former is improved from 12.6 to 10.6 K by the reduction of the heat conduction and radiation to produce the robust pellets. The latter is optimized from 5.41° to 1.73° at the maximum by the low-angle pellet path to mitigate the destruction of pellets in the guide tube. It has been confirmed by these conversion that the quality of pellets is significantly improved, i.e., the probability of intact pellets is increased to ∼100%. Also, the controllability of the pellet size is desired as needs of pellet injection experiments. It has been demonstrated by the property of pellet production related to the cooling temperature and the pressure of supplied hydrogen gas.