Repetitive pellet fuelling for high-density/steady-state operation on LHD

A repetitive pellet injector has been developed and pellet refuelling experiments have been launched on the Large Helical Device (LHD) in a new regime characterized by high density steady state operation. In order to suppress the disturbance to the core plasma as much as possible, we employ relatively small and slow pellets. The size and velocity of the pellets are typically 2.5 mm in diameter and 250 m s-1, respectively. The pellets were injected into neutral beam heated plasmas with repetition frequency of 10 Hz. The density rise Δn-bare per pellet is 1.5 ? 1019 m-3 and the pellet penetration depth, which is normalized by the plasma minor radius, is typically less than 0.5. Quasi-steady-state operation was achieved at plasma parameters of ne=0.8x1020m-3, Te = Ti = 1.0 keV. Repetitive pellet fuelling showed improved energy confinement compared with massive gas puff fuelling in the high density regime in common with previous transient pellet injection experiments, and it can sustain such an improved confinement property