Advanced Operational Regime with Internal Diffusion Barrier on LHD

An interesting high-density operational regime with an internal diffusion barrier (IDB) has been extended to the helical divertor configuration in the Large Helical Device. The IDB is characterized by steep density gradient in core plasma and the attainable central density exceeds 1 × 10²¹ m^-3 at the moderate magnetic field [approximately]2.5 T while keeping relatively low density mantle plasma surrounding the core. In the IDB discharge, significant central pressure rise is observed, and the maximum central pressure attains 150 kPa by optimizing magnetic configuration. Such a high central pressure causes very large Shafranov shift, more than half radius, even at high magnetic field. Core fueling is absolutely essential for the IDB formation, and the IDB is reproducibly obtained by employing intensive multiple-pellet injections. The attainable density is restricted by lack of heat deposition at core plasma due to strong attenuation of a neutral beam in the high-density plasma.