Temperature dependence of the thermal diffusivity in high-collisionality regimes in the large helical device

The positive density dependence of energy confinement times, tau(E), as expressed in the international stellarator scaling 95 (ISS95), where tau(E) proportional to n(e)(-0.51) P-0.59 in is the line-averaged density and P is the heating power), declines in high-collisionality regimes in the large helical device experiments. In the low-collisionality regime, where parameter dependences in ISS95 agree well with the experiment, the temperature dependence of thermal diffusivity is as strong as predicted by the gyro-Bohm model and/or the neoclassical theory. As the collisionality increases to the plateau regime, the temperature dependence becomes moderate, where the thermal diffusivity is proportional to the square root of the electron temperature. Furthermore, the thermal diffusivity is inversely proportional to the magnetic field strength and the electron temperature gradient is proportional to the electron temperature in the high-collisionality regime. Compared with ISS95, the energy confinement time expected from these observations has a weaker density dependence together with mitigated power degradation: tau E proportional to n(e)(-1/3) P-1/3.