Characteristics of the Global Energy Confinement and Central Pressure in LHD

Abstract

Global energy confinement in the Large Helical Device has been proved to be comparable to those of tokamaks in ELMy H-mode. It shows a gyro-Bohm-like property as seen in international stellarator scalings. This implies that the anomalous transport dominates the neoclassical transport. At least in the configurations with small helical ripples, no significant collisionality dependence predicted by the neoclassical theory has been observed. Confinement degradation compared with the international stellarator scalings often takes place in the plasmas with high peripheral density. In many cases, this is due to the shallow penetration of heating beams. Adding to this, deviation from gyro-Bohm can be caused by the emergence of a “weak temperature dependence” of the thermal diffusivity. It depends on the plasma parameters whether this weak temperature dependence deteriorates or improves the thermal transport. The central pressure that is also an important parameter for envisioning a fusion reactor is not necessarily proportional to the global confinement. The central pressure is insensitive to the variation of magnetic configuration that strongly affects the global confinement.