Density Limits for the Core and Edge Plasmas Related to the Local Temperatures in LHD

Easy access to the high-density regime without fatal disruptive phenomena is one of the important characteristics of the Large Helical Device (LHD). The operational density is considerably higher than the Greenwald density limit for tokamak plasmas. The density limit in LHD is reached when the edge density at the last closed flux surface exceeds a value approximately equivalent to the Sudo density limit that increases with the square root of the heating power. Extremely high central density of >1 × 10²¹ m^-3 is achievable with a peaked density profile, as long as the edge density is kept lower than the Sudo limit. Furthermore, the central heating power must be larger than the radiation loss in the core region to avoid the "cold-core" phenomenon. As for the plasma edge, complete detachment takes place when the edge density exceeds the limit. Then, reattachment/Serpens mode/radiative collapse will follow, depending on the recycling condition.