Comparative studies of a spherical tokamak and a conventional tokamak: Magnetic turbulence-induced transport

The paper reports experiments on turbulence-induced transport in plasmas with aspect ratios A of 1.4 ∼ 2.5 in a single machine. There is little aspect ratio dependence in electrostatic fluctuations. The relative levels of magnetic fluctuations in the core region are 0.01% and 0.05% for A=1.4 and 2.5, respectively. The electron heat transport estimated from the magnetic fluctuation level of 0.01% is 0.2 kW/m2, which is less than 2% of the electron heat flux of 13 kW/m2 estimated from the global power balance. Thus in spherical tokamaks, the present results show that magnetic fluctuations are not responsible for electron heat transport, as in conventional tokamaks. (author)