Density regime of complete detachment and operational density limit in LHD

The highest central density in net current free helical plasmas of 5 x 1020 m-3 has been demonstrated in the recent Large Helical Device (LHD) experiment. The volume-averaged electron density reaches 3 x 1020 m-3, in spite of a small power density of less than 0.5 MW/m3 and a magnetic field strength of less than 3 T. These are attained in the plasmas with strongly peaked density profiles generated by pellet injection. In the case of gas-fueled plasmas, complete detachment takes place when the hot plasma boundary shrinks below the last-closed-flux-surface (LCFS), or in other words, the edge temperature decreases to a critical value. The edge density that results in this shrinking corresponds to the maximum edge density achievable under the attached condition. At detachment, the ionization front moves from the ergodic region to inside of the LCFS and the particle confinement effectively improves. Under an appropriate recycling condition, complete detachment is self-sustained, which is called the Serpens mode. The discharge is finally terminated by radiating collapse, unless gas fueling is stopped. Radiating collapse is triggered even at a small radiation loss fraction of about 0.3, while it ranges from 0.3 to 1 at complete detachment. It is therefore difficult to determine a threshold radiation loss fraction that determines the operational density limit. Both of the critical densities for complete detachment and radiating collapse are dependent on the square root of the heating power, as is predicted by the conventional Sudo density limit scaling. This is explained by the edge temperature property that is a function of the square root of the heating power divided by the density, i.e. the density that results in a critical temperature increases with the square root of the heating power. Even though the Sudo fraction, which is the ratio of the volume-averaged density to the Sudo scaling, reaches as high as 3.5 in pellet-fueled plasmas, the edge density in these cases are similar to those in gas-fueled plasmas of which the Sudo fraction is ∼0.8 at attachment. Indeed, the Sudo fraction is proportional to the peaking factor defined by the ratio of the volume-averaged density to the edge density, which exceeds 3 in pellet-fueled plasmas. In detached plasmas, edge densities are roughly twice as large as those in attached plasmas, reflecting the improved particle confinement. (author)