The perturbative analysis is recognized as a powerful tool to study an effect of turbulence on transport and its unique feature makes possible to study the transport in the almost flat electron temperature region, where the usual analysis based on the calculated power deposition and measured gradients is not applicable. To study the transport features of the confined plasmas, the heat pulse propagation experiments are performed by on-axis electron cyclotron heating (ECH) power modulation on the LHD plasmas with neutral beam injection (NBI). The heat pulse propagation can not be explained by transport features obtained from the steady state analysis. Requirement of larger (3-5 times) heat diffusivity to reproduce the heat pulse propagation in Co-and balanced NBI plasmas indicates the non-linearity of the heat transport. The analysis method for the heat pulse propagation based on the non-linear dependence of heat diffusivity X

The effect of the rational surface on the heat transport is found to be more important in the counter dominant NBI heated plasmas. A unique feature of heat pulse propagation is observed near the m/n = 2/1 rational surface (m, n are the poloidal and toroidal mode numbers, respectively). A simultaneous response of the temperature perturbation on radially separated flux surfaces is observed. This non-monotonic heat pulse propagation can not be explained even if the heat transport is strongly non-linear. The change in the magnetic field topology due to enlargement of a magnetic island structure is used to explain this non-monotonic heat pulse propagation phenomenon. The estimated O-point position of the island is located near the m/n = 2/1 rational surface. The m/n = 2/l island healing with decrease in electron collisionality is also observed as was predicted by theories. The magnetic island enlargement is considered to be related to a direction and profile of plasma current mainly driven by NBI. The simple equation of heat pulse propagation in slab geometry with time-dependent boundary conditions is used to evaluate the heat diffusivity inside the magnetic island. The estimated electron heat diffusivity inside an m/n = 2/l magnetic island has same order as X

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